Control Tools

• Diagnosis is the basis for prevention and control of AT. The combination of better, more sensitive and specific parasitological, molecular and serological methods and the integrative approach to diagnose this disease complex will result in better surveillance and prevention. Parasitological techniques are inexpensive and can be used in the field or in itinerant laboratories, but require trained technical personnel and lack sensitivity. Other diagnostic techniques available for AT (PCR and ELISA) are mostly applicable at laboratory level, which is suitable for epidemiological studies, and monitoring at population level, provided that local laboratories may acquire specific reagents and capacities. Efforts are still needed to accomplish and generalize such practice, especially in the context of the Progressive Control Pathway (PCP). Support from WOAH, as well as national and local authorities in the affected areas would facilitate the collection and storage of reference isolates/strains and biological samples that are required for adequate validation and verification of the performance of novel diagnostic tools.

  Point of care diagnostics are lacking and are required, primarily to support decision treatment, because laboratory diagnosis generates delays and costs that are most often not compatible with field requirements.

  Particularly for surra and dourine in horses, several aspects need to be improved: (i) specificity of the tests (PCR and ELISA) which currently do not allow to distinguish species amongst Trypanozoon subgenus, (ii) deliberate lack of data reporting from some countries. Similarly, for surra and nagana in camels, because treatment is not the same, there is need to distinguish Trypanosoma species (T. evansi / T. vivax) especially in Middle East and North Africa, where apparently, camel trypanosomosis is systematically considered as surra, while nagana may already be significantly prevalent.

  The absence of vaccine leaves only two options for control: drug use and vector control.

  A few drugs are available to treat AT, but effectiveness is limited by a number of problems that need to be addressed, including inefficient supply chains, presence of fake drugs in the market, incorrect use and wrong dosage, lack of support from veterinary services, chemo-resistance and a limited number of private services. The development of new drugs and their arrival on the market would increase the therapeutic arsenal and provide an option in the event of chemo-resistance.

  Vector control is an efficient way to control AAT by reducing or stopping parasite transmission. Several methods can be used to reduce or eliminate tsetse fly populations. These methods include: i) insecticide-treated cattle, ii) insecticide-impregnated attractive systems to kill the tsetse (e.g. traps and targets) and iii) area-wide interventions using aerial spraying or the sterile insect technique.

  However, control, or even elimination, of the tsetse fly in affected areas is complex, costly, and needs permanent effort unless elimination is reached, which has been achieved in a few areas. Optimal sustainable and cost-effective intervention strategies need to be established and adapted to epidemiological contexts and agro-ecological settings. The role of mechanical vectors in surra epidemiology is generally overlooked and control methods for mechanical vectors are not considered efficient so far, due to the very high prolificity of these oviparous biting flies. However, new traps are in the prototype stage and new tools may emerge from research and development efforts.

  As far as basic knowledge is concerned, there is a lack of fundamental knowledge about livestock immune response to trypanosomes and its role in protection or pathogenesis. How host, parasite and environmental factors interact and contribute to disease severity is badly understood. More particularly, research on trypanotolerance mechanisms in cattle and parasite pathogenic and virulence factors could pave the way to innovations in order to further develop a vaccine that could be a major breakthrough. There is also a lack of understanding of the role of trypanotolerant host, or more generally asymptomatic carriers, in the epidemiological cycle of AT and the transmission rate of parasites to vectors, and how they should be taken into account, depending on whether they are potential dead end or reservoir.

  Guidelines for the control of nagana have been established (FAO, CIRDES) but are insufficiently adopted and applied. There is a lack of funding and concertation between stakeholders to collectively address AT problems, and global commitment of stakeholders is needed. This is often amplified by the transboundary characteristics of AT epidemiology, linked to pastoralism and animal movements, as transboundary agreements between veterinary services on protocols for diagnostics and treatments are lacking. Also, the impacts of disease control and elimination on changes in livelihood, livestock sector, value-chains, trade, environment, including green house gas emissions, have been insufficiently assessed. Thus, interdisciplinary studies are needed. Finally, the effect of global climate change on vectors and AT is still not fully understood.

  HAT: In the last decade, there has been significant improvement in vector control and diagnostics and drugs for g-HAT. For r-HAT however, improvement has been more modest with the introduction of fexinidazole, an oral and safe and effective drug. However, children still need to undergo lumbar puncture and if in meningo-encephalitic stage, their treatment still relies on the extremely toxic melarsoprol. Simple diagnostic tests for r-HAT are lacking. The introduction of reliable rapid diagnostic tests for malaria reduces the number of microscopic examinations, lowering chances of unexpected trypanosome detection, which risks increasing underreporting of r-HAT further. Improved diagnosis of r-HAT in the livestock reservoir would be highly valuable, as well as increased modelling efforts to help predicting future outbreaks.

• Diagnostics availability

• Commercial diagnostic kits available worldwide

  Numerous diagnostic methods are available to detect trypanosomes or diagnose trypanosomoses, but few are commercially avaible (methods are described in the following sections).

  • One commercial kit is available for surra and has been validated by WOAH: the Card Agglutination Test for Trypanosomes CATT/T. evansi. The antigen used in the CATT/T. evansi consists of fixed and stained T. evansi Rode Trypanozoon antigen type (RoTat) 1.2 parasites produced in rats. The test mainly detects IgMs, which are early-circulating antibodies with short half-life making it a good indicator of a recent infection, or at least of a recent circulation of trypanosomes in the blood. This kit is rather inexpensive, fast, simple and can be implemented in the field on any host species. Its sensitivity is generally high in equids, camels, and dogs, with a medium specificity. It is of low sensitivity in cattle and sometime in buffaloes. This kit is available from the Institute of Tropical Medicine, Antwerp, Belgium (WOAH reference laboratory for surra), but its format is better suited to herd testing than individual testing.
  • HAT: For g-HAT, tests for serological screening include a card agglutination test for trypanosomiasis (semi-commercial CATT/T.b. gambiense) and two rapid diagnostic tests, HAT Sero K-Set (Coris Bioconcept, Gembloux, Belgium) and Abbott Bioline HAT 2.0 (Abbott, Seoul, South Korea). The mAECT is a parasitological test used as confirmatory test after a positive serology in g-HAT and is produced and distributed by the Institut National de Recherche Biomedical (INRB, Kinshasa, RD Congo).

  GAPS

  There is an urgent need for rapid diagnostic tests of AT, especially for nagana and surra. Ideally, such tests need to detect active infections, based on antigen or DNA/RNA detection of trypanosomes. Such test would allow targeting treatment on actually infected animals and would really improve management of animal health, while reducing and rationalizing use of drugs.

  Nagana: One rapid serological test, VerY Diag, is produced by CEVA (CEVA Santé Animale, Libourne, France) but is currently not commercially available and must be requested to CEVA. This serological test detects antibodies directed against one protein of T. vivax and one of T. congolense, it is thus not recommended for infection by T. brucei and does not indicate active infection. It has not been validated by WOAH.

  Although WOAH recommends ELISAs for the detection of immunoglobulin G (IgG), for nagana and surra, none are commercially available.

  HAT: For r-HAT, no simple screening test is available. Reference laboratory tests, both immunological and molecular tests, are usually in house although attempts are made to uniformize protocols between laboratories. Immune trypanolysis has so far been used as a reference test for presence of specific antibodies against T.b. gambiense but is carried out in only a few reference laboratories and a recent evaluation in pigs has evidenced lack of specificity (Ilboudo et al., 2022).

• Diagnostic kits validated by International, European or National Standards

  For AT: CATT/T. evansi is recommended by WOAH for surra diagnostic. No commercial diagnostic kits have been officially validated or certified by International, European or National Standards otherwise. For g-HAT, target product profiles (TPP) have been published by WHO.

  GAPS

  Lack of reliable, commercial kits with stable long-term commercialization for diagnosis of nagana and dourine.

• Diagnostic method(s) described by International, European or National standards

  Animal trypanosomoses: different techniques exist and are summarised in the WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals for animal diseases (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-manual-online-access/).

  Current diagnostic tests vary in their sensitivity and specificity, the ease with which they can be implemented and their costs. The choice of one or several tests is guided by epidemiologically adapted diagnostic requirements, availability of equipment, and expertise and economic principles. Characterization of available diagnosis techniques was reviewed recently (Desquesnes et al, 2022a). It should be noted that in most cases, a single diagnostic method applied at a given time does not unequivocally identify the various parasitological and disease statuses of a host which can be “non-infected”, “asymptomatic carrier”, “sick infected”, “cured/not cured” and/or “multi-infected”. Thus, the diversity of hosts affected by these animal trypanosomoses is such that integrative, diachronic approaches are needed that combine: (i) parasite detection, (ii) DNA, RNA or antigen detection and (iii) antibody detection, along with epizootiological information because no single diagnostic method can detect all active infections and/or trypanosome species or subspecies (Desquesnes et al. 2022b).

  Parasite detection: the sensitivity of the tests is globally low since parasitaemia is highly variable and low in average during infection course. Several techniques are available: thin blood smear and centrifugation technique (HCT).

  Giemsa-stained thin blood smear (GSBS): low sensitivity (10⁵–10⁶ trypanosomes/mL of blood); subgenus and sometimes species-specific; the species can also be deduced from epizootiological information. When positive, the GSBS brings diagnostic certainty. Parasitaemia being highly fluctuating, the sensitivity of the test is highly variable and generally considered as low.

  Animal trypanosomoses: different techniques exist and are summarised in the WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals for animal diseases (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-manual-online-access/).

  Current diagnostic tests vary in their sensitivity and specificity, the ease with which they can be implemented and their costs. The choice of one or several tests is guided by epidemiologically adapted diagnostic requirements, availability of equipment, and expertise and economic principles. Characterization of available diagnosis techniques was reviewed recently (Desquesnes et al, 2022a). It should be noted that in most cases, a single diagnostic method applied at a given time does not unequivocally identify the various parasitological and disease statuses of a host which can be “non-infected”, “asymptomatic carrier”, “sick infected”, “cured/not cured” and/or “multi-infected”. Thus, the diversity of hosts affected by these animal trypanosomoses is such that integrative, diachronic approaches are needed that combine: (i) parasite detection, (ii) DNA, RNA or antigen detection and (iii) antibody detection, along with epizootiological information because no single diagnostic method can detect all active infections and/or trypanosome species or subspecies (Desquesnes et al. 2022b). Parasite detection: the sensitivity of the tests is globally low since parasitaemia is highly variable and low in average during infection course. Several techniques are available: thin blood smear and centrifugation technique (HCT). Giemsa-stained thin blood smear (GSBS): low sensitivity (10⁵–10⁶ trypanosomes/mL of blood); subgenus and sometimes species-specific; the species can also be deduced from epizootiological information. When positive, the GSBS brings diagnostic certainty. Parasitaemia being highly fluctuating, the sensitivity of the test is highly variable and generally considered as low.

  HCT: medium sensitivity (10²–10³ trypanosomes/mL of blood); genus and sometimes subgenus specific; examinations must be carried out within a short time after blood sampling (preferably 1– 2 hours). When positive, sub-genus or species must be complemented by PCR to specify the species.

  PCR: sample preparation for PCR must be done on blood, or preferably on buffy coat, after blood centrifugation, using a commercial DNA purification kit or a Chelex resin preparation method. Recommended primers (Gold standard) are those targeting the satellite DNA (Masiga et al., 1992). The primers predominantly used are: TVW (T. vivax), TBR (Trypanozoon) and TCS (T. congolense type savannah). PCRs using these primers are highly sensitive and species-, sub-genus- or type-specific, respectively. When positive, PCR alone is not a diagnostic certainty; it must be complemented by other tests or information (see below), because it may give false positive results due to sample contaminations. When negative, PCR alone cannot ascertain the absence of infection due to false negative results obtained from animals with low parasitaemia.

  Antibody-detection tests (ELISA and IFAT) become positive 1–6 weeks after infection, thus with an incubation period of 2 weeks on average, and the persistence of antibodies after parasite elimination can last from 1 to 13 months and is 3 months on average.

  (i) ELISA: Four ELISAs using soluble antigens from whole cell lysates of animal trypanosomes are recommended: ELISA T. vivax, ELISA T.b. brucei and ELISA T. congolense type savannah (for nagana), and ELISA T. evansi (for surra); depending on the context (Asia/America/Africa), 1 to 3 tests can be recommended; they all cross-react. ELISAs exhibit high sensitivity (> 90%) and genus specificity (>95%), but subgenus specificity is not consistent, and none of them is species-specific. A positive sample reveals an immune response in the host to the parasite, but it does not indicate an active infection due to the persistence of antibodies after parasite elimination (as stated above), thus: (i) it must be complemented by other tests or information (see above) if active infection is to be confirmed, and (ii) once an animal is seropositive to one or several ELISAs (or IFATs), it is not possible to determine whether it is or has been harbouring one or several trypanosome species. Even when a positive serology is associated with a positive species-specific agent detection test, other Trypanosoma species may be suspected in a mixed infection. A sample is positive when its relative percentage of positivity (RPP) is above the cut-off value established for the host species (appropriate conjugates are defined for each species; see below). Its negative predictive value is very high, unless the host was very recently infected. Providing it is associated with negative results to sensitive agent detection tests, when negative, ELISA is reliable, due to its high genus specificity. Both tests must be repeated at 30-day intervals due to the delay in seroconversion; however, longer delays may be observed occasionally. Conjugates to be used in ELISA for each host species are: cattle and buffalo: anti-bovine IgG whole molecule; pig and elephant: Protein G conjugate; camels: Protein A conjugate; dog: anti-dog conjugate; goat and sheep: anti-goat and sheep conjugates. Conjugates remain to be defined for other host species.

  ii) IFATs can be used in the same conditions as ELISAs.

  iii) CATT/T. evansi: serum or plasma are diluted 1:4 and tested as described by the manufacturer. Positive samples are samples presenting results = or > to one + (doubtful samples are considered negative samples). Positive predictive value is high; however, nonspecific agglutination may occur. It is generally recommended to combine ELISA T. evansi and CATT/T. evansi to increase sensitivity and diagnosis reliability; however, in case of discrepancy, it is recommended to repeat sampling and testing

  (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.01.21_SURRA_TRYPANO.pdf ).

  iv) The complement fixation test (CFT) is used to confirm clinical evidence and to detect latent infections in equids infected with T. equiperdum (dourine) since humoral antibodies are present whether they display clinical signs or not. Uninfected equids, particularly donkeys and mules, often give inconsistent or nonspecific reactions because of the anticomplementary effects of their sera. In the case of anticomplementary sera, or for clinical diagnostic purposes, the indirect fluorescent antibody test (IFAT) and enzyme-linked immunosorbent assays (ELISAs) may provide additional information.

  There are no internationally adopted protocols. Cross-reactions are possible due to the presence of other trypanosomes in some countries (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.06.03_DOURINE.pdf).

  GAPS

  Parasitaemia is highly fluctuating, therefore the sensitivity of the parasite detection tests is highly variable depending on the infection course and is globally very low.

  Because of the persistence of the antibodies, positivity in any serological test does not mean the presence of an active infection.

  Cross-reactions may occur with any serological test employed so the exact trypanosome species infecting an animal may not be established.

  Whole Cell Lysate Antigens (obtained from in vivo or in vitro produced Trypanosomes) for ELISA T. vivax, ELISA T. brucei brucei, ELISA T. congolense savannah and ELISA T. evansi are produced by the WOAH nagana reference laboratory (CIRAD Montpellier, France) and can be requested to it though not commercially available sensu stricto.

  Nagana, surra and dourine: Rapid diagnostic tests (RDT) are currently unavailable.

• Commercial potential for diagnostic kits worldwide

  The market is potentially large since Trypanosoma species of African origin that affect animals are present in all continents or at risk of invasion. However, it must be underlined that farmers in most endemic settings are not ready to pay for a diagnostic test, and even animal health professionals do not include diagnosis as part of the usual services they bring to farmers/livestock keepers/owners.

  GAPS

  In low- and middle low-income countries affordability is limited.

  It is key to build strong public-private partnership as diagnostic test will not be soon economically sustainable and will remain companion tests in the short and mid-term. Use of diagnostic tests must be supported by veterinary services.

  Market research is necessary to assess the demand and affordability of new diagnostics, including their access to end-users. Sensitisation to the need of diagnosis directed to end users is needed.

  As long as the cost of a diagnosis will be equal or above that of treatment, there is little hope that diagnosis will become part of the regular medical path.

  Research is needed to develop and validate highly effective tests, particularly rapid-diagnostic tests. In addition, the conditions for delivery and conservation may be an issue, so the thermostability and robustness of diagnostic assays are of great importance.

• DIVA tests required and/or available

  DIVA is intended for eradication of disease or economic control of disease/need and nature of desired DIVA test.

  No DIVA test exists since no DIVA vaccines are available.

  GAP

  In absence of any vaccine, the development of a vaccine combined with a DIVA diagnostic test is likely unfeasible

• Vaccines availability

  There is currently no vaccine available for trypanosomosis (human or animal).

  There has been a long history of vaccine research in trypanosomes, but this has largely served to highlight the significant barriers that the biology of the parasites presents to this ambition (see section “Main means of prevention, detection, and control – Vaccines” for more details).

  As described previously, trypanosomes possess a highly elaborated system of antigenic variation. The variable antigens are immunodominant, and the scale of potential variants that can be generated by the antigenic variation system means that the variable protein (variant surface glycoprotein – VSG) is not a feasible vaccine target. The surface VSG ‘coat’ also sterically prevents access of antibodies to other invariant proteins on the parasite surface. Many such invariant antigens have been assessed as vaccine candidates, but do not result in appreciable protection (or at best provide moderate protection), even when non-conventional antibody approaches (e.g. single chain nanobodies) are used. However, a recent study that immunised groups of mice with most of the predicted surface-expressed invariant antigens in T. vivax did identify one protein, named the ‘invariant flagellum antigen from T. vivax’ or IFX, which reproducibly resulted in sterile immunity to T. vivax infectious challenge in immunised mice. The protein was localised to tether the flagellum to the cell body, and this location/function perhaps represents a particular vulnerability in the parasite to antibody access. Notably, in limited experiments in goats, this protection was not recapitulated. However, this proof of principle of the ability to protect following immunisation has revived interest in vaccination against trypanosomosis. A vaccine must be considered only a long-term possibility given the current status of knowledge and research.

  Other than the very challenging search for antigens that may result in protection from infection, some consideration has also been given to identifying candidates, which following immunisation result in a moderation of clinical severity (i.e. ‘anti-disease vaccines’). While not preventing infection (or transmission), such approaches have been postulated to potentially play a role enabling the deployment and productivity of susceptible animals in endemic areas. A T. congolense cysteine proteinase was shown to enable immunised cattle to continue to gain weight despite being infected – demonstrating the potential for such approaches. However, there has not been recent research effort in this area, perhaps due to the difficulty in marketing of anti-disease vaccines more generally.

  It has been well characterised in mice that trypanosomes induce a profound immunosuppression targeting B cells and the antibody response – trypanosomes drive natural killer (NK) cells to kill B cells in the lymphoid organs. This results in There is currently no vaccine available for trypanosomosis (human or animal).

  There has been a long history of vaccine research in trypanosomes, but this has largely served to highlight the significant barriers that the biology of the parasites presents to this ambition (see section “Main means of prevention, detection, and control – Vaccines” for more details).

  As described previously, trypanosomes possess a highly elaborated system of antigenic variation. The variable antigens are immunodominant, and the scale of potential variants that can be generated by the antigenic variation system means that the variable protein (variant surface glycoprotein – VSG) is not a feasible vaccine target. The surface VSG ‘coat’ also sterically prevents access of antibodies to other invariant proteins on the parasite surface. Many such invariant antigens have been assessed as vaccine candidates, but do not result in appreciable protection (or at best provide moderate protection), even when non-conventional antibody approaches (e.g. single chain nanobodies) are used. However, a recent study that immunised groups of mice with most of the predicted surface-expressed invariant antigens in T. vivax did identify one protein, named the ‘invariant flagellum antigen from T. vivax’ or IFX, which reproducibly resulted in sterile immunity to T. vivax infectious challenge in immunised mice. The protein was localised to tether the flagellum to the cell body, and this location/function perhaps represents a particular vulnerability in the parasite to antibody access. Notably, in limited experiments in goats, this protection was not recapitulated. However, this proof of principle of the ability to protect following immunisation has revived interest in vaccination against trypanosomosis. A vaccine must be considered only a long-term possibility given the current status of knowledge and research.

  Other than the very challenging search for antigens that may result in protection from infection, some consideration has also been given to identifying candidates, which following immunisation result in a moderation of clinical severity (i.e. ‘anti-disease vaccines’). While not preventing infection (or transmission), such approaches have been postulated to potentially play a role enabling the deployment and productivity of susceptible animals in endemic areas. A T. congolense cysteine proteinase was shown to enable immunised cattle to continue to gain weight despite being infected – demonstrating the potential for such approaches. However, there has not been recent research effort in this area, perhaps due to the difficulty in marketing of anti-disease vaccines more generally.

  It has been well characterised in mice that trypanosomes induce a profound immunosuppression targeting B cells and the antibody response – trypanosomes drive natural killer (NK) cells to kill B cells in the lymphoid organs. This results in disruption of the B cell and antibody response, extending to destroying memory and recall responses to non-trypanosome immunisations (Radwanska et al., 2008; Frenkel et al, 2016; Moon et al., 2022). This has potential implications for impacting vaccine efficacy in target animals if infected with trypanosomes. While there is some data to suggest that a similar phenotype occurs in cattle, it has not been fully characterised in the clinically relevant host species – this has clear potential relevance to vaccination strategies, including if a vaccine against trypanosomes can be developed (Radwanska et al., 2018).

  GAPS

  Can the protective effect of IFX, a candidate antigen of T. vivax, be translated from the mouse model to clinically relevant host species?

  Can other antigens be identified that would result in similar sterile protection, including in other trypanosome species?

  Is there a market for a vaccine that only protects against one species of trypanosome (e.g. T. vivax)?

  With better characterisation of clinically relevant trypanosome species (T. congolense, T. vivax, T. evansi, T. equiperdum) genomes, proteomes and secretomes, can other candidate proteins that may confer protection be identified?

• Commercial vaccines availability (globally)

  There is no vaccine currently available (see section on “Vaccines (inactivated, attenuated, sub-unitt, vectored, GMO…).

  GAPS There is no vaccine candidate that provides the prospect of a commercial vaccine in the short to medium term, and certainly not against all three species of disease-relevant trypanosome. The recently published candidate (IFX) that provides sterile protection in mice, only provides protection against T. vivax, and there is no direct orthologue of the gene in T. congolense or T. brucei (Autheman et al., 2021). There may be opportunities in exploring proteins that have the same function/location (i.e. are involved in structural tethering of the flagellum to the trypanosome cell body) in T. congolense and T. brucei, in the event that this function/localisation represents a particular vulnerability in trypanosomes. Additionally, protection post-IFX vaccination has only been demonstrated in mice, and therefore further work is required to validate if efficacy can also be demonstrated in cattle or small ruminants.

• Marker vaccines available worldwide

  There are currently no vaccines available for trypanosomoses, and while there is a recent revival of interest in trypanosome vaccinology, there is no prospect for a vaccine in the short to medium term. In this context, there is currently no marker vaccine.

  GAP

  The main obstacle is the identification of antigens that would represent a viable commercial vaccine prospect (see more detailed discussion in sections Main means of prevention, detrection and control-Vaccines” and “Introduction to Vaccines availability”-. Until this is changed, it is highly unlikely that a marker vaccine approach will be considered.

• Effectiveness of vaccines / Main shortcomings of current vaccines

  Not applicable as no vaccines available.

• Commercial potential for vaccines in Europe

  Given the scale of AT (both in Africa for nagana generally, in South America for T. vivax, and in South America, North Africa, the Middle East and Asia for T. evansi, and world over for T. equiperdum), there would be a clear market for an effective vaccine, or effective vaccines, against the disease(s). The scale of the contemporary market requires full assessment, which is tied into improving surveillance and improving our understanding of the current distribution and scale of disease caused by the relevant species of trypanosomes. It also needs to be determined that if a candidate vaccine approach can only be developed against one of the trypanosome species (given the genetic diversity of the species, it may be very challenging indeed to develop a vaccine against all pathogenic Trypanosoma species), would there be a market for a single-species vaccine (Morrison et al., 2023; Magez et al., 2010).

  GAPS

  Updated contemporary epidemiological and incidence/prevalence data from disease-endemic countries.

  Contemporary market assessments are required for the following:

  A vaccine against African trypanosomosis (i.e. all three species)

  A vaccine that is only effective against T. brucei (including T. evansi and T. equiperdum

  A vaccine that is only effective against T. vivax

  A vaccine that is only effective against T. congolense

• Regulatory and/or policy challenges to approval

  With no current vaccine prospect, it is not possible to predict regulatory/policy challenges until a vaccine antigen and formulation is identified.

  GAP

  Lack of vaccine antigen candidate of sufficient maturity.

• Commercial feasibility (e.g manufacturing)

  With no current vaccine prospect, it is not possible to predict manufacturing challenges until a vaccine antigen and formulation is identified.

  GAP

  Lack of vaccine antigen candidate of sufficient maturity.

• Opportunity for barrier protection

  With no current vaccine prospect, it is not possible to predict manufacturing challenges until a vaccine antigen and formulation is identified, and the characteristics of the level and duration of protection are defined.

• Pharmaceutical availability

• Current therapy (curative and preventive)

  Trypanocides are arguably a major component of the available control measures for nagana, surra and HAT.

  Nagana: Currently, in the African continent, the two main used compounds for the treatment of Trypanosoma congolense, T. vivax and T. brucei brucei in livestock are diminazene aceturate (DA) and isometamidium chloride (ISM). The use of homidium salts (HOM) are much more restricted and should be strongly discouraged due to its proved carcinogenic potential.

  It is not easy to get an accurate estimate for the use of trypanocides in Africa, however it is commonly accepted that the use of about 160 million trypanocide treatments per year (20 million - isometamidium chloride and 140 million – diminazene aceturate) is presently the best approximation of this value. See also section “Costs of above measures” for further details.

  Diminazene aceturate (DA) is used for the treatment of AT, babesiosis and theileriosis caused by Theileria annulata. DA is used as curative agent and not for prophylaxis, as the drug is rapidly eliminated. Pharmacologically, diminazene is an aromatic diamidine derivate, as pentamidine, a drug used to treat first stage infections with Trypanosoma brucei gambiense. Its first commercial name was Berenil, but it currently appears under a variety of brand names. The most common formulation of diminazene aceturate is a yellow powder presented in sachets that should be dissolved in sterile water to a volume of 15 mL (small sachet), or 150 mL (large sachet). The dissolved powder gives a clear yellow solution, with a final concentration of 7% DA. Once in solution, DA alone is stable only for 2-3 days. To circumvent this short stability, commercial liquid formulations usually contain phenyldimethyl pyrazolone (an analgesic) as stabilizer. Moreover, combinations of diminazene and vitamin B12, or B6 are also available. A combination of diminazene and levamisole, with a trade name Sanimix is available since 2017. This mixture aims at treating trypanosome and gastro-intestinal parasite infections, in order to eliminate both important causes of anaemia simultaneously.

  DA is effective against both T. congolense and T. vivax with relatively few side effects, even at doses two or three times higher than the normal therapeutic dose of 3.5 mg/kg. For these reasons, added to its low cost (see “Costs of above measures” section for further details), DA is the most used trypanocide in cattle and small ruminants. The drug is also applied to horses, donkeys, pigs and dogs, but mild to severe toxicity have been observed, even when DA is used at the recommended dosage. Data of DA use in pigs are discordant. DA is not effective against T. simiae, the cause of an hyperacute disease in pigs, but it eliminates T. congolense infections in this species at a dose of 3.5 mg/kg.

  DA should be administered through a deep Intramuscular (IM) injection, preferably targeting the muscles of lower commercial value. Treatment with doses up to 7 mg/kg have been routinely applied, mainly in commercial farms, when using the “sanative pair approach”, where one injection of DA per year is used to eliminate possible resistant strains to isometamidium, when this drug is used at regular intervals for prophylactic purposes. Furthermore, doses higher than 7mg/kg of DA have been tried in attempts to eliminate resistant strains. If high doses are applied, it is advisable to divide the injection into two separate sites to avoid local pain and adverse local reactions. Similarly, in experimental infection of T. vivax in sheep in French Guyana, 7 mg/kg was the minimum dose to eliminate the parasite, and higher doses were sometime necessary. Nowadays, in most of the cases, 7 mg/kg of DA seems to be the minimal dose to apply for curative treatments against T. vivax.

  T. b. brucei and T. evansi are less susceptible to DA and a double dose of 7 mg/kg BW should be used to treat infections with these species. DA does not penetrate the central nervous system, and consequently it is not effective against any case once this particular location is reached.

  Isometamidium chloride (ISM), which entered the market as Samorin/Trypamidium is a trypanocide with both curative and prophylactic properties, used to treat and prevent infection in livestock and companion animal species. The drug is a hybrid compound which contains a homidium subunit linked to a moiety of m-amidinophenyl-azo-amine, from the DA molecule. Commercial formulations are impure mixtures of four chemical isomers with different anti-trypanosomal activity, of which ISM is the principal component. An analysis of the different isomers revealed that the mixture of them was significantly more active than the single components, suggesting a synergistic action of the individual components.

  The drug is presented as a dark red powder to be dissolved at 1% or 2% concentration in sterile water prior to use. ISM is commonly sold as single-dose or multi-dose sachets. For prices see the “Costs of above measures” section. Isometamidium is mainly used to treat or prevent infections by T. congolense and T. vivax in livestock and companion animals. Furthermore, the use of isometamidium for T. vivax infections has the advantage to effectively preventing relapses caused by sub-populations of this parasite, which hide temporarily in tissues other than blood. Conversely, ISM is not the drug of choice for the species of the Trypanozoon sub-genus given its reduced activity against T. brucei spp. and T. evansi infections. Moreover, as DA, it does not cross the blood-brain barrier, making it ineffective for infections in the central nervous system. Despite the above-mentioned reduced activity, there are reports documenting the clearance of T. brucei infections, using ISM at 1mg/kg.

  As a curative drug, ISM is administered to cattle at a dose of 0.25-0.5 mg/kg as a deep IM injection. The deep IM injection is required as ISM will cause inflammation at the injection site and can cause severe lesions with the formation of fibrotic scars. The successful intra-venous use of ISM has been reported and might be an alternative to IM in certain animal species.

  For prophylaxis, ISM is used at 0.5-1 mg/kg intramuscularly. Protection in cattle normally lasts 3-4 months but it can vary markedly, up to 6 months. As mentioned for DA, if large volumes of drug are required, it is recommended to divide them in two separate injection sites.

  The use of ISM as a prophylactic is only economically justifiable when the disease risk is medium to high (see FAO guidelines) and herd health cannot be maintained with curative treatments alone. This is the case in extensive wildlife-livestock interface areas surrounding conservation areas in Africa. Prophylaxis is also recommended to particularly susceptible exotic high production breeds, especially during the rainy season, when the biting flies, including tsetse flies’ challenge is at its highest. Prophylaxis with ISM, is also recommended in the case of transhumance, when areas with high risk of trypanosome infection are crossed.

  Surra: DA is largely used for the control of surra, both in Latin America and Asia. Although the normal dose of DA for the treatment of a Trypanozoon infection (the sub-genus to which belongs T. evansi) is 7mg/kg, in Southeast Asia, veterinaries are mainly using the dose of 3.5mg/kg, which brings a clinical improvement, but, when follow-ups are established, obviously persistent infection do occur. Nevertheless, the strategy of using low doses of DA is still appreciated by veterinaries and farmers, even if it does not clear the infection (Desquesnes et al. 2022). Nevertheless, in some cases in Thailand for example, persistent infection have been reported even when using 10mg/kg DA.

  The combination of quinapyramine sulfate and chloride salts is very efficient to cure early cases of T. evansi infection both in horse and cattle. The advantage of this preparation is that circulating parasites are immediately killed in the bloodstream, while those hidden in other body fluids, will be killed as soon as they enter the bloodstream. Probably, step by step, Quinapyramine salt combination can get rid of the parasites over a 3–4-month period. It is the only sustainable treatment for horses living under a risk of infection (in close vicinity with cattle and buffaloes). Melarsomine hydrochloride (Cymelarsan®), has been developed for the control of T. evansi in camels (0.25mg/kg); its efficacy has been evaluated in horses, buffaloes, and cattle in which the dose should be increase at least to 0.5-1mg/Kg; results are not always consistent.

  Dourine: Attempts to treat T. equiperdum infections with melarsomine hydrochloride give inconsistent results. Guidelines for treatment of AT are available on the CIRDES Website (https://www.cirdes.org/wp-content/uploads/2018/12/F03-Trypanocides-Ang.pdf).

  HAT: Drugs to treat HAT are not commercially available in the market but can be obtained through WHO. Treatment mainly depends on the form of the disease (T.b. gambiense or rhodesiense) and is relatively toxic. Detailed guidelines are available in the “Guidelines for treatment of human African trypanosomiasis” published by WHO (https://www.who.int/publications/i/item/9789240096035, June 2024). Six components are available.

  Fexinidazole: First line treatment for g-HAT and r-HAT patients ≥ 6 years and ≥ 20 kg (except severe late-stage g-HAT). 10 days oral treatment consisting of 4 days loading dose (1800 mg daily, 1200 mg for children) and 6 days maintenance dose (1200mg daily, or 600mg for children). Tablets are to be taken with solid food.

  Pentamidine (pentamidine isethionate): for g-HAT in hemo-lymphatic stage (mainly children and 2^nd line treatment). Intramuscular injection of 4 mg/kg once daily for 7 days.

  Suramin (suramin sodium powder): for r-HAT in hemo-lymphatic stage (mainly children). Five intravenous injections of 20 mg/kg (maximum 1 g) given weekly for 5 weeks. Treatment is started with a test dose of 4-5mg/kg, which is completed a few hours later.

  Nifurtimox Eflornithine Combination Treatment (NECT): For g-HAT in meningo-encephalitic stage, mainly children or patients in severe late stage (>100 white blood cells/µl in CSF). Oral nifurtimox 15 mg/kg per day in three doses for 10 days combined with intravenous eflornithine (α-difluoromethylornithine or DFMO) 400 mg/kg per day in two 2-h infusions for 7 days.

  Melarsoprol (Arsobal): mainly children with r-HAT. Slow intravenous injections of 2.2 mg/kg per day (maximum: 5 mL) once daily for 10 days. Reactive encephalopathy occurs in up to 5-18% of patients and can be fatal 10-70% of them.

  GAPS

  A key gap in the fight against AT is the access of farmers to good quality products, good practices, and good diagnosis.

  In some sub-Saharan African countries, the veterinary network is not sufficiently dense and efficient, with the result that the farmers usually administer the treatments by themselves and have only access to informal markets, where fake and counterfeited products are a commonplace. Informal markets can reach 60% of the total volume of trypanocides sold in some countries of West and Central Africa. Even sterile, or at least clean water is sometime lacking to suspend powdered trypanocides and is negatively affecting the treatment effectiveness.

  Better information-sharing and surveillance networks should be set up between stakeholders (veterinary services, producer organizations, research institutes, private sector) so that decision-makers can have reliable, accurate data on the health situation and the measures that can be taken. By way of example, in most of the Southeast Asian countries, only DA is available, while it is known to be poorly efficient against T. evansi, it is toxic for equids and is generally used at a half of the recommended dose (3.5 mg/kg instead of 7 mg/kg), therefore, surra will not be under control in this region in the near future.

  The key leverage in the coming years will be to significantly invest in veterinarians and animal technologist network to advance good veterinary practices, including the correct use of curative and prophylactic trypanocides. In this manner, it will be possible to have an efficient fight against Animal Trypanosomoses, as well as preventing resistance to trypanocidal drugs.

• Future therapy

  Private sector: only two private laboratories are still active in the research for future trypanosomosis therapy: CEVA Santé Animale & Boehringer Ingelheim (BI).

  BI: Since 2023, The Bill and Melinda Gates Foundation, the UK Government Foreign Commonwealth and Development Office, Boehringer Ingelheim and GALVmed have joined forces to find a new trypanocide to AAT. Research is ongoing.

  CEVA: ongoing research on diagnosis, treatment and prevention. Some public research laboratories are testing components and have published results based on in vitro assays or in vivo experiments in mice.

  GAPS

  Massive disengagement of all the other private laboratories. The most recent trypanocide is melarsomine hydrochloride (1985). Last innovations in the field of trypanosome therapy were done by Ceva: Veriben B12 in 2008 and Sanimix/Veryl (diminazene+levamisole) in 2017 by combining adding components (vitamin, anthelminthic) to old trypanocides. Fexinidazole per oral proved to be efficient against T. evansi in rats, but its use is restricted to humans.

• Commercial potential for pharmaceuticals

  Currently in Africa, the need for trypanocide treatments for AT is estimated around 160 million doses/year at a mean cost of 0.5-5 USD/dose. As described above (section “Current Therapy”), drugs are available and distributed on the market and widely used, but some treatment failures are observed, and resistance to current drugs is appearing. A new drug that could overcome current treatment failure would be needed. However, financial means of the farmers are limited, and they may not be able to buy products more expensive than those already existing. New products should thus be affordable and accessible.

  Nevertheless, there are also needs in Latin America and Asia for the control of T. vivax and T. evansi infections; in some cases, such as for T. vivax in Brazil, or for T. evansi in horses in general, farmers and breeders would certainly have the financial means to buy trypanocides providing efficient drugs would be available.

  GAPS

  The lack of investments from private companies is mainly due to the fact that products already exist and are globally efficient and the potential market appears unattractive.

  Moreover:

  - Business environment is complex and unfriendly (informal market, price-oriented market, complicated access to end users, corruption, geopolitical tension, high payment risk).

  - Regulatory processes are very costly and long procedures are required to register products. Moreover, the informal market is important, and the fake drugs are not controlled in a majority of African states.

  Thus, priority is to allow end users to access products that are already available more than creating new therapeutics.

• Regulatory and/or policy challenges to approval

  The regulatory process is very costly and long and laborious procedures are associated to product registration. Most African governments are currently not able to effectively control the import of veterinary products, and the informal market is quite dominant in these settings, which can contribute to hinder investisment in R&D.

  GAPS

  Several national authorities in Africa demand that trypanocides are packaged in “Ready To Use solution” (vial) or sterile powder in vial + sterile water, making injections safer for animals.

  Co-packing sachets with Water for Injection would have the following implications:

  Increased cost of production: This would require a change of manufacturing procedures, product containers and packaging materials. Including carrying out stability studies in the new container closure

  Compliance complexity: This change would necessitate submitting variation of all the products with long and varied approval timelines, this will not only be a very costly process but will have a great impact on commercialization of the products. Knowing that not all countries will switch to new packaging regulations at the same time.

  Impact on affordability of product:

  • The cost of production: Apart from additional production cost, since now, this requirement does not exist for all African countries where trypanocidal drugs are sold, batches will have to be produced specifically for some countries, which would increase the cost further due to the low volumes.
  • The cost of the packaging material: A specific package will need to be created for the sterile water and will be much more expensive.
  • The cost of transport: As an example, for a classic pack of 10x2,36g of Diminazene aceturate, the quantity of sterile water needed is 125mL, what would roughly double the volume, and massively increase the weight. Therefore, this will double the cost of sea freight and road transport and at least triple the cost of air freight on these goods, the latter depending on both weight and volume.
  • The selling price: higher production, packaging and transport costs will automatically translate into higher selling prices.

  Field impact: The difficulty to carry the sterile water packs when treating large herds and risk of breaking the packs open.

  The environmental cost: Producing and transporting the sterile water which is very bulky will increase the carbon footprint of the products to reach end-users.

• Commercial feasibility (e.g manufacturing)

  GAPS

  The production costs of trypanocides in Europe are constantly increasing (overheads, workforce, raw material). All private laboratories are now manufacturing in Asia except CEVA/Laprovet, which is still manufacturing in France, or in Morocco (MCI Santé animale).

  In some parts of oversea Europe, for example in French Guyana, there is no clear regulation for Trypanocides (which are theoretically not allowed) although the disease and the need for trypanocides are present; such situation may lead to a raise of black-market products from neighbouring countries

• New developments for diagnostic tests

• Requirements for diagnostics development

  Nagana, Surra and Dourine: Several promising methods can be developed to meet the needs for field applicable AT diagnostic tests. They include pan-Trypanosoma IgG ELISA based on whole cell lysate WCLA (mixed Trypanosoma spp. or T. evansi antigens) or on recombinant antigens, species-specific point-of-care diagnostic (POCD) method based on IgG detection for AT, POCD based on IgM detection, POCD based on antigen detection, POCD based on molecular detection of 7-SL sRNA, visual polymerase spiral reaction (PSR) assay. Applications, advantages, limitations, and challenges of each method are discussed by Desquesnes et al. (2022a).

  HAT: WHO has set the following priorities for diagnostic tests: A test for r-HAT usable in peripheral health facilities was considered highest priority, followed by a diagnostic tool to identify individuals with suspected but microscopically unconfirmed g-HAT, to receive treatment. Furthermore, an individual test to assess T.b. gambiense infection in low prevalence settings and a high throughput g-HAT test for verification of elimination were also considered important (Priotto et al., 2023). Corresponding target product profiles were established (Priotto et al., 2023a; 2023b; 2023c; 2023d).

  GAPS

  Nagana, Surra and Dourine: Due to the high diversity of Trypanosoma species and wild and domestic hosts, a large number of specimens, from infected and non-infected animals are required to validate tests. A collaborative platform or structure created, funded and managed to collect and maintain a repository of field trypanosome isolates, and promote the exchange of DNA and serum samples would be of great value.

  HAT: due to the low prevalence, sensitivity of new diagnostics will be difficult to evaluate and specimens from biological repositories will have to be used (Franco et al., 2012).

• Time to develop new or improved diagnostics

  Time and cost depend on the type of test. In general, the development of tests is much faster and less expensive than developing vaccines. Yet, development and validation up to commercial availability does usually take several years. A new test must be evaluated by several laboratories in several epidemiological context before validation and possibly recommendation by the WOAH.

• Cost of developing new or improved diagnostics and their validation

  Creating and validating new tests is a lengthy and demanding process, leading to significant expenses. The exact cost varies based on the nature of the test and the expenses involved in making the necessary reagents and providing the equipment needed for analysis. Evaluation of each test on the other hand requires numerous reference samples to assess sensitivity and specificity. Experimental infection of different livestock is also important to assess various factors related to the test, e.g., the first time an animais shows positivity, agreement with the level of parasitemia, persistence of positivity after treatment, and cut-off levels. The animal experiments and laboratory analyses need equipment and standard facilities.

  For PCR tests, the major factor having both technical and economical constraints is the cold chain required for shipment, highly impacting the test price. Recent work and breakthroughs in technology such as portable PCR machines, FTA cards, filter papers and freeze-drying of reagents might help to overcome these issues.

  The lack of clear market expectations and figures makes it difficult for tests, particularly rapid diagnostic tests, to be readily available. This is because manufacturers can only produce small quantities due to limited demand, which in turn prevents them from making tests more affordable. However, for a disease with a wide geographical distribution like Surra, finding a commercial company interested in marketing the test should be easier.

  HAT: European registration of new diagnostics before commercialisation is complicated and expensive.

  GAPS

  Validation by WOAH of new diagnostic tests before they are recommended by WOAH is complicated, time-consuming and costly.

  Once a newly developed diagnosis test is validated, if the cost is higher than the cost of treatment, diagnosis would most probably not be adopted unless there is a strong institutional framework supporting a control strategy based on a test & treat strategy; also, unless diagnostics are used for research purposes, e.g. in lagre scale surveys aiming to assess prevalence.

• Research requirements for new or improved diagnostics

  Nagana, Surra and Dourine: For serological tests, the use of recombinant antigens is generally favoured as it allows standardization, however, such test generally exhibit a lower sensitivity compared to native antigen-based assays. It has been shown that the sensitivity of recombinant antigen-based ELISA could be markedly enhanced by combining several recombinant antigens, which opens the door to further improvements. However, the WOAH-recommended ELISA based on WCLA currently remains the best tool for antibody detection with optimal sensitivity. The improvements required on this technique can now be fulfilled, thanks to the use of lyophilized reagents and dry samples, and to the in vitro production of most of the African trypanosomes. It can then be expected, shortly, that these WCLA will be prepared from well-standardized in vitro-produced parasites, which will allow ELISA to be more largely implemented for epidemiological studies, herd surveillance and monitoring. There is an urgent need for rapid diagnostic tests of AT, especially for nagana and surra. Ideally, such tests need to detect active infections, based on antigen or DNA/RNA detection of trypanosomes. Such test would allow targeting treatment on actually infected animals and would really improve management of animal health, while reducing use of drugs. HAT: The Human African trypanosomiasis specimen biobank contains biological specimens from T.b. gambiense and T.b. rhodesiense infected HAT patients and makes those available to support research of new diagnostics (Franco et al., 2012).

  GAPS

  Nagana, Surra and Dourine. Due to strict export, import and biosafety regulations in most countries, the availability of validated sample panels from experimentally and naturally infected animals is currently very limited. Which platform could be created to facilitate access to this necessary resource?

  HAT: the specimen bank does not contain specimens suitable for development and evaluation of RNA based diagnostic tests. For g-HAT, specimen collection has been biased by screening with CATT. The r-HAT collection contains a limited number of specimens.

• Technology to determine virus freedom in animals

  Due to the possible extra-vascular refuge of some parasites (out of reach of the immune system), notably species belonging to the sub-genus Trypanozoon, it is sometime impossible to definitely prove an animal is free of these parasites. As a consequence, no technology is currently able to indicate parasite freedom. At best, 3 monthly iterative examinations using the three diagnosis technologies (parasitological, molecular and serological) may provide substantial diagnosis reliability.

  GAPS Development of a rapid diagnostic test based on detection of circulating antigens should be a priority. However, given that trypanosomosis is characterized by low parasitemia and that parasites, especially Trypanozoon, can hide in tissues, it is highly unlikely that such a test could achieve enough sensitivity to provide a high negative predictive value to reasonably affirm the absence of parasites. There is an ultimate “gap” in the establishment of parasite freedom for Trypanosoma infections since existing extravascular foci have been identified, especially for surra in camels, but also for T. vivax in sheep. These foci may be aqueous humour of the eye, synovial liquid and nervous system, where quiescent para sites may stay for an undetermined period before multiplying and invading again the rest of the organism. In these extravascular foci parasites are out of reach of the immune system, leaving the animals seronegative and also negative to all other diagnostic tests. The consequence is that an animal coming from infected area may forever be considered at risk of being a healthy and undetected parasite carrier.

• New developments for vaccines

  No vaccines are available at the present time, and the prospects for a vaccine in the short to-medium term are limited.

  Recent data (see below) have revived the prospect of vaccines in the long term, although this will require substantial further work to bring to fruition, if successful. However, there are increasing resources available for both the disease-relevant trypanosome species and the clinically relevant bovine host that mean vaccine research can be undertaken with approaches and resolution that was not previously possible, providing hope that the revival of trypanosome vaccinology will continue to progress.

  The recently published candidate (IFX) that provides sterile protection in mice (Autheman et al., 2021), only provides protection against T. vivax, and there is no direct orthologue of the gene in T. congolense or T. brucei. There may be opportunities in exploring proteins that have the same function/location (i.e. are involved in structural tethering of the flagellum to the trypanosome cell body) in T. congolense and T. brucei, in the event that this function/localisation represents a particular vulnerability in trypanosomes. Additionally, protection post-IFX vaccination has only been demonstrated in mice, and therefore further work is required to validate if efficacy can also be demonstrated in cattle or small ruminants.

  There is a wealth of genomic, transcriptomic, proteomic (including secretome) and metabolomic data now available for the disease-relevant trypanosome species, including data from multiple strains/subtypes within species (Moreira et al., 2023; Oldrieve et al, 2022; Steketee et al., 2021; Silva Pereira et al. 2020; Tihon et al., 2017; Odongo et al., 2016; Jackson et al., 2012; Grébaut et al. 2009; Holzmuller et al., 2008). There is the potential within these data to much more fully explore the identification of potential protective antigens.

  Approaches that have induced any protection have all involved the induction of B cell and antigen-specific antibody response – for example, immunization and challenge with trypanosomes expressing homologous surface antigens (variant surface glycoproteins; VSGs) (Morrison et al., 1982), and IFX in mice for T. vivax (Autheman et al., 2021). While the isotype that mediates complement-mediated trypanosome destruction in mice has been identified (IgG2a), antibody isotype effector function in cattle has only been characterised very recently (Noble et al., 2023), meaning that, for example, which isotype and effector function is critical in the cattle antibody response in cattle, has not been studied. However, the tools and resources to analyse B cell approaches in cattle are maturing (Roos et al., 2023; Ramirez-Valdez et al., 2023), meaning that it will be possible to more fully characterize the features and dynamics of an effective B cell and antibody response in cattle.

  Relatively little attention has been paid to the role of T cells in the development and maintenance of a protective response, largely due to the limited number of antigens that have been demonstrated to induce protection.

  Trypanosomes that reproduce clonally (i.e. have no involvement of sexual recombination in their life cycle), such as T.b. gambiense, T. evansi and mechanically transmitted T. vivax, should in theory have a more limited and less diverse antigen repertoire than species that do undergo sexual recombination (T. congolense and T.b. brucei). Whether this represents a potential vulnerability is an area worthy of further investigation (Silva Pereira et al, 2020; Weir et al, 2016; Carnes et al., 2015; Koffi et al, 2009).

  GAPS

  Can the findings of protection induced by IFX for T. vivax be translated from mice to cattle?

  Can the antigenic properties of IFX be used to identify protective antigens in other trypanosome species?

  With better characterisation of clinically relevant trypanosome species (T. congolense, T. vivax, T. evansi, T. equiperdum) genomes, proteomes and secretomes, can other candidate proteins that may confer protection be identified?

  What antibody isotypes confer protection in cattle?

  What are the characteristics of a protective B cell and antibody response in cattle?

  What role does the T cell response play in development and maintenance of a protective immune response in cattle?

  Overall, in areas of multiple Trypanosoma species infections (T. vivax, T. congolense & T. brucei in cattle for example), a vaccine should also be multivalent to be really efficient and therefore be adopted.

• Requirements for vaccines development / main characteristics for improved vaccines

  With the current state of knowledge, a vaccine is not a realistic prospect in the short to medium term (further details can be found in sections on Vaccines, Commercial vaccines availability and Introduction to new developments for vaccines”). Research is still either at the antigen discovery stage or limited to the mouse model.

  GAP

  Lack of vaccine antigen candidate of sufficient maturity.

• Time to develop new or improved vaccines

  With the current state of knowledge, a vaccine is not a realistic prospect in the short to medium term (see sections on “Vaccines, Commercial vaccines availability and Introduction to new developments for vaccines”). Research is still either at the antigen discovery stage or limited to the mouse model.

• Cost of developing new or improved vaccines and their validation

  With the current state of knowledge, a vaccine is not a realistic prospect in the short to medium term (see sections “Vaccines and Commercial vaccines availability). Research is still either at the antigen discovery stage or limited to the mouse model. Therefore, substantial investment is required in basic research before consideration can be given to the costs relevant to the development of a vaccine as a product.

  GAP

  Lack of vaccine antigen candidate of sufficient maturity.

• Research requirements for new or improved vaccines

  See sections on “Vaccines, Vaccines availability and Effectivemess of vaccines/main shortcomings of current vaccines” for detailed discussion of limitations around current vaccine prospects.

  GAPS

  Can the findings of protection induced by IFX for T. vivax be translated from mice to cattle?

  Can the antigenic properties of IFX be used to identify protective antigens in other trypanosome species?

  With better characterisation of clinically important trypanosome species (T. congolense, T. vivax, T. evansi, T. equiperdum) genomes, proteomes and secretomes, can other candidate proteins that may confer protection be identified?

  What antibody isotypes confer protection in cattle?

• New developments for pharmaceuticals

• Requirements for pharmaceuticals development

  Nagana: Since 2023, The Bill and Melinda Gates Foundation, the UK Government Foreign Commonwealth and Development Office, Boehringer Ingelheim and GALVmed have joined forces to find a new medical solution to AAT and research is ongoing to test a Benzoxaborole compound. The next step will be to translate the proof of concept of this promising new compound to a pharmaceutical product available for stakeholders. Some academic research is also ongoing to assess new compounds in vitro or on mouse model.

  HAT: Acoziborole is a single dose oral drug effective for both stages of g-HAT. Its efficacy has been demonstrated in a multicentre, open-label, single-arm, phase 2/3 trial (Betu Kumeso et al., 2023). If safety is confirmed, use of acoziborole might be extended to screening tests-positives patients without parasitological confirmation (so-called screen and treat strategy).

  GAPS

  HAT: although acoziborole should be theoretically effective for r-HAT, a clinical trial has not yet been carried out. Effectiveness of acoziborole in children infected with T.b. gambiense is being assessed in a clinical trial. Additional safety data and drug interactions are being studied.

• Time to develop new or improved pharmaceuticals

  Nagana: validation of Benzoxaborole could be implemented within two years if the drug is made available and fundings provided for experimental and field assessment.

  HAT: submission of acoziborole for treatment of g-HAT to the European Medicines Agency is foreseen at earliest mid-2025.

  GAP

  The time required to develop a product is difficult to assess for animal trypanosomes but, along with the cost of development, this is undoubtedly the main constraint.

• Cost of developing new or improved pharmaceuticals and their validation

  GAP

  The cost required to develop a product is difficult to assess for animal trypanosomes but, along with the time of development, this is undoubtedly the main constraint.

• Research requirements for new or improved pharmaceuticals

  GAPS

  Investment from private and public sector, along with stakeholder’s engagement and clear definition of needs.

  New high throughput biotechnologies are available to test quickly and efficiently numerous compounds in vitro and could be applied on trypanosomes.

Disease details

• Description and characteristics

• Pathogen

  Species of the genus Trypanosoma are flagellate protozoans belonging to the order Kinetoplastida, family Trypanosomatidae. African trypanosomoses are mammal parasitic diseases due to several pathogenic Trypanosoma species originating from Africa; in animals, they are responsible for nagana, also called “Animal African Trypanosomosis” (AAT), surra and dourine, and in humans, they are responsible of Human African trypanosomiasis (HAT), also called sleeping sickness. These parasites inhabit the blood, the plasma, the lymph, and various tissues of their mammalian hosts, and they are also found in insects acting as vectors. African trypanosomes are mainly cyclically transmitted by tsetse flies; they belong to the Salivarian section with three main subgenera: Nannomonas, containing T. congolense, which includes three types: savannah, forest and Kilifi, T. simiae and T. godfreyi; Duttonella containing T. vivax and T. uniforme; and Trypanozoon, which contains T. brucei, T. evansi and T. equiperdum. Within T. brucei, three subspecies have been described: T. brucei brucei (infective only for animals), T. b. gambiense, the agent of the chronic form of HAT and T. b. rhodesiense, the agent of the acute form of HAT. Additionally, T. suis, sometimes described in pigs, belongs to the subgenus Pycnomonas. Some Trypanosoma spp cannot be transmitted by tsetse flies, such as the agent of surra, T. evansi, which is mainly transmitted mechanically by other biting flies, and T. equiperdum, the agent of dourine, which is transmitted sexually in equids. T. vivax can be transmitted by biting flies other than tsetse flies.

  GAPS

  Defining species, sub-species, and types within the Trypanosoma genus is complex (and some knowledge is incomplete), especially within Trypanozoon. The classification of parasites, based on clinical, epidemiological and morphological information, is now being called into question by phylogeny based on molecular markers.

  The classification of T. evansi and T. equiperdum within the Trypanozoon subgenera has been clarified with the advent of genomic data (Oldrieve et al., 2021; Carnes et al., 2015), but this illustrates the difficulty of reconciling epidemiological and genomic data. T. evansi and T. equiperdum have transmission and clinical characteristics that result in a very rational basis for the historical treatment as two “species”. However, genomic evidence shows that they are variants of T.b. brucei, and the named species are not monophyletic, i.e. the mechanical transmission of T. evansi has arisen independently on at least three occasions, and indeed the venereal transmission in equids, classically treated as T. equiperdum, has also been manifested by an isolate that genetically is most similar to T. evansi (Oldrieve et al., 2021). This complexity is one reason why specific markers (e.g. for diagnosis of T. evansi from T. brucei) have not been, and probably cannot be, clearly identified. T. evansi and T. equiperdum illustrate well why a more complete understanding of the genetics and taxonomy of Trypanosoma in general, particularly of isolates circulating and causing clinical disease, would be welcome.

  A similar complexity exists among the African trypanosomes that cause the human disease. Different genetic lineages have been identified in West and Central Africa, particularly T.b. gambiense Group 1, causing most human disease, a genetically and epidemiologically distinct form and mostly anthroponotically transmitted, and T.b. gambiense Group 2. However, no T.b. gambiense Group 2 cases have been reported for several decades. T.b. rhodesiense, causing human disease in East and Southern Africa, is again genetically distinct. In contrast to T.b. gambiense Group 1, T.b. rhodesiense and T.b. gambiense Group 2 are genetically indistinguishable from co-circulating T.b. brucei and are considered human-infective variants of T.b. brucei. The gene that enables T. b. rhodesiense to infect humans has been identified as SRA. Some key knowledge gaps with human-infective trypanosomes remain; for example, whether T.b. gambiense Group 1, identified in animals (e.g. pigs) can be infective to humans via the tsetse vector and which role do animal reservoirs play in its epidemiology.

  Based on ITS1 sequence and varying pathogenic effects, the three types of T. congolense could be considered as three different species. However, a genomic study by Tihon et al., 2017 indicates that there is evidence for hybridisation, at least between T. congolense Forest and Savannah. Although this paper was also discussed by Tibayrenc and Ayala (2018), it is unarguable that there are significant gaps in our understanding of the epidemiology of these three types (especially for Forest and Kilifi), and the extent of the role they play in disease.

  Among T. vivax species, while it is clear that there are tsetse and non-tsetse transmitted lineages, there does not appear to exist substantial genetic differences at the genomic level (Silva Pereira et al., 2020), and the clinical picture across the distribution range is relatively homogeneous. However, early evidence showed the existence of several serodemes of T. vivax in Latin America and the loss of the ability to undergo a cyclical development in tsetse among these mechanically transmitted T. vivax isolates (Dirie et al., 1990).

• Variability of the disease

  Trypanosomoses correspond to several diseases caused by different species of trypanosomes that affect differentially a wide range of mammalian species. Each Trypanosoma species has common pathogenicity traits, which are described below, but a range of symptom severity is observed. Most trypanosome species causing trypanosomoses are biologically transmitted by tsetse flies, and some are also mechanically transmitted by biting flies out of the tsetse belt. Mechanical transmission of T. vivax and T. evansi by biting flies has been demonstrated experimentally, as well as with T. congolense, but with a lower incidence for the latter due to lower parasitaemia. However, three diseases are considered as animal trypanosomoses (AT): nagana (AAT), surra and dourine.

  Nagana is due for the most part to T. congolense, T. vivax and T. brucei, though other pathogens such as T. simiae, T. suis and T. godfreyi are also considered etiological agents; these parasites are mainly transmitted cyclically by tsetse flies. T. vivax is also spread by mechanical transmission, as is observed in Latin America.

  Nagana: For the nagana disease complex, the most pathogenic and economically important tsetse transmitted Trypanosoma for a wide range of hosts, including livestock, are T. congolense type savannah, followed by T. vivax. Infection with the former can be lethal in acute cases, while those with the latter may be chronic, slowly leading to emaciation and death. However, in both cases, chronic evolution and even healthy carriage are possible after recovering from the acute phase. Particularly acute cases of haemorrhagic T. vivax infection have been reported in the past (Gardiner, 1989; Wellde et al, 1983) in East Africa. T.b. brucei is variably pathogenic in livestock. Under experimental infections, in some cases no clinical signs are observed, whereas in others, sudden death has been reported; still, T. brucei is generally considered as a mild pathogen in livestock. Other types of T. congolense (forest and Kilifi) are considered of lower pathogenicity than savannah type. T. simiae and T. godfreyi are reported to more specifically affect pigs. All African trypanosomes have been identified in a wide range of wildlife species.

  Surra is the disease of mammals caused by T. evansi, and it is included in the WOAH Terrestrial animal listed diseases for multiple species. T. evansi is transmitted by biting flies (see “Species involved. Vectors cyclical/non-cyclical).

  T. evansi and T. vivax are pathogenic for a very wide range of mammalians, and they can both cause acute to chronic diseases. T. evansi is known to mainly affect camelids (as well as equids and dogs) in Africa, but it affects a large range of wild and domestic hosts in Asia, including camels, equids, cattle, water buffaloes, sheep, pigs, goats, elephants, deer, rhinos and dogs, and, in Latin America, equids, cattle, buffaloes, dogs, and a large range of wild animals including vampire bats, capybaras and other rodents. T. vivax is limited to cattle, water buffalo, deer, equids (in Africa and Latin America), and can cause serious disease outbreaks, particularly in Latin America, with high mortality rates. T. vivax has recently been reported to occur in the Middle East, as demonstrated by parasitological and molecular techniques (Asghari and Rassouli, 2022).

  Dourine is an equine disease caused by T. equiperdum. It is rarely found in the host’s bloodstream but rather shows a remarkable tropism for the mucosa of the genital organs, subcutaneous tissues, and the central nervous system. It is therefore known to be only venereally transmitted amongst Equidae. There are no pathognomonic clinical signs of dourine but venereal transmission, genital oedema and cutaneous “douros” are characteristics of dourine. It is considered that T. equiperdum affects only equids, mainly by sexual transmission with a mortality rate that can reach 75%. It is distributed worldwide and is a WOAH terrestrial animal listed disease in equines.

  The Non-tsetse transmitted Animal Trypanosomoses (NTTAT) correspond to diseases due to mechanically transmitted T. evansi (causing surra), T. vivax, as well as sexually transmitted T. equiperdum (causing dourine).

  Human African Trypanosomiasis (HAT) or sleeping sickness in humans is due to T. brucei gambiense, responsible for a chronic disease (g-HAT) in West and Central Africa, and to T.b. rhodesiense that causes an acute disease (r-HAT) in East and Southern Africa. These two subspecies are cyclically transmitted by tsetse flies and are also found in animals, particularly T.b. rhodesiense for which the animal reservoir is a key component of disease epidemiology.

  GAPS

  Knowledge gaps persist regarding the various factors and their interactions that influence pathogenesis of trypanosomoses and their epidemiology. While it is established that multiple variables, including prior exposure, maternal immunity, drug resistance, host genetics, and environmental conditions, impact the outcomes of the infection, the precise mechanisms underlying these interactions remain unclear.

  Information on the pathogenesis of T. brucei in indigenous African cattle breeds is limited, and the potential for central nervous system involvement in these hosts requires further investigation.

  Additionally, the role of natural reservoirs and mechanical vectors in trypanosome transmission warrants more comprehensive study. The possibility of non-vectorial transmission routes, including sexual transmission, for certain trypanosome species deserves exploration.

  The origins of haemorrhagic T. vivax strains are unknown.

  The reasons for a more restricted host range of T. vivax compared to the pan-mammalian T. evansi are not fully understood. The disparate pathogenicity of T. evansi in different geographic and host contexts is puzzling, as is the precise genetic differentiation between T. evansi and T. equiperdum. Comprehensive genomic and kinetoplast sequencing of these trypanosome species from diverse regions could provide critical insights.

• Stability of the agent/pathogen in the environment

  Salivarian trypanosomes essentially survive in the cyclical vector and in the mammalian host.

  In their cyclical vectors, they can survive for very long periods; once a tsetse fly is infected, it is considered that it will remain infective for the rest of its life.

  Within their mammalian hosts, Salivarian trypanosomes can be found in various biological compartments, mainly in liquids such as blood, lymph, cerebrospinal fluid (CSF), joint fluid, but also in other parts of the body such as genital organs and skin. Trypanosomes establish long-term infections in their mammalian hosts. Although the maximum lifespan of infection in cattle remains undetermined, experimental studies have confirmed persistent infections lasting for hundreds of days to several years.

  Salivarian trypanosomes do not survive for long periods outside the host and vector. They can survive in blood samples (cooled) for at least 12 hours, and up to 72 hours. Their survival in infected meat is estimated at 24-72 hours but has not been extensively studied.

  For mechanically transmitted trypanosomes, survival in the biting fly proboscis is estimated at a few minutes, and consistently below two hours. However, this is questionable for Stomoxys because they can store the blood in their crop, in which the physico-chemical conditions are favourable to trypanosome survival. Experimental studies have demonstrated potential transmission after 24-48 h, possibly due to crop regurgitation during blood feeding.

  Overall, beside the above-mentioned circumstances, Salivarian trypanosomes cannot survive in the environment.

  GAPS

  It is unknown how long mammals can remain healthy carriers of trypanosomes in the absence of treatment.

  Diagnostic tests that can distinguish a “complete cure” from some cryptic trypanosomes still being present are needed.  

  It is unknown how long parasites may remain infective within an animal carcass and consequently the risk posed by butchering meat or feeding carnivores with meat from an infected animal.

  Delayed transmission of T. evansi (and possibly other species) by Stomoxys needs to be investigated.

• Species involved

• Animal infected/carrier/disease

  Trypanosomes can infect a wide range of domestic and wild animals. Clinical cases have been described in cattle, water buffalo, sheep, goats, camels, horses, donkeys, alpacas, llamas, pigs, dogs, cats, elephants, as well as other domestic and wild animal species. Clinical expression of the infection, ranging from asymptomatic to severe symptoms, depends on the host and parasite species/population pairing.

  In cattle, trypanosomosis is mainly caused by T. congolense savannah and/or T. vivax and to a lesser extent by T. brucei. The disease may be acute or chronic depending on the host genetics and physiology, the fitness of the host in the context of the environment (especially food availability), the pathogen and co-existence in that host of multiple pathogens. In some cases, cattle can be carrier of trypanosomes at low and fluctuant parasitaemia and without showing any symptoms, this is especially true for the so-called trypanotolerant cattle. An animal may be infected with different strains and species of trypanosome and is likely to be infected with other pathogens.

  Horses are highly susceptible to infection by T. evansi and T. equiperdum.

  In domestic pigs, T. simiae produces a hyperacute, fulminating haemorrhagic disease, while T. suis, a rarely and insufficiently described parasite of the Pycnomonas subgenus, is responsible for a chronic disease (Rodrigues et al., 2020). Pigs can carry T. congolense and T. brucei, including T. brucei gambiense, but have been reported to be refractory to T. vivax (Hoare, 1972).

  Both domestic and wild mammals may carry T.b. rhodesiense, the agent of r-HAT, usually exhibiting mild disease.

  Besides humans, considered as the main hosts for T.b. gambiense, domestic and wild animals can also be carriers of T.b. gambiense, but their epidemiological role is poorly characterized.

  Surra affects particularly camels (often as chronic disease), horses (acute disease) in Africa, but also buffaloes and cattle in Asia; it is Trypanosomes can infect a wide range of domestic and wild animals. Clinical cases have been described in cattle, water buffalo, sheep, goats, camels, horses, donkeys, alpacas, llamas, pigs, dogs, cats, elephants, as well as other domestic and wild animal species. Clinical expression of the infection, ranging from asymptomatic to severe symptoms, depends on the host and parasite species/population pairing.

  In cattle, trypanosomosis is mainly caused by T. congolense savannah and/or T. vivax and to a lesser extent by T. brucei. The disease may be acute or chronic depending on the host genetics and physiology, the fitness of the host in the context of the environment (especially food availability), the pathogen and co-existence in that host of multiple pathogens. In some cases, cattle can be carrier of trypanosomes at low and fluctuant parasitaemia and without showing any symptoms, this is especially true for the so-called trypanotolerant cattle. An animal may be infected with different strains and species of trypanosome and is likely to be infected with other pathogens.

  Horses are highly susceptible to infection by T. evansi and T. equiperdum.

  In domestic pigs, T. simiae produces a hyperacute, fulminating haemorrhagic disease, while T. suis, a rarely and insufficiently described parasite of the Pycnomonas subgenus, is responsible for a chronic disease (Rodrigues et al., 2020). Pigs can carry T. congolense and T. brucei, including T. brucei gambiense, but have been reported to be refractory to T. vivax (Hoare, 1972).

  Both domestic and wild mammals may carry T.b. rhodesiense, the agent of r-HAT, usually exhibiting mild disease.

  Besides humans, considered as the main hosts for T.b. gambiense, domestic and wild animals can also be carriers of T.b. gambiense, but their epidemiological role is poorly characterized.

  Surra affects particularly camels (often as chronic disease), horses (acute disease) in Africa, but also buffaloes and cattle in Asia; it is mainly mechanically transmitted by biting flies but also can be transmitted perorally to carnivores. It has a very large potential reservoir in domestic animals and wildlife. In Latin America, T. evansi is also biologically transmitted by vampire bats (Desmodus rotundus).

  GAPS

  Parasitaemia of infected animals is highly fluctuating; after early peaks of parasitaemia, some animals exhibit low or undetectable parasitaemia which impairs diagnosis sensitivity. The impact and role of this ‘carrier state’ within individual hosts, and upon disease epidemiology, needs to be further elucidated, as these animals can act as reservoirs of parasites.

  It is still unknown why some mammal species are highly susceptible to some parasite species and not to others. Similarly, the mechanisms of trypanotolerance are not elucidated.

  The bases for CNS (Central Nervous System) invasion by some trypanosomes remain to be understood.

  The ability to distinguish infections due to different Trypanozoon species remains low: a clear distinction between surra and dourine is not possible in equines, the same applies for surra and nagana in camels in parts of Africa. Similarly, it is difficult to distinguish T.b. brucei from T.b. gambiense in animals in HAT foci.

  Host specificity of T. equiperdum can be questioned, in relation to the difficulty to discriminate T. equiperdum from T. evansi.

  A better characterization of clinical signs and pathological features induced by different clades of T. evansi and T. equiperdum should be undertaken using natural and experimental infections.

  What is the pathogenicity of trypanosomes in wildlife?

  What is the pathogenicity of trypanosomes other than T. simiae and T. godfreyi in pigs?

• Human infected/disease

  Human serum contains trypanolytic factors that protect humans from most Trypanosoma infections, but two subspecies of T. brucei have developed a strategy to circumvent these factors and are thus responsible for Human African Trypanosomiasis (HAT).

  Trypanosoma brucei gambiense is found in West and Central Africa; it is responsible for a chronic disease. Trypanosoma brucei rhodesiense is found in Eastern and Southern Africa; it is responsible for an acute disease.

  The two forms of HAT are focal diseases (i.e. they tend to occur within particular geographic areas or foci). Notably, Uganda has both forms, but the diseases do not currently overlap.

  Other Trypanosoma species or sub-species are normally not infective to humans, although there have been cases of T. evansi diagnosed in India and Vietnam (Truc et al., 2013; Van Vinh Chau et al., 2016). Very rare cases of T.b. brucei, T. congolense and T. vivax have also been reported in humans in the past. Such infections mainly concern people with specific mutations, notably lack of functional Apolipoprotein L1 which is part of the trypanolytic factors, or people who have immune deficiencies. Cases of human trypanosomiasis caused by T. lewisi, considered as a commensal in rats, have also been reported.

  GAPS

  Some rare cases of atypical human infections by animal trypanosomes have been reported: what are the factors making humans susceptible to T. evansi, T.b. brucei, T. congolense, T. vivax and T. lewisi? What is the proportion of human population with immune deficiencies or genetic mutations that could be susceptible to T. evansi, T.b. brucei, T. congolense, T. vivax and T. lewisi?

• Vector cyclical/non-cyclical

  Cyclical vectors: Tsetse flies belong to the Glossinidae family, comprising a single genus, Glossina, divided into three subgenera or groups (palpalis group: riverine species; morsitans group: savannah species; fusca group: forest species). Glossinidae are closely related to Muscidae and belong to Diptera. Thirty-one species and subspecies of tsetse are generally recognized. Both sexes of tsetse are haematophagous (Solano et al. 2010).

  Tsetse flies are only found in parts of Sub-Saharan Africa.

  Tsetse require humidity, and are found in vegetation near rivers and lakes, in forest-galleries and in wooded savannah. All species are potential vectors of Salivarian trypanosomes, but only a few (of the palpalis and morsitans groups) are important as disease vectors in the field.

  Tsetse flies have a particular mode of reproduction named “adenotrophic viviparity,” which is only encountered in a small number of Diptera grouped in the biological group of pupiparous insects (Hippoboscidae, Streblidae, Nycteribiidae). In this group, the female does not lay eggs but gives birth to a larva (larviparity) as large as the mother (Haines et al, 2020), since it has a pocket similar to the uterus of the mammals, in which it preserves its larva until maturity. During its intra-uterine life, the larva is fed by secretions of lactiferous glands annexed to the uterus, part of which relies on symbiotic bacteria (Wigglesworthia spp.).

  Females live longer than males. Some twelve-month survival records were obtained in the laboratory but, in natural conditions, as a mean, females are believed to live for about three months and males between one and two. The lifespan is very variable depending on the season: optimal in the rainy season, it decreases in the cold season, and even more in the hot season. Despite a sex-ratio close to one at hatching, females are generally more numerous in a population due to their longer lifespan.

  It is believed that only the first mating results in fecundation, although they may engage in subsequent copulations. One of the characteristics of tsetse flies is that every female will give birth to no more than 4-5 descendants throughout its life in nature. This very low number is compensated by their long-life span, a high probability of reaching the adult stage, and a very high resilience and ability to survive at low densities, especially for tsetse species of the palpalis group.

  Tsetse cannot survive at elevated temperatures and require a shady environment. This is why they are being affected by global warming (Lord et al., 2018), although it may also enable them to colonise new areas.

  Probably linked to their very low rate of reproduction, insecticide resistance has so far never been demonstrated in tsetse.

  Mechanical vectors: hematophagous flies that start feeding on one host and continue to feed on another host, can contribute to mechanical transmission of trypanosomes, when probing and injecting saliva prior to blood-sucking, since they may inoculate some infected material remaining on their mouthparts. They mainly belong to tabanids and stomoxyine flies, although some Musca spp., Glossina spp., and some hippoboscids are also suspected, notably in camels for the latter.

  The Tabanidae family encompasses more than 4,400 species. Different species of tabanids occupy different kinds of landscapes, latitudes and altitudes (Baldacchino et al., 2014). They require humidity, and are found in vegetation near rivers and lakes, world over, from arctic to tropical areas. There are 144 genera, however, the most frequent ones are Tabanus (horse flies), Chrysops (deer flies), Haematopota (clegs), Atylotus, Hybomitra and Ancala. Tabanids are large size insects (6-35mm). Only females are hematophagous, and they feed mainly on mammalian hosts, once every 3-5 days to insure reproduction. Tabanids are very prolific oviparous insects since females can lay 1,000-5,000 eggs in a 1–3-month period during their adult life. Larval development is long, from 3 months to 3 years, with 7-13 larval stages before nymph and adult stages. Adults are seasonally abundant with a marked nychthemeral activity. Tabanids are present in all environments and are much attracted by large mammals such as cattle, buffaloes, horses and camels, but also wild animals such as deer, antelopes and elephants.

  Stomoxyine flies include more than 50 species belonging to 10 genera, amongst which the most abundant flies belong to genera Stomoxys and Heamatobia. One Stomoxys species is cosmopolitan: S. calcitrans, the others are distributed everywhere. Wild species are found in forest or vegetation in moist environments, but S. calcitrans is a more “domestic” species, developing on manure and vegetal wastes, therefore highly abundant in livestock farms and in sugar cane and pineapple plantations. Stomoxys are medium size insects (5-15mm). Females and males are hematophagous, and they feed on mammalian hosts, from 1-2 times a day to every 1-2 days. Stomoxyine flies are very prolific oviparous insects since females can lay 300-1,000 eggs over a 1–3-month period during their adult life. Larval development is short, from 2-4 weeks, with 3 larval stages before pupae and adult stages. Adults are seasonally abundant with a marked nychthemeral activity. Stomoxys spp are present in livestock environments and are attracted by the same hosts as tabanids. Haematobia spp. are smaller flies which rather stay on their hosts and lay 30-50 eggs per batch on cattle or horse dung (200-500 eggs per female in a lifetime). Their density is very high in some livestock farms. They are also present in all types of environments, always close or on their hosts.

  Overall, tabanids and Stomoxyine flies are present almost everywhere and they present a very high prolific potential with 200-5,000 eggs per female per life. Tabanids are potentially the best mechanical vectors due to their large size, seasonal abundance, and persistent feeding behaviour.

  GAPS

  What is the impact of climate change on tsetse distribution? What is the impact of climate change on mechanical vectors distribution? (see also “Links to climate. Distribution of diseases or vector linked to climate” section)

  an tsetse develop insecticide resistance? Investigations are needed in areas where control campaigns are on-going using insecticide.

  The exact role of tsetse in the environment (with regards to predation and parasitism for instance) has not been investigated for a very long time.

  The infection rate (i.e. mature salivary gland infections) in tsetse in g-HAT foci has always been surprisingly low, which raises questions.

  The resilience of tsetse populations (of the palpalis group) to elimination campaigns, despite their apparent vulnerability, is a paradox that remains to be understood.

  The recent move towards molecular analysis of tsetse flies trapped in the wild may artificially inflate calculation of infection rates due to amplification of DNA of dead, non- viable or non-infective trypanosomes in the tsetse gut material. This needs to be quantified since data are needed for modelling.

  What is the role of transmission by non-tsetse vectors within the tsetse belt?

  Being able to predict areas and time periods of mechanical transmission remains challenging.

  Factors driving high densities of mechanical vectors remains to be understood.

  What is the capacity of Stomoxys for delayed mechanical transmission by crop-regurgitation in natural conditions?

  Is insect trapping able to reduce or even control mechanical transmission and biting fly populations?

• Reservoir (animal, environment)

  Wild and domestic mammals that carry pathogenic tsetse-transmitted trypanosomes

  represent

  important reservoirs

  of parasites for their vectors, and thus for livestock as trypanosomes can be infective for

  tsetse flies at low parasitaemia. Wildlife, for which no control measure is undertaken, can have an important epidemiological role at the interface between protected areas and livestock grazing areas. Some reptiles are also suspected to carry pathogenic trypanosomes for mammals, e.g.,

  T. brucei

  in varan (Njagu et al., 1999).

  Regarding T. evansi and T. vivax transmitted by biting flies, mostly sick animals presenting high parasitemia can act as active reservoirs since mechanical vectors are efficiently infected from only high parasitaemic animals. Equids are sole known reservoirs for T. equiperdum.

  Tsetse, as cyclical vectors, are per se reservoirs of Salivarian trypanosomes, since they are reported to be infected, and infective, throughout their life.

  Biting flies are not considered as reservoirs, since trypanosomes cannot survive more than 2 hours in Tabanids (minutes, usually), and 2 to 72 hours in the Stomoxys crop. For the latter, whether they would be naturally regurgitated still requires confirmation.

  HAT: A broad range of wild and domestic animals can act as reservoirs of the human-infective parasites especially T.b. rhodesiense. Although animals can be infected with T.b. gambiense, there is limited evidence that animals form a maintenance reservoir and mathematical modelling suggests that animals are unlikely to substantially hinder elimination of transmission of g-HAT, at least in the Democratic Republic of the Congo (DRC, the country with highest g-HAT burden) and one focus in Chad (Antillon et al, 2024; Crump et al., 2022; Rock et al., 2022).

  Trypanosomes cannot survive in the environment.

  GAPS

  The role of animals as reservoirs of T.b. gambiense to humans in endemic areas in West Africa remains unquantified, especially for domestic pigs which can be found infected by trypanosomes of Trypanozoon subgenus including, but not restricted to T.b. gambiense, but whether they act as active reservoir or as a protective screen for the humans around, is still under debate.

  The role of wildlife reservoir for r-HAT has been examined only in limited foci.

  The same applies for the role of wildlife as reservoir for nagana.

  The role of wild host as reservoir for livestock infection by T. evansi is not known.

  Due to the ability of Stomoxys to keep trypanosomes alive in their crop for 1-3 days, what is their epidemiological role in delayed transmission? Can they act as temporary reservoirs of T. evansi?

  Overall, particular care should be taken to avoid over-interpreting positive PCR results amplifying trypanosome DNA before concluding that there are live trypanosomes capable of sustaining disease transmission.

• Description of infection & disease in natural hosts

• Transmissibility

  Trypanosomosis is mainly a vector-borne disease that involves three interacting organisms: the host (human, wild or domestic animal), the blood sucking vector (tsetse flies for cyclical transmission; biting flies for mechanical transmission) and the pathogenic parasite. Salivarian trypanosomes are essentially transmitted to mammals by hematophagous insects that get infected during a previous blood meal on an infected animal.

  Cyclical transmission of trypanosomes is due to infected tsetse flies since they remain infective all their life.

  Mechanical transmission occurs when a biting fly picks up trypanosomes from an infected host, interrupts its blood meal, and then transfers living trypanosome to a new host during a subsequent bite. In that case, the insect is not a permanent carrier of trypanosomes. This process depends on the contamination of the biting site with viable parasites from the previous host. However, the efficiency of mechanical transmission is limited by the short survival time of trypanosomes in the fly’s proboscis, typically ranging from 30 to 120 minutes.

  Direct infection, through contaminated blood and other body fluids is also possible. Therefore, iatrogenic transmission via serial injections or surgery tools is considered as potential transmission route for all trypanosomes (Silva et al., 1996).

  Members of Trypanozoon show a general tissue tropism and exhibit a special ability to penetrate mucosae thus allowing peroral infection to carnivores and vampire bats (T. evansi), as well as vertical trans-placental and venereal transmission for T. equiperdum. Although vertical transmission of T.b. gambiense has been proposed (Lindner and Priotto, 2010), modelling suggests that the dynamics of case data in different settings can be readily explained by the known human-vector transmission pathway (Crump et al., 2024; Rock et al.,2022).

  Further details are provided in the “route of transmission” section.

  GAPS

  Overlooked routes of transmission: the impact of sexual and congenital transmission for T. evansi should be further explored, as well as the importance of peroral transmission, especially in wild carnivores.

  The following factors that influence the interactions between the three actors (vector- parasite-host) and thus the transmission remain poorly understood:
  • for the vector: effect of fly age, genetic susceptibility, physiological condition (fly’s immune system), environment and microbiota on vector competency, naming its ability to get infected and be infective

  • for the parasites: role of trypanosome genotype on transmissibility, effect of trypanocidal drug resistance on transmissibility

  • for the host: effect of host susceptibility on the vectors infection rates.

• Pathogenic life cycle stages

  When a tsetse fly feeds on the blood of a parasitized host it also ingests bloodstream forms of the trypanosomes. For T.b. brucei, T.b. gambiense and T.b. rhodesiense, infective forms for tsetse flies are the short stumpy forms of blood trypomastigote. These bloodstream forms will transform in the fly’s gut into procyclic trypomastigotes and multiply before migrating towards salivary glands (T. brucei) or proboscis (T. congolense), where they transform into epimastigotes and multiply again, prior to transforming into metacyclic trypomastigotes (mammalian-infective form). Development of T. vivax takes place in the mouthparts only. This process takes 5-13 days for T. vivax, 15-23 days for T. congolense and 22-33 days for T. brucei. After this period, trypanosomes will be injected into a host as the fly feeds. Once infected, flies remain infective for the rest of their life.

  In mechanical transmission, there is no cyclical evolution of the parasite, it stays as a bloodstream form on the mouthparts of the mechanical vector and restarts its binary fission still as bloodstream form in a new host.

  In vampire bats, which act as biological vector, only blood forms, trypomastigotes, are found in bats’ organs including salivary glands.

  GAPS

  Biological basis of distinction between mechanically and cyclically transmitted T. vivax (speed of induction, reversibility, synchrony of both ways of transmission, genetic bases).

  Amastigote forms of trypanosomes have been observed in some hosts and were suggested to be latent forms of the parasite; they should be explored as a potential relapsing source of infection.

• Signs/Morbidity

  Nagana can take the form of an acute or a chronic disease that causes intermittent fever and is accompanied by anaemia, oedema, lacrimation, enlarged lymph nodes, abortion, decreased fertility, delayed sexual maturity, loss of appetite and weight, leading to early death in acute forms or to digestive and/or nervous signs with emaciation and eventually death in chronic forms. The severity of symptoms is related to the gradient of susceptibility of the host (see below) to trypanosome infections and to the pathogenicity of species and strain of the infective trypanosome (e.g.: pathogenicity of T. congolense savannah > T. vivax > T. brucei > T. congolense forest…). Morbidity and mortality rates can be high in naïve or debilitated animals.

  Generally, wild mammals and indigenous humpless taurine breeds (Bos taurus taurus) possess a certain degree of tolerance to the infection: they appear to be able to mitigate symptoms, especially anaemia, control parasitaemia faster than susceptible animals, and can eventually recover (Hanotte et al., 2002; Murray et al.,1984). Tolerant cattle breeds, called trypanotolerant, are represented by the N’Dama breed (long-horn) and West African Shorthorn breeds like Lagune and Baoulé (Berthier et al., 2016). Indicine (Zebu) humped cattle and European taurine breeds are susceptible to the disease and cannot be raised in highly enzootic areas in the absence of treatment. Some local crossbred cattle breeds (Bos indicus x Bos taurus) such as Méré, Borgou and Sheko present an intermediate susceptibility between African taurine and Zebu (Berthier et al., 2016; Mekonnen et al., 2019). Dwarf breeds of small ruminants, Djallonké sheep and Dwarf West African goat, are able to live in highly enzootic areas and also seem to be trypanotolerant.

  In addition to genetic factors, environmental factors like food and water availability, concurrent diseases, and breeding systems can impact on animal susceptibility. Even trypanotolerant cattle can suffer of the disease if nutrition is suboptimal.

  As per HAT, T.b. gambiense induces a chronic form in humans, and T.b. rhodesiense an acute form, but they are usually not very pathogenic to livestock.

  For surra, infections by T. evansi in horses and dogs are generally fatal without, and even with, treatment; in camels it may be fatal or remain chronic. In cattle it may be inapparent or evolve towards chronic stage; more rarely it can be acute and lead to death.

  As concerns dourine in horses: T. equiperdum can induce fever, swellings and local oedema of the genital organs and mammary glands, oedematous eruptions of the skin and depigmentation anaemia, emaciation, ocular lesions, lack of coordination in the limbs, facial paralysis, but no marked loss of appetite. Pregnant mares may either abort or foal normally.

  GAPS

  Genetic determinisms responsible for trypanotolerance in cattle are not known; the molecular and cellular mechanisms leading to trypanotolerance are not elucidated, neither the basis of an efficient immune response.

  Trypanotolerance in small ruminants remains to be better characterized as publications gave contradictory results depending on the breeds and trypanosome species studied.

  Role of interspecific immunity in trypanosome infections, and consequences on the susceptibility of the animals when aging, should be investigated. Effects of two to three serial infection/treatments at one-month interval to enhance immune capacity of livestock could be explored.

  The impact of co-infections, due to different trypanosomes or other pathogens, on the disease evolution is poorly understood.

  Pathognomonic “plaques” for T. equiperdum-caused dourine are not always observed – artifact or dermal tropism? Histological study should be implemented when observed.

  Distinction between T. evansi/T. equiperdum infections in horses is very difficult.

• Incubation period

  Nagana becomes patent in cattle, small ruminants and equines generally within 7 to 21 days following the bite of a trypanosome-infected tsetse fly. However, incubation periods can vary widely, ranging from 4 days up to as long as 8 weeks. Animal infected with highly virulent strains may develop symptoms even sooner, with clinical signs appearing 7 to 10 days after an infective tsetse bite in cattle, and within 2 weeks, with possible death, in small ruminants.

  The incubation period for surra in horses, dogs, buffaloes, and cattle typically ranges from one to three weeks following infection by mechanical vectors. Nonetheless, longer incubation periods have been documented. In regions where surra is endemic, determining the exact incubation period is challenging as it can vary significantly based on factors such as the animal's age, overall health, exposure to stress, and whether the infection is a first occurrence or a relapse after a prolonged asymptomatic phase. This variability can range from days to years. The incubation period for dourine is highly variable. It can range from three weeks to several months or even longer. Due to this significant variation, the World Organisation for Animal Health (WOAH) has established a standard incubation period of 6 months for the purposes of its Terrestrial Animal Health Code.

  g-HAT-infected patients may remain asymptomatic for months or even years. Typically, the mean duration of the hemo-lymphatic stage has been estimated at 18 months (Checchi et al., 2008) and one of the meningo-encephalitic stage to about 8 months (Checchi et al, 2015), but there is considerable variation, with a documented case probably infected for 29 years (Sudarshi et al., 2014). The evolution of T.b. rhodesiense infection is faster, with an incubation period of 1 to 3 weeks. However, genetic differences in the parasite have been associated to different clinical phenotypes (MacLean et al., 2004; Nambala et al., 2024).

  GAPS

  In camels infected with surra, long asymptomatic periods can occur due to the parasite's ability to hide in extravascular locations, such as eyes and joint fluids. Consequently, pinpointing an incubation period is challenging because of this potential for prolonged latency.

  A similar phenomenon might exist in humans infected with T.b. gambiense, but further research is needed to confirm this hypothesis.

• Mortality

  The mortality rate due to AAT varies widely, depending on the animal species, breed, and the specific trypanosome involved. In untreated cattle and horses infected with certain virulent strains, death can occur within weeks or months, especially when animals are malnourished or weakened by other diseases, with a mortality rate that can reach 50-100%. Adult mortality rates in cattle are typically 3 – 6 percentage points higher, with herd death rates for adult cattle often three times those observed in uninfected animals or in low challenge areas. Field studies consistently show that nagana increases mortality rates in exposed cattle populations and especially increases calf mortality by 0-10% in tolerant breeds of cattle and by 10-20% in susceptible breeds of cattle (Swallow, 2000). See also “Socio-economic impact. Direct impact on a) production” section.

  Surra is highly lethal to horses, with mortality rates nearing 100% regardless of treatment, most of them being inefficient. While rare exceptions exist in Southern Latin America, horses typically succumb to T. evansi infection. In contrast, camels experience a more chronic form of the disease, which may culminate in death after approximately three years.

  Mortality rates from T. equiperdum vary widely, influenced by the severity of the disease, which in turn depends on the trypanosome strain and the horse's overall health. While infections typically persist for one to two years, approximately half of affected horses succumb during this period. In some cases, the infection may linger for up to three to five years.

  r-HAT is an acute disease that leads to death within a few weeks or months if left untreated; g-HAT is a more chronic form that eventually leads to death, months or years after infection, in the vast majority of cases.

  GAPS

  Accurate estimates of fatality cases and rate are missing.

  The factors contributing to endemic trypanosomosis (high morbidity but low mortality) in livestock are not fully understood. Possibly, multi-species infections may contribute to a general “anti-trypanosomal immunity”.

  Only a few studies on the socio-economic impact of NTTAT have been performed, nearly all in South-East Asia. They are neglected in Africa. Due to their theoretical (T. vivax) or demonstrated (T. evansi) ability to establish in Europe, the potential impact of mechanically transmitted trypanosomes should be evaluated, and emergency control measures be defined.

• Shedding kinetic patterns

  As indicated in the “Description of Infection & Disease in natural hosts” section, Salivarian trypanosomes cannot survive for long periods outside their cyclical vector and host organs and fluids. Their transmission mostly depends on cyclical and mechanical vectors, or iatrogenic contamination.

  However, transmission of trypanosomes can occur through host body fluids from a host to another; this makes possible (i) peroral transmission of T. evansi to carnivores and vampire bats, via trans-mucosae penetration of the parasite in mouth, oesophagus and possibly stomach (in the case of vampire bats); (ii) the direct transmission from mother to foal, when the foal licks contaminated secretions from the mother, and (iii) venereal transmission between males and females when vaginal or seminal fluids are contaminated; this is suspected for Trypanozoon in general, including T. brucei ssp. and T. evansi, and it is the main way of transmission of T. equiperdum.

  GAPS

  Conditions and duration of survival of infective forms of trypanosomes in meat, meat products and sperm are not clearly known.

• Mechanism of pathogenicity

  How trypanosomes influence the clinical signs in infected animals remains an area of study. Some aspects are well understood, but in most cases this knowledge pertains to T. brucei in experimental mouse models – how well such information translates to T. congolense and T. vivax, and to clinically relevant ruminant hosts, is still largely unaddressed. However, some key factors central to trypanosome pathogenicity have been well characterised. Prominent amongst these is the trypanosome system of antigenic variation, whereby trypanosomes exploit a very large gene family (>1,500 genes in T. brucei) to change the identity of the immunodominant variant surface glycoprotein on the cell surface in order to evade the development of variant-specific antibodies. This incredibly elaborate system is central to the establishment of chronic infections, and the VSG surface coat also hinders antibody access to other invariant surface proteins – and is a key challenge to overcome for vaccination strategies (further information can be found in the “Vaccines (inactivated, attenuated, sub-unit, vectored, GMO...”) section. A further well characterised process that contributes to chronicity of infections is the density-dependent differentiation from replicative to non-replicative insect-transmissible forms that occurs to limit parasite growth in the mammalian host. The mechanisms underpinning this are very well defined in T. brucei, but it is not clear how this translates to other trypanosome species. The mechanically transmitted T. evansi has lost the ability to transmit cyclically through tsetse flies – and therefore does not undergo this density-dependent process, providing one explanation as to why parasitaemias during infections with T. evansi are higher than in T. brucei infections (probably also enhancing the potential for mechanical transmission in itself).

  A key clinical sign in AT, in all hosts, is anaemia. How trypanosomes induce anaemia is not clear, with parasite-induced lysis of red blood cells, suppression of erythropoiesis, and dysregulation of host inflammatory processes all being reported as contributing to anaemia. Similarly, immunosuppression is a factor in animal trypanosomosis. Trypanosome-mediated destruction of B cells and B cell memory, which are responsible for mature and effective antibody responses, is well characterised to occur in mice (with the three trypanosome species), but while there are indications that this phenomenon also occurs in trypanosome-infected cattle, the extent and impact of this process in clinically relevant hosts is not currently clear.

  However, immunosuppressive effects of T. evansi infection were proved to interfere with vaccination in buffaloes (Singla et al., 2010).

  The balance and timing between pro- and anti-inflammatory immunological processes are critical in determining infection severity and duration – again this is well-established in mouse models, but how well this translates to clinically relevant host models is not currently clear. These examples highlight the need for more studies in disease-relevant host models, as identifying virulence mechanisms may provide a route to the development of future interventions that can reduce disease severity and burden.

  GAPS

  To what extent do the pathogenicity mechanisms identified in T. brucei translate to T. congolense and to T. vivax?

  To what extent can the information obtained in the mouse model be translated to clinically relevant hosts, including cattle, camels and other small ruminants?

  How do trypanosomes (and the different trypanosome species) cause anaemia? What are the extent and mechanisms of immunosuppression in livestock?

  What is the impact of immunosuppression due to trypanosome infections on response to vaccination in livestock and on intercurrent diseases?

• Zoonotic potential

• Reported incidence in humans

  r-HAT is a true zoonotic disease, in which T.b. rhodesiense from either domestic cattle or wildlife can be transmitted to humans via the tsetse fly, causing the acute form of the disease. g-HAT is mainly considered as a human disease, but importance of animal reservoirs is not clearly defined/understood (see below). Other animal trypanosomes do not normally infect humans, except in case of specific genetic mutations or strong immunosuppression in the patient allowing trypanosomes to multiply.

  As reported by WHO (Franco et al., 2024), a total of 802 cases of Human African Trypanosomiasis were reported in 2021 and 837 in 2022, 94% of the cases were caused by infection with T.b. gambiense. The areas reporting 1 HAT case/10 000 inhabitants/year in 2018–2022 cover a surface of 73 134 km², with only 3013 km² at very high or high risk. This represents a reduction of 90% from the baseline figure for 2000–2004, the target set for the elimination of HAT as a public health problem. Ongoing mapping of HAT cases has helped to estimate the population at risk (41.5 million for 2018–2022) (Franco et al., 2024). Six percent of the at-risk population is for r-HAT.

  For the surveillance of the disease, 4.5 million people were screened for g-HAT with serological tests in 2021–2022. In 2025, the elimination of HAT as a public health problem was validated in Togo, Benin, Uganda, Côte d’Ivoire, Equatorial Guinea, Chad, Ghana and Guinea for g-HAT and in Rwanda for r-HAT. To reach the next target of elimination of transmission of g-HAT, countries have to report zero cases of human infection with T.b. gambiense for a period of at least 5 consecutive years. The criteria and procedures to verify elimination of transmission have been recently published by WHO (2023).

  Between 2013 and 2022, the number of r-HAT patients reported ranged between a few dozens to more than one hundred cases per year, but the number of undetected cases is poorly known, and the disease has a high outbreak potential.

  Incidence in humans of other animal trypanosome infections (T.b. brucei, T. congolense, T. vivax, T. evansi, T. lewisi) is documented but remains extremely rare.

  GAPS

  While the zoonotic potential of T.b. gambiense in animals has been documented (Mehlitz and Molyneux, 2019), its transmission dynamics remains to be investigated (Büscher et al., 2018). This question, which may have been of minor importance when the number of cases was high, is becoming one of the top key questions in a context of elimination of transmission for g-HAT.

  Being able to geographically predict future local epidemics of r-HAT is becoming a key question because of the high mortality of this disease. Climate change might influence occurrence of such outbreaks, by influencing the distribution and fertility of tsetse flies (Nnko et al., 2021; Lord et al., 2018). See also section on “Climate change”.

  The capacity of national programs to detect, diagnose and treat new cases remains a critical challenge in an elimination context where country resources go to other priorities.

  Evaluation of human-parasite contact in exposed human population (veterinarians, farmers, slaughterhouse technicians, rural population / ingestion of raw meat and blood, etc) is needed.

  The potential of T. evansi or other typical animal trypanosomes to cause infections in humans remains elusive, as positive tests do not mean active infection.

• Risk of occurence in humans, populations at risk, specific risk factors

  g-HAT occurs in West and Central Africa, but in 2021-22, 61% (940/1546) of all g-HAT cases were detected in the Democratic Republic of the Congo (DRC). In Central Africa, Angola (218 cases) and the Central African Republic (155 cases) followed the DRC. In West Africa, almost all cases were detected in mangrove areas in Guinea (58 cases) (Franco et al.,2024). The risk for infection with T.b. gambiense is determined by contact with tsetse flies, which usually occurs in humid forest areas or along rivers, lakes or mangroves.

  For r-HAT, the vast majority of recent cases originated from Malawi (73 out of 93 detected in 2021-22) (Franco et al., 2024). In 2022 a r-HAT outbreak occurred in Ethiopia, after 23 years of absence of the disease (Abera et al., 2024). The risk for infection with T.b. rhodesiense is associated with the proximity of tsetse flies and wild animals or cattle, a condition which is often met in protected areas. For this reason, tourists may also get infected (Franco et al, 2022).

  GAPS

  Although a strong decrease in HAT prevalence occurred in the last decade, in particular areas with an unstable security situation or that are difficult to reach, HAT surveillance activities remain a concern as they are difficult to implement.

  Due to the lack of simple diagnostic tests for T.b. rhodesiense infection in humans, diagnostic capacity remains low, leading to an unknown degree of under-detection. Improved diagnosis in animals could also help predict future outbreaks.

  In areas of g-HAT where vector control is being implemented, it may be interesting to evaluate the one-health impact on AT incidence.

• Symptoms described in humans

  Although there is a certain degree of overlap, symptoms and clinical signs for HAT are associated to the stage of infection, the first one being the hemo-lymphatic stage and the second one, the meningo-encephalitic stage.

  In the hemo-lymphatic stage, trypanosomes mainly spread and proliferate in lymph and blood and most symptoms and signs are not disease-specific. One of the earliest symptoms is swelling of the lymph nodes in the neck, typical for HAT due to T.b. gambiense, and occurrence of a chancre at the site of the infective bite for T.b. rhodesiense infection. The latter is mainly observed in cases originating from non-endemic areas. A lymph node or chancre aspirate may allow early diagnosis. Other symptoms and signs include irregular fever, headache, weakness, weight loss, pruritus. Once the patient enters in the meningo-encephalitic stage (cerebrospinal fluid white blood cell count of >5 cells/µl or presence of trypanosomes in the cerebrospinal fluid), symptoms and signs of neurological involvement may become apparent. The disruption of the sleep-wake cycle has been at the origin of the name “sleeping sickness”. Other neurological symptoms include psychiatric disturbances, motor function abnormalities and sensory disorders. If patients do not receive treatment, they fall into coma and eventually die. Some r-HAT patients may suffer from cardiac failure, multi-organ failure, and die even before neurological involvement.

  GAP

  Simple algorithms based on clinical symptoms and signs alerting health facilities to carry out tests for diagnosis of HAT are missing.

• Likelihood of spread in humans

  The parasite T.b. rhodesiense is mainly circulating in animals, and human infection (via the tsetse bite) can be considered accidental. T. b. gambiense on the other hand, is well adapted to humans, which are considered as its main host, but it can also infect animals in areas where tsetse flies occur.

  Other trypanosomes species are unlikely to spread in humans.

  GAPS

  The role of the animal reservoir of T.b. gambiense remains a major question mark: How long can it maintain transmission without passing through human hosts, how important is transmission from animals to humans?

  To detect re-emergence of r-HAT early would be extremely important but early detection is hindered by the remoteness of the disease and underdiagnosis.

• Impact on animal welfare and biodiversity

• Both disease and prevention/control measures related

  The animal form of the disease poses a welfare problem, unless animals are rapidly diagnosed and efficiently treated. This is most often a debilitating chronic disease, and trypanocides are both costly and sometimes inefficient, other times they are unobtainable or may be counterfeited, so that animals may be ill and suffering for considerable periods, and if not treated, will often die in miserable conditions.

  Vector control measures and campaigns may bring benefits to the animals and to their owners by, reducing both disease incidence and biting fly nuisance. It may provide additional one-health benefits also to humans in areas where both human and animal forms of trypanosomosis co-occur.

  Early trypanosomosis control methods involved culling wild animal reservoir, which needless to say, had a highly negative impact on animal welfare and biodiversity. Likewise, vector control through aerial or ground spraying is likely to have an impact on insect biodiversity.

  GAPS

  Globally, there is a need for more efficient prevention (vector control, better food and breeding system) and control measures (diagnostic and treatment) to limit suffering induced by AT. This includes better awareness and training for farmers and better support by various stakeholders (zootechnicians, veterinary services, policy makers, NGOs), as well as better access, quality, and proper use of trypanocides to cure infected animals. In Asia, for example, most often the Ministries of Agriculture only approve diminazene aceturate for the control of surra, while obviously chemoresistance is largely distributed in cattle, and the chemical is not adapted for treatment in horses. Therefore, other drugs such as isometamidium chloride for the treatment in cattle, and quinapyramine for the treatment of surra in horses, should be made available in these countries.

• Endangered wild species affected or not (estimation for Europe / worldwide)

  Most wild animals are known to be sensitive to Salivarian trypanosome infection. However, as the natural hosts of tsetse, African wild animals are generally trypanotolerant, unless under stress. The disease is therefore not thought to be implicated in threatening the survival of endangered species in Africa.

  T. evansi can infect all mammals with serious pathology in deer, ocelot, howler monkeys, tapir, tiger, orangutan, lion etc, and was responsible for fatal cases of surra in rhinos, deer and elephants in Asia. Surra remains a potential danger for any endangered mammal species. Australia has introduced drastic control measures to prevent the introduction of T. evansi into its territory. Walabies proved to be highly sensitive in experimental conditions.

  GAPS

  Prevalence in wild hosts, including wild carnivores, is not known, as well as potential health impact in interactions with stress factors.

  There is need to get more data on prevalence of trypanosomes in wildlife when there is a potential risk for cattle at wildlife -cattle interface areas.

• Slaughter necessity according to EU rules or other regions

  For T. equiperdum infections, the control can only be brought by slaughtering the infected animals. However, treatment attempts have been made and are currently being made.

  For other Trypanosoma spp, there is no rule for slaughtering, however, in case of multi-chemo-resistant strains of trypanosomes, slaughtering the infected animals remains the only way of control and to prevent spreading of such strains.

  In the EU, the management of potential animal trypanosomoses outbreaks is described in the Animal Health Law (https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32018R1882).

  Currently, Dourine and Surra are classified under the Category of listed disease: D + E.

  “Category D disease” means a listed disease for which measures are needed to prevent it from spreading on account of its entry into the Union or movements between Member States.

  “Category E disease” means a listed disease for which there is a need for surveillance within the Union.

  Therefore, there is currently no official obligation of stamping-out policy in EU for nagana, dourine and surra. However, any member state can take measures it deems appropriate to control an AT outbreak.

  GAPS

  T. equiperdum, and to a lesser extent T. evansi: is it necessary or not to slaughter the animals to eliminate the parasite/disease? Are there some chemicals and treatment protocols that could completely cure the animals?

• Geographical distribution and spread

• Current occurence/distribution

  To the best of our knowledge, Trypanosoma congolense and T. brucei s.l. are confined to the tsetse-infested belt in sub-Saharan Africa.

  T. vivax also occurs in tsetse-infested areas, but mechanical transmission and animal movement enabled this parasite to spread beyond these areas, especially in many tsetse-free areas and countries in Africa but also in Latin America and, as recently reported, in the Middle East (Asghari and Rassouli, 2022).

  T. evansi mainly occurs outside the African tsetse-belt, and especially in northern Africa, the Middle East, South and Southeast Asia, and South America. T. evansi was also able to establish itself in the Canary Islands and it gave rise to outbreaks in metropolitan Spain and France, but its presence has not been reported since 2018, which suggests that the sanitary measures implemented were adequate (Tejedor-Junco et al., 2023). In Sub-Saharan Africa the parasite does also occur, and it is likely to be present in more countries than presently reported, as suggested by recent studies that availed themselves of molecular detections techniques (e.g. in Rwanda) (Gashururu et al., 2021).

  Dourine is a disease that was historically present in many regions of the world. During the 20th century, the implementation of control measures based on serological diagnosis and eradication policies led to the elimination of this disease in most industrialized countries. Currently, we do not know the exact distribution of the disease, which remains predominantly present in remote areas. The analysis of the countries that reported cases of dourine to the WOAH from 2005 (WAHIS) to the present provides an overview of the regions where this disease may circulate (keeping in mind that some of these countries may have since controlled the disease):

  • Sub-Saharan Africa (Botswana, Ethiopia, South Africa and Namibia),

  • The Middle East (Iran),

  • Asia (China, Kyrgyzstan, Mongolia, Pakistan, Russia and Turkmenistan).

  It is important to keep in mind, however, that some of these countries may have implemented effective disease control measures since these cases were reported.

  The occurrence of a dourine outbreak in Italy in 2011 (contained through an eradication policy) highlights the importance of maintaining border control for this disease.

  GAPS

  Although there is a broad understanding of the global range of trypanosomes and their associated diseases, the accuracy of the information and how up-to-date it is vary depending on the species of trypanosomes and the geographical regions, which hinder surveillance and control. The diseases are not always reported to WOAH by countries, whose maps are thus affected by gaps in space and time.

  The continental atlas of vector-borne AT for Africa is in the process of assembling the current knowledge of the occurrence and distribution of T. congolense, T. brucei, T. vivax and T. evansi for the African continent (Cecchi et al., 2014). The atlas is based on a review of the scientific literature over the past thirty years, and mapping is at the site level. The initiative, implemented by the Food and Agriculture Organization of the United Nations (FAO), also includes a component on the distribution of tsetse flies (Cecchi et al., 2015 and 2024), and it has the ambition of being regularly updated. In parallel, initiatives at the country level, i.e. national atlases, are being developed by several countries; these include both published and unpublished data and therefore provide a more complete picture of the disease distribution than the continental atlas. National atlases have already been developed and published by six countries: Burkina Faso (Percoma et al., 2022), Ethiopia (Gebre et al., 2022), Kenya (Ngari et al., 2020), Mali (Diarra et al., 2019), Spain (Henríquez et al., 2024), Sudan (Ahmed et al., 2016) and Zimbabwe (Shereni et al., 2021), and they are presently being developed in several additional African countries (Boulangé et al., 2022). These or similar initiatives in all endemic countries would greatly enhance our knowledge of disease distribution, and they would also allow to generate improved occurrence maps at the continental and global level.

  A review of the global distribution of T. evansi as reported in the scientific literature was provided by Aregawi et al., (2019), and a similar one was provided for T. vivax (Fetene et al., 2021). Similar reviews have not been conducted for T. congolense, T. brucei and T. equiperdum, however, all African trypanosomes geographical distributions are available in Desquesnes et al., 2022. In terms of limitations, it should be stressed that in these reviews mapping is limited to the country level, and it stops short of mapping the exact location of the reported disease occurrence. Furthermore, these reviews are normally one-off exercises, and they are not regularly updated.

  T. evansi presence is generally not looked at within the tsetse belt, which therefore, limits information availability on its geographical occurrence.

  Due to the potential economic impact of T. equiperdum infection on international movement and trade of equids, some countries appear reluctant to provide data and update the geographic distribution of dourine.

• Epizootic/endemic- if epidemic frequency of outbreaks

  Nagana is mainly an enzootic disease in Africa, as demonstrated by the fact that most surveys and studies in the endemic countries will find nagana-causing parasites at any point in space and time. Still, variations in disease prevalence and incidence are vast across the endemic areas, outbreaks do occur, and disease-free areas at certain points in space and time are a possibility. Seasonality also plays a role in driving disease prevalence and incidence.

  The epidemiology of T. vivax in Latin America is rather different from that in Africa, and much more prone to periodic epizootic outbreaks.

  T. evansi/surra is also an endemic disease, although outbreaks of surra have also a broader relevance and are more frequently reported in the literature.

  HAT: The disease in humans is typically focal with low prevalence of 0.1% or less. As g-HAT infection in humans lasts a long time, disease dynamics are slow and sudden large outbreaks are not expected. This contrasts with the acute form of the disease (r-HAT) where small, clustered outbreaks are observed, although case numbers are not generally high. Historically, HAT epidemics have been devastating leading to the depopulation of some affected areas.

  GAPS

  Because of the enzootic nature of nagana, it is possible that outbreaks may be underreported, and therefore not adequately tackled. As an example, all atlases being developed focus on cross-sectional surveys and, to a smaller extent, longitudinal surveys, but they do not specifically focus on possible existing data on reported outbreaks. This challenge is compounded by the fact that some outbreaks may fail to be confirmed with diagnostic tests and therefore fail to meet the requirement for data inclusion in the atlases. A strengthening of disease surveillance and an increase in the use of confirmatory tests could shed more light on the relative importance of nagana outbreaks over its enzootic occurrence.

  Timely reporting of outbreaks to WOAH, especially in non-endemic countries, would strengthen knowledge and surveillance.

• Speed of spatial spread during an outbreak

  As outbreaks of nagana mostly occur in areas where the disease is already endemic, it is not easy to study the possible spatial spread of the outbreak. The outbreak may manifest itself as a spike in disease incidence in each location that may or may not spread to other areas, depending on the factors that triggered it.

  Outbreaks of T. evansi were reported in previously uninfected areas, e.g., Canary Islands and France. Speed of spatial spread was not studied per se but spread requires movement of infected animals, camels for instance, and presence of vectors to sustain transmission. In France, the outbreak was limited to one farm and controlled by rapid diagnostic and treatment. A recent outbreak of surra in horses in Sumba Island (Indonesia) is showing a very quick and lethal spreading of the infection with acute and fatal disease (Wardhana, 2023).

  GAPS

  Specific studies following possible reported nagana outbreaks could shed light on the issues of the speed of spatial spread during an outbreak.

  NTTAT: better report of past outbreaks and modelling would be useful to anticipate potential outbreaks.

  HAT: The recently reported r-HAT outbreak in Ethiopia is cause for concern. Surveillance in livestock and wild animals, as well as modelling, may help to predict future area of outbreaks.

• Transboundary potential of the disease

  Trypanosomoses can and have spread across country boundaries, due to vector and animal movement. In the prevailing enzootic situations of nagana, when the disease is already present on both sides of a border, the transboundary component of disease circulation and spread is normally more difficult to monitor. By contrast, in those relatively rare but existing situations when an area free from the disease or at a very low prevalence has been created, the transboundary influx of parasites may become more apparent. One example is provided by the area in north-eastern Zimbabwe, bordering Mozambique, where tsetse flies have been eliminated and the sporadic positive cases of nagana have been ascribed to imported, transboundary infections (Shereni et al., 2021).

  Several transboundary/imported cases of T. evansi have also been reported, including in Europe where the disease is not endemic.

  HAT: There are known transboundary risks, e.g. for g-HAT between Uganda and South Sudan and between Chad and Central African Republic, with endemicity sometimes at different levels on either side of the border.

  GAPS

  A good understanding of the degree to which transboundary circulation of trypanosomes impacts the epidemiology of the disease would require more detailed studies. Transboundary circulation of trypanosomes is likely to contribute to the diversity of the species and strains of trypanosomes circulating in an area or a country and thereby contribute to making disease control more difficult. Transboundary partnerships are thus required to improve epidemiological knowledge and disease control.

  Beside transboundary concept, “inter-island disease” should also be considered, especially in multi-insular countries such as Indonesia and The Philippines.

• Route of Transmission

• Usual mode of transmission (introduction, means of spread)

  Introduction of trypanosomes in a herd of herbivores can be made by parasites inoculation by tsetse flies (cyclical transmission) or through the introduction of an infected host (further on expanded by tsetse flies or by mechanical vectors). Introduction and transmission by vampire bats may also occur for T. evansi. Carnivores can get the infection when feeding on infected prey or meat. Dourine may be introduced in a herd through mating of stallion and mare.

  Spreading depends on the parasite species:

  Parasites responsible for nagana are mainly cyclically transmitted through the bite of tsetse flies; however, T. vivax can be transmitted both by tsetse flies (cyclical) and by biting flies (non-cyclical). Mechanical transmission of trypanosomes can occur when hosts exhibit high parasitaemia in the presence of high density of biting flies such as Tabanidae, Stomoxyinae and Hippoboscidae. T. vivax strains that are regularly mechanically transmitted may lose their ability for cyclical transmission. Mechanical transmission of T. vivax may occur in Africa and is the main mode of transmission in Latin America and anywhere out of the tsetse belt.

  Mechanical transmission is the main mode of transmission of T. evansi both in Africa, Asia and Latin America. As other members of the Trypanozoon sub-genus, T. evansi has the ability to penetrate mucosae, therefore it can also be transmitted perorally to carnivores (penetration through mouth or oesophagus mucosae), and, in Latin America, to the vampire bats (Desmodus rotundus), which can act as host, reservoir and biological vector. It is thought that survival and therefore penetration of T. evansi through the mucosae of vampire bats can occur in the mouth, the oesophagus and even the stomach, as it is not acidic. As other mammals, once vampire bats are infected by T. evansi they develop peaks of parasitaemia during which parasites invade all body fluids including salivary glands; from there, vampire

  bats become infective and can act as biological vectors. Besides, due to mutual bites inside bats colony, vampire bats are considered as a true reservoir, able to maintain T. evansi inside the colony, even in the absence of a main host, such as horses and cattle. Vertical and venereal transmission of T. evansi are also suspected. Finally, although rare, transmission of T. evansi to human may result from butchering an infected bovine through transcutaneous penetration.

  Iatrogenic transmission of T. evansi and T. vivax may happen; epizootic outbreaks of T. vivax have been described in Bolivia and Brazil during vaccination campaigns (Silva et al., 1996). Even transmission through the use of the same procedure glove during rectal palpation was observed for T. vivax in cattle (Leal et al., 2025).

  T. equiperdum is venereally transmitted from stallion to mare, but vertical transmission from mare to foal is also possible.

  In HAT, trypanosomes are cyclically transmitted through the bite of the tsetse fly. g-HAT can be vertically transmitted, but this does not play a major role in the disease epidemiology.

  GAPS

  Role of mechanical transmission in tsetse-infested areas.

  The role of vertical transmission and silent reservoirs of infection needs examination.

  Mechanically transmitted trypanosomes: periodic dynamic of transmission by mechanical vectors should be better known in areas where tsetse have been recently eliminated.

  A better understanding of the connections between wild and domestic animals is also necessary.

  As concerns mechanical transmission, the exact role of each potential vector (stomoxyine flies, tabanids, other hematophagous flies such as Musca crassirostris) is to be determined. The exact role of Stomoxyinae in the transmission of T. evansi remains to be elucidated, especially regarding immediate and delayed transmission (crop regurgitation).

• Occasional mode of transmission

  For species other than those belonging to the Trypanozoon subgenus (for which they are usual ways of transmission), possibly:

  • Carnivores can be infected by ingesting meat or organs from infected animals (T. vivax, T. congolense), through the mucosa of the mouth in which bone splinters make wounds through which the parasites penetrate.

  • Animals can be infected sexually or vertically either by transplacental infection, or by mother to foal contamination. For humans, infection can be vertical but does not play a significant role in disease epidemiology, while the existence of sexual transmission remains controversial.

  • Latrogenic transmission during blood sampling, serial surgery, rectal palpation or vaccination with contaminated needles for all species.

  GAPS

  Mechanical transmission is possible, but its specific role for T. congolense and T. brucei is not known. Sexual transmission of T.b. gambiense has been claimed (Rocha et al., 2004) but remains controversial.

  Whether and for how long time the meat and other products from a Trypanozoon-infected livestock is potentially infective after a regular slaughtering process is not clearly defined.

  Can T. equiperdum be occasionally transmitted by hematophagous flies?

  How important is sexual transmission in the transmission of T. evansi?

  Could peroral transmission play any role in herbivore’s infection? (similar to trichinosis in horses, for example).

• Conditions that favour spread

  The spread of trypanosomoses is influenced by several factors. Vector and host density and distribution play critical roles. Increased contact between vectors and hosts at shared resources, such as watering points, can accelerate transmission. Host susceptibility to infection is enhanced by factors like malnutrition, lack of water, intercurrent diseases, and stress from transportation, partum, or surgery. Vector competence and fly population density also significantly impact disease transmission.

  Introduction of an infected animal into a previously unexposed population, in the presence of vectors, can initiate an outbreak.

  Stress-induced high parasitemia in an infected animal can transform it into a “super spreader,” rapidly disseminating the infection, particularly in areas with high densities of mechanical vectors.

  Low initial prevalence of infection can favor rapid outbreak dynamics, as commonly observed in outbreaks of mechanically transmitted trypanosomoses in Latin America and Asia.

  GAPS

  Nagana and HAT: Factors driving the population dynamics of tsetse populations, and their vector capacity are not fully understood.

  Surra: Factors driving the population dynamics of Tabanidae (or biting flies in general) populations are not fully understood.

  International regulations and quarantine rules need to be refined as well as proper diagnostic schedules.

  The impact of uncontrolled domestic and wild trans-frontier animal movements needs to be defined.

• Detection and Immune response to infection

• Mechanism of host response

  The immune response to nagana is a complex interplay between host defences and parasite evasion. Initial infection triggers innate immunity, involving neutrophils, macrophages, and complement, but the parasite is able to escape this first line of defence. The adaptive immune response, including antibody production and cellular immunity, is mounted, but parasites escape this through rapid antigenic variation, and adaptive immune response is often hindered by parasite-induced immunosuppression. Trypanosomes manipulate host responses by inducing regulatory T cells and altering cytokine profiles, which is mainly described in the mouse model. While some host species exhibit trypanotolerance, the mechanisms underlying this tolerance remain largely elusive. Understanding the complex

  interplay between the host immune system and the trypanosome is essential for developing effective control strategies.

  Nagana, Surra, Dourine, g-HAT and r-HAT: The immune response is generally unable to completely eliminate trypanosomes (although self-cure has been reported in a few cases – in particular in studies with cattle infected with T. vivax only). Animals can die or recover and become unapparent carriers. These unapparent infections can be reactivated if the animal is stressed.

  Nagana, Surra, Dourine and g-HAT: They induce a strong humoral response with high levels of IgM and IgG that are essential to mitigate the disease. However, due to the ability of trypanosomes to change their surface glycoproteins, the parasite population is able to escape host immune response.

  Nagana and Surra: Immune control is possible for T. vivax and T. evansi after some time and after trypanocidal treatments. Self-cures are observed. It could be linked with the limited polymorphism of mechanically transmitted parasites outside the tsetse belt. For surra in horses, although in most of the cases the issue of the infection is death, in some cases, surra appears to be chronic and to allow survival of horses, possibly in large horse populations such as in India.

  GAPS

  Elucidating the mechanisms of parasite immune evasion and the factors contributing to host resistance would allow novel targets for vaccine and drug development to be identified.

  Nagana and HAT: Specific features of immune response in the relevant hosts are poorly understood, especially the role of the different cell types like macrophages, granulocytes, dendritic cells and γδ T cells or NK cells or the contribution of pro- and anti-inflammatory cytokines, as well as their potential role in trypanotolerance. The functional effect of tsetse flies salivary antigens and microbiota in immune response of dermis is poorly known. It requires further investigation in the target host species.

  Surra and Dourine: Immunization with modified parasites could be attempted.

• Immunological basis of diagnosis

  Animals develop humoral immune response with IgM and further on with IgG, and these antibodies can be detected by different techniques, especially ELISA (Enzyme-Linked Immunosorbent Assay), although strong cross-reactions are observed amongst Salivarian trypanosomes. Because of the appearance of new variable antigen types in each new wave of parasitaemia, IgM are serially produced.

  Nagana: Several antibody detection techniques have been developed to detect trypanosome antibodies for the diagnosis of trypanosomosis, ELISA, immunofluorescence antibody test (IFAT) and complement fixation test (CFT), with variable sensitivity and specificity. Indirect ELISA provides generally good sensitivity and specificity (at the genus level). Attempts to develop rapid tests have been made for T. vivax and T. congolense (Boulangé et al, 2017), and one was commercialised (VerY Diag from CEVA) that is claimed to differentially detect antibodies directed towards both parasites though it is generally not widely available. However, due to the persistence of antibodies in the bloodstream for long periods after treatment or self-cure, specific IgG detection tests do not necessarily diagnose ongoing infection, thus limiting such tests to epidemiological surveys and control campaigns follow-up.

  g-HAT: Good antibodies level that can be detected by rapid tests (CATT, RDT), or by ELISA, IFAT and trypanolysis

  Surra: Semi-commercial rapid kits exist for T. evansi (CATT T. evansi)

  Dourine: As for nagana, several antibody detection techniques have been developed (CFT, ELISA, IFAT). Production of specific antigens is limited and relies on just a few laboratories.

  GAPS

  Need to carry out performance evaluation and validate serological tools to ensure they are fit for purpose and define target product profile.

  Need of efficient, affordable point of care tests. This should ideally be based on the detection of parasite antigens or molecular markers rather than specific antibodies but performance in low parasitemia context requires accurate evaluation.

  Need of standardized ELISA test that can be easily distributed for epidemiology and monitoring purposes r-HAT: No simple antibody or antigen detection tests (RDT format) are currently available.

• Main means of prevention, detection and control

• Sanitary measures

  Nagana and Surra: Protecting animals is challenging in endemic areas due to several factors. Preventing insect bites (including those from tsetse flies) is possible, but difficult, and the efficacy of trypanocide prophylaxis and treatments is often limited. Moreover, the unreliability of diagnostic tools means that all animals in endemic areas should be considered as potential carriers. In case of high level of chemoresistance or cross-resistance to trypanocides, culling may be the only option to control the spreading of such resistant strains.

  For international trading, according to the Sanitary code, WOAH:

  (i) Providing they have been collected /prepared under controlled conditions (see Sanitary code, WOAH), animal products (meat, milk,

  semen etc) are considered to be safe products regardless of the status of the exporting country, zone, or compartment.

  (ii) Importation of susceptible animals from non-infected country, zone or compartment is allowed if the animal is in good health and was kept all its life in non-infected country, zone, or compartment.

  Dourine: All equids with suspected or confirmed T. equiperdum infection, as well as serologically positive animals, are considered potential carriers, and, according to the WOAH Terrestrial Manual, should be culled (chapter 3.6.3. Dourine in horses; Trypanosoma equiperdum infection): “the only effective control is through the slaughter of infected animals”. However, while slaughter is the recommended control measure, individual countries have the discretion to implement this policy. Treatment for dourine is generally prohibited in most countries.

  The Complement Fixation Test (CFT) is currently the sole diagnostic method endorsed by the WOAH for certifying individual animal freedom from dourine infection prior to movement. The current CFT protocol described in the WOAH Terrestrial Manual requires improvement of titration procedures and validation of reagents. While alternative diagnostic tests, such as ELISA and Indirect Fluorescent Antibody (IFA), are available, they require further validation to meet WOAH standards “for pre-movement certification of individual animal health status."

  GAPS

  Exact conditions and duration of potential infectivity of trypanosomes in a carcass is not determined.

  Potential presence of African trypanosomes in semen has never been clearly confirmed.

  For dourine, the efficacy of a treatment to cure T. equiperdum has not been demonstrated yet.

  For dourine the validity of ELISA versus CFT needs to be demonstrated, as well as the need for species specific antigens (equiperdum/evansi).

• Prevention through breeding

  Nagana: Trypanotolerant cattle breeds are traditionally raised under low input systems in areas with high trypanosome pressure even in the absence of trypanocides and vector control. Trypanotolerant breeds include the N’Dama (long-horn taurine) and short-horn taurine like Lagune or Baoulé. These breeds have the ability to reduce the symptoms of infection, especially mitigate anaemia, contrary to susceptible breeds represented by Zebu cattle and European cattle. It should be noted that tolerance does not mean resistance as these animals may also be affected by the trypanosomoses, particularly if they are poorly fed.

  GAPS

  Though it is known that trypanotolerance has a genetic basis and some chromosomal regions were demonstrated to be linked to trypanotolerance or were selected in trypanotolerant cattle breeds, current knowledge and tools do not allow implementing genetic improvement programs. Further studies are needed to accurately identify genetic markers and translate the results into innovation.

  Tolerance to nagana, surra or dourine in some breeds or populations of species other than cattle has been poorly investigated.

• Diagnostic tools

  Whatever the disease, diagnostic tools are based on two pillars: direct parasite observation and indirect detection methods.

  Direct parasite observation: Only the direct visualization of the parasite through microscope can bring a diagnosis of certainty. Parasitological methods aim at detecting the parasite itself mainly in blood (but possibly in lymph, CSF, chancre fluid, genital fluids, etc) through microscopical examination of thick- or thin blood-stained smears (with a very low sensitivity, which requires parasitaemia to be above 105 parasites/mL). Concentration methods by centrifugation of blood in capillary tubes allow a more sensitive detection by microscopic examination of the interface buffy coat / plasma, where trypanosomes are concentrated (parasitaemia > 100-500 parasites/mL can be detected depending on the Trypanosoma species). Using fresh samples collected less than 4 hours ago, parasites can be isolated by injection to laboratory animals (mouse or rat, or other rodents

  depending on trypanosomes species/strains); sensitivity can reach 10-50 parasites/mL). For g-HAT in humans, due to the small number of parasites in blood, mini-anion exchange centrifugation columns are best suited to diagnose the infection. The column retains the blood cells while trypanosomes flow through. The column eluate is collected in a special collection tube, which is centrifuged to enrich the sample and trypanosomes are detected directly in the point of the collector tube. This increases the sensitivity up to 20-50 parasites/mL.

  Beside the observation of parasites, indirect techniques allow DNA, RNA and antibody detection.

  DNA detection through PCR is highly sensitive (can detect down to 1-10 trypanosomes/ml), and the level of specificity can be selected from sub-genus to species, sub-species and sometime types or clades. For HAT infection, SHERLOCK and various reverse transcriptase quantitative PCRs have been proposed (Sima et al., 2022; Van Reet et al., 2021), but they are not yet fully validated.

  Antibody detection methods are the best methods to evaluate the prevalence of infections in a population, or the success of a tsetse control campaign, however they cannot confirm active infection due to the persistence of antibodies in the blood several months after parasite elimination. Among various serological techniques, Enzyme-Linked Immunosorbent Assay (ELISA) has emerged as a reliable and widely used method. ELISA displays high sensitivity and specificity (at the genus level), rapid results, and suitability for large-scale screening. A variety of antigens, such as whole-cell lysates, soluble antigens, and recombinant proteins, have been employed in ELISA tests. The combination of ELISA with other diagnostic techniques, such as PCR, can provide a more comprehensive approach to AAT diagnosis.  

  GAPS

  All diseases: Sensitive rapid diagnostic tests (RDT) for detection of active infections and treatment decision are needed. Tests to distinguish within the brucei clade is needed. For animal, it concerns T. brucei /T. evansi / T. equiperdum. For human disease and the ability to detect animal reservoir: T. brucei brucei / T. brucei gambiense. Ideally, a sensitive RDT, or molecular tests.

  Even when combined, the commonly used diagnostic methods (parasite isolation, parasitological methods, serology, DNA detection) do not provide an absolute certain and definite answer, as false positive and/or false negative cases occur. This situation is intrinsically linked to the low sensitivity or low specificity (or both) of used method(s); also, low level(s) of parasitaemia below the detection threshold are often encountered in field

  conditions. In addition, parasite isolation, DNA or RNA detection and serological diagnostic tests require well equipped laboratories and skilled personnel. Cost is also a gap for access to universal AT diagnosis, which would in turn lead to improved surveillance and better management of disease outbreaks.

  As concerns T. evansi: is there an extravascular focus which does not stimulate the immune system; if so, how can we detect such infections?

• Vaccines

  No vaccines are available at the present time, and the prospects for a vaccine in the short-medium term are limited. Recent data (see below) has revived the prospect of vaccines in the long term, although this will require substantial further work to bring to fruition, if successful.

  The surface variable antigen coat of trypanosomes is a fundamental biological property that has impeded vaccine development. The system of antigenic variation is so elaborated (Morrison et al., 2009), that the responsible, immunodominant protein, the variable surface glycoprotein (VSG), is not a feasible target for vaccines – while genomic data suggest that the mechanisms of VSG variation may differ between the three lineages of African trypanosome (T. congolense, T. vivax and T. brucei s.l.) (Jackson et al., 2012; Silva Pereira et al. 2020). It is therefore highly unlikely that the VSG represents a viable vaccine prospect in any species. Several proteins that are conserved and surface expressed (e.g. invariable surface glycoprotein, ISGs), which in principle may represent attractive targets, have been tested with limited evidence of protection (e.g. Lança et al., 2011; Magez et al., 2021). This is likely to be for two reasons – first, the VSG coat forms a very dense monolayer that is thought to inhibit access of antibodies to conserved antigens/epitopes on the surface. Second, trypanosomes have evolved a mechanism whereby antibodies that are bound to the surface are engulfed and removed by the flagellar pocket, the cellular site for endocytosis in trypanosomes (Engstler et al., 2007). This means that induced antibody levels need to be high enough to overcome this mechanism to be effective. Therefore, the combination of properties of the VSG coat presents a substantial challenge to vaccine discovery in trypanosomes, which must be overcome irrespective of choice of antigen or delivery method/formulation.

  While the above studies have focused on parasite proteins that result in protection following immunisation, there have also been attempts to generate vaccine approaches that result in moderation of the pathology (so-called ‘anti-disease vaccines’), not necessarily protection against infection. Immunisation with parasite secreted/excreted cysteine proteinases were shown to confer an effect in terms of ameliorating the severity of clinical signs during an experimental infection with T. congolense – importantly this was shown in cattle – providing evidence that this approach is feasible (Authié et al., 2001). However, efficacy was insufficiently convincing, and it is unclear whether there would be a market with key stakeholders (e.g. farmers or vaccine producers) for such anti-disease vaccines for trypanosomes.

  Camel-derived ‘nanobodies’, which are single domain smaller proteins when compared to the larger antibody proteins expressed by most mammalian hosts, have aimed to overcome these challenges. It was hypothesised that these nanobodies might be able to access antigens or epitopes that are not accessible for conventional mammalian antibodies. This was trialled through delivery of antigen-specific nanobodies in mice, and although some protective effect was demonstrated in mice, it was not sufficient for a feasible vaccine or therapeutic approach (Baral et al., 2006). Recently, a study expressed all annotated surface expressed proteins in T. vivax, and immunised mice followed by infectious challenge. This resulted in the identification of one protein (Invariant Flagellum Antigen from T. vivax; IFX), that reproducibly gave sterile protection following immunisation (Autheman et al, 2021). Protection was shown to be conferred by antibody mediated complement lysis. The protein function is thought to be in tethering the flagellum to the antibody surface – this function is perhaps the cause of its vulnerability; in that it will be fixed in position and therefore not subject to the hydrodynamic removal of other surface-bound proteins. This latter study highlights biological properties that can potentially be the focus of vaccine investigations going forward (Romero-Ramirez et al.,2022). However, it should be noted that almost all this work is in mice and needs to be translated to the clinically relevant bovine model (and note that an initial study in goats with IFX did not result in protection).

  GAPS

  Is there a market for anti-disease vaccine approaches with relevant stakeholders (i.e. farmers, veterinarians, animal health pharmaceutical companies)?

  Until recently, the antigenic variability of the parasite was considered an insurmountable barrier to the development of an effective and mass field applicable vaccine. Research on specific and conserved immunogenic antigen(s) (see above) may help in progressing towards the development of a vaccine. However, almost all work has been done in the mouse model, and knowledge needs to be generated in the clinically relevant bovine or small ruminant models.

  Can the findings of protection induced by IFX for T. vivax be translated from mice to cattle?

  Can the antigenic properties of IFX be used to identify protective antigens in other trypanosome species?

  With better characterisation of pathogenic trypanosome species (T. congolense, T. vivax, T. evansi, T. equiperdum) genomes, proteomes and secretomes, can other candidate proteins that may confer protection be identified?

  Can advances in vaccine technology (e.g. DNA or RNA vaccination) result in improved protective responses for candidate antigens?

• Therapeutics

  Trypanocides are a major component of the control measures for nagana and surra.

  For use in cattle, trypanocidal drugs are limited to the three following compounds:

  1. Diminazene aceturate (Berenil®, Hoechst; Veriben®, CEVA and other various generic formulations) (DA). Dose for Trypanozoon infection is double (7mg/kg) than for T. vivax and T. congolense (3.5mg/kg). However, under controlled conditions, few T. vivax and T. congolense are still sensitive to 3.5mg/kg DA. Practically, 7mg/kg is the regular attack dose used by veterinarians for any Trypanosoma species.

  2. Homidium bromide (Ethidium®, Laprovet) and homidium chloride (Novidium®, Mérial). These chemicals being cancerogenic should be avoided because of serious risk for humans handling drugs or possibly ingesting drug residues.

  3. Isometamidium chloride (ISMC) (Samorin® /Trypamidium®, Mérial; Veridium®, CEVA) can be used both as a curative (0.5mg/kg) or preventive drug (1-2 mg/kg). To avoid or to destroy resistant strains to one or the other chemical; ISMC is generally used as a sanative pair, alternated with DA. Such sanative pair is generally recommended for transhumance periods in enzootic areas.

  For use in equines and camels, trypanocidal drugs are limited to the two following compounds:

  4. Melarsomine hydrochloride (MH) (Cymelarsan®) has been developed for the control of T. evansi in camels (0.25mg/kg). Its efficacy has been evaluated in horses, buffaloes, and cattle in which the dose should be increased at least to 0.5-1mg/kg. Results are not always consistent. Attempts to treat T. equiperdum infections with MH also give inconsistent results. Overall, MH should be used in first intention and as early as possible after infection.

  5. Quinapyramine salts (Q) can be used to treat certain trypanosome infections, but their effectiveness varies. While single salts may provide some relief, they often lead to relapses. Due to the risk of developing drug resistance, especially in cattle, their use should be limited to treating T. evansi infections in horses, camels, and dogs. However, a combination of sulfate and chloride Quinapyramine salts has shown significant promise. This combination effectively kills parasites both in the bloodstream and as they are released from other body tissues. This gradual elimination of parasites can occur over a 3–4-month period. To ensure a complete cure, it is recommended to administer two treatments, spaced three months apart.

  Drugs to treat HAT are not commercially available in the market but can be obtained through WHO. They are not allowed to be used in animals. Detailed guidelines are available in the “Guidelines for treatment of human African trypanosomiasis” published by WHO (https://www.who.int/publications/i/item/9789240096035, June 2024). Six components are available.

  1. Fexinidazole: recently developed and of low toxicity, fexinidazole is the first line treatment for g-HAT and r-HAT patients ≥ 6 years and ≥ 20 kg, except severe late-stage g-HAT. The 10 days oral treatment consists of 4 days loading dose (1800 mg daily, 1200 mg for children) and 6 days maintenance dose (1200mg daily, or 600mg for children). Tablets are to be taken with solid food. Theoretically, some HAT patients can take the drug ambulatorily (although under strict supervision), but in practice, most, if not all patients are hospitalized. Patients that are excluded from fexinidazole treatment need to undergo lumbar puncture and are treated with the older, disease stage and disease form dependent drugs, which show higher toxicity or are more complicate to administer than fexinidazole.

  2. Pentamidine (pentamidine isethionate): for g-HAT in hemo-lymphatic stage (mainly children and 2^nd line treatment). Treatment consists of intramuscular injections of 4 mg/kg once daily for 7 days.

  3. Suramin (suramin sodium powder): for r-HAT in hemo-lymphatic stage (mainly children). Five intravenous injections of 20 mg/kg (maximum 1 g) given weekly for 5 weeks. Treatment is started with a test dose of 4-5mg/kg, which is completed a few hours later.

  4. Nifurtimox Eflornithine Combination Treatment (NECT): For g-HAT in meningo-encephalitic stage, mainly children or g-HAT patients in severe late stage (>100 white blood cells/µl in CSF). Oral nifurtimox 15 mg/kg per day in three doses for 10 days combined with intravenous eflornithine (α-difluoromethylornithine or DFMO) 400 mg/kg per day in two 2-h infusions for 7 days. Hospitalisation is needed.

  5. Melarsoprol (Arsobal): mainly children with r-HAT. Slow intravenous injections of 2.2 mg/kg per day (maximum: 5 mL) once daily for 10 days. Melarsoprol is highly toxic, and reactive encephalopathy occurs in 5-18% of patients and can be fatal for 10-70% of them. Hospitalisation is needed.

  GAPS

  Nagana, surra and dourine: None of these drugs can ensure 100% efficacy, there is always potential for relapse of trypanosome infections.

  Serial treatments (3 boosts at 1-month interval), using both potential efficacy of the drug to kill parasites, and stimulating the immune system of the host should be evaluated as regular treatments for AT.

  Treatment failure is widely reported in AT. However, the scale of this problem, and what proportion of treatment failure is due to (i) poor drug quality (i.e. counterfeit or poor-quality drugs), (ii) inappropriate dosing (i.e. concentration or route of administration), and/or (iii) drug resistance, is poorly understood (Richards et al., 2021). All three of these issues undoubtedly occur, but improving our understanding (and any regional variation) will require application of resources, and the development of tools for both research and surveillance (e.g., markers for drug resistance, tools and infrastructure to enable drug testing) (FAO, 2022; Morrison et al., 2024). If we understand the causes of treatment failure, strategies can be designed and implemented to mitigate them and improve the quality of trypanosome control.

  New drugs are sorely lacking; apart from melarsomine, which is “only” 30 years old and only licensed for treatment of surra (Ungogo and de Koning, 2024), the main drugs used for animal trypanosomosis (diminazene aceturate, homidium bromide and isometamidium chloride) are all closely related and have been used for an extensive period of time (70-100 years). The combination of relatedness and long-term use means that resistance is a real risk, and is clearly occurring (Richards et al., 2021). Additionally, it was recommended some time ago that homidium bromide should not be used due to potential toxicity to humans (Sutcliffe et al., 2014). A potential new drug (benzoxaborole) is currently in development (Giordani et a.l, 2020) but requires further assessment to be translated into a marketable product.

  HAT: Acoziborole is a new, safe, oral single 960 mg dose drug for g-HAT with high efficacy (Betu Kumeso et al., 2023) that underwent a phase 2/3 clinical trial. Further clinical trials are ongoing for children and to document safety. The drug has not yet been approved.

• Biosecurity measures effective as a preventive measure

  To prevent the introduction of trypanosomosis into unaffected areas, it is crucial to control the movement of animals. This can be achieved through the implementation of a comprehensive diagnostic program. This program should involve the repeated use of a combination of diagnostic tests, including parasitological, molecular, and serological methods. Furthermore, the introduction of animals suspected of having trypanosomiasis should be strictly prohibited to safeguard unaffected areas. For further details regarding the prevention and control measures, refer to the “Border/trade/movement control (management) sufficient for control” section.

  Vector control helps preventing parasite transmission in endemic areas or during outbreaks.

  For dourine: all equids with suspected or confirmed T. equiperdum infection, as well as serologically positive animals, are considered potential carriers, and, according to the WOAH Terrestrial Manual, should be culled (chapter 3.6.3. Dourine in horses; Trypanosoma equiperdum infection): “the only effective control is through the slaughter of infected animals”. However, while slaughter is the recommended control measure, individual countries have the discretion to implement this policy. Treatment of dourine is generally prohibited in most countries.

  GAPS

  Lack of point of care tests

  Lack of 100% fully curative treatments.

• Border/trade/movement control sufficient for control

  Nagana: No specific rules laid down in the WOAH Animal Health Code although trypanosomosis due to T. congolense in bovines was included in the former List B of WOAH. See the recommendations of the WOAH Code chapter 8.19: Infection with Trypanosoma brucei, T. congolense, T. simiae and T. vivax (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2023/chapitre_tryp_brucei_congolense_simiae_vivax.pdf). First adopted in 2021.

  In T.b. rhodesiense areas, livestock surveillance should be performed and animals treated with trypanocides to prevent the spread of HAT.

  Surra: The implementation of the Animal Health Law included surra in the list of diseases regulated for entry into the EU for cattle, small ruminants, equids and other ungulates (https://eur-lex.europa.eu/EN/legal-content/summary/the-eu-animal-health-law.html). See the recommendations of the the WOAH Code chapter 8.23: Infection with Trypanosoma evansi (Surra) (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_tryp_evansi.htm). First adopted in 2024.

  Nagana: No specific rules laid down in the WOAH Animal Health Code although trypanosomosis due to T. congolense in bovines was included in the former List B of WOAH. See the recommendations of the WOAH Code chapter 8.19: Infection with Trypanosoma brucei, T. congolense, T. simiae and T. vivax (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2023/chapitre_tryp_brucei_congolense_simiae_vivax.pdf). First adopted in 2021.

  In T.b. rhodesiense areas, livestock surveillance should be performed and animals treated with trypanocides to prevent the spread of HAT.

  Surra: The implementation of the Animal Health Law included surra in the list of diseases regulated for entry into the EU for cattle, small ruminants, equids and other ungulates (https://eur-lex.europa.eu/EN/legal-content/summary/the-eu-animal-health-law.html). See the recommendations of the the WOAH Code chapter 8.23: Infection with Trypanosoma evansi (Surra) (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_tryp_evansi.htm). First adopted in 2024.

  Dourine: The implementation of the Animal Health Law included dourine in the list of diseases regulated for entry into the EU for equids (The implementation of the Animal Health Law included surra in the list of diseases regulated for entry into the EU for pigs, cattle, small ruminants and equids (https://eur-lex.europa.eu/EN/legal-content/summary/the-eu-animal-health-law.html). According to WHOA recommendations, a negative dourine CFT result is required before an equine can travel internationally. However, specific agreements can be drawn up between different countries to implement specific rules. See: the recommendations of the WOAH Code Chapter 12.3 : Dourine (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2023/chapitre_dourine.pdf). First adopted in 1968.

  GAPS

  Nagana: No regulation is available for most of the trypanosome infections as far as border/trade/movement control of domestic or wild animals are concerned. Climate changes and changes in the agro-ecological conditions, together with international or regional animal movements could contribute to the geographical expansion of trypanosome species to new biomes and livestock-agricultural areas.

• Prevention tools

  Nagana and human infective species: Prophylactic use of trypanocidal drugs to prevent the disease in animals protects people as well as animals from illness since in some r-HAT areas domestic cattle are the main reservoir of the human infective T.b. rhodesiense.

  Tsetse control, as it interrupts transmission, is one of the most efficient preventive strategies, both for human and animal trypanosomoses.

  For surra, physical or spatial separation of host species acting as potential reservoirs, such as bovines and buffaloes, on one side, and more susceptible species such as horses, on the other side, is a necessary and efficient compartmentalization.

  (See also Sections “Main means of prevention, detection and control. - Vaccines and –Therapeutics”.

  Surra: Regular use of trypanocidal drugs on the animal.

  Surra and dourine: compliance with rules for animal movement and quarantine.

• Surveillance

  Nagana: The antibody detection tests are useful for large-scale surveys to determine the distribution of tsetse-transmitted trypanosomosis and to follow-up prevalence. Sample collection and storage is made easy using filter papers (Wen et al., 2016).

  HAT: In zones with high g-HAT prevalence, active screening of the entire population at risk is performed by specialized mobile teams. Elsewhere, passive screening is carried out on HAT suspects who present themselves with symptoms at fixed health infrastructures. For g-HAT initial screening with antibody detection tests is, for seropositives, followed by parasitology, or if unavailable, by remote testing in specialized laboratories. Suspects for r-HAT are examined directly for presence of trypanosomes.

  Trapping of insects using reference traps such as Vavoua and Nzi traps enables the determination of vector pressure, for both mechanical and cyclical vectors. For tsetse flies, dissection and parasitological examination for the detection of trypanosomes can provide further insights.

  Investigation in wildlife is sometimes requested to evaluate a risk of spill over. Biting flies may act as flying syringes.

  Sentinel animals such as dogs, are sometime useful to detect the circulation of some parasites such as T. evansi.

  T. equiperdum: Regular clinical control of horses and donkeys. CFT testing of all equines for export.

  GAPS

  For nagana, there is a lack of large-scale surveys that would allow to have more accurate information on the geographical distribution and prevalence of the disease.

  Dourine: Need for standardisation and validation of CFT test. Alternative tests exist but need validation to replace CFT (est. 1915).

• Past experiences on success (and failures) of prevention, control, eradication in regions outside Europe

  Tsetse flies/Nagana: elimination of tsetse flies may lead to elimination of trypanosomes and trypanosomosis, providing mechanical transmission does not relay cyclical transmission. Although elimination is not possible in the majority of the enzootic areas, for several reasons including cost, sustained reductions in tsetse populations greatly reduce transmission, in combination with the use of trypanocides. Integrated control based on a combination of cattle treatment and vector control has been more frequent. At the scale of the farmer, trypanocide treatment remains the most widely used tool.

  The various methods are listed in section above: “Main means of prevention, detection and control – mechanical and biological control”.

  By definition, tsetse-transmitted AT can be eliminated should the tsetse vector be eliminated. Success stories of permanent elimination of tsetse flies are few but do exist. Permanent deliberate elimination of tsetse over large areas has been achieved in Nigeria and Zimbabwe in the past, more recently in the Okavango Delta in Botswana (Kgori et al, 2006), and in the smaller areas of Unguja Island, Zanzibar, Tanzania and the Niayes in Senegal, using various strategies and tools (Vreysen et al., 2000; Seck et al., 2024). But there have been other experiences which, although not having fully eliminated the tsetse flies, have managed to decrease the prevalence of AAT: examples with published results include the Pan African Tsetse and Trypansomiasis Eradication Campaign (PATTEC)- African Development Bank funded programs in Ghana and Burkina Faso thanks to existing monitoring programmes and analyses (e.g. Adam et al., 2017), or more recently and still on-going, national activities conducted in many countries under the PCP umbrella, targeting continuous control of AAT with targeted objectives (Diall et al., 2017).

  Regarding HAT, the use of Tiny Targets (TT) in an isolated focus of g-HAT in the Mandoul, in Chad, combined with diagnostic and treatment of the patients, has led to elimination of transmission (Rock et al., 2022).

  Historically, residual insecticides were applied from knapsack sprayers by teams on the ground or by helicopters. In recent years aerial spraying with non-residual insecticides using SAT (the sequential aerosol technique) was undertaken in Botswana and SIT (the sterile insect technique) was used in Burkina Faso, Tanzania, and Senegal.

  Tsetse control by applying insecticide to cattle (insecticide-treated cattle or ITC) has been shown to greatly reduce the numbers of tsetse in an area which in turn means fewer cattle will be bitten. The insecticides used are usually the ones that also affect ticks, thus having the added benefit of reducing damage from ticks and the incidence of tick-borne diseases. The use of insecticide-impregnated fencing or netting is effective in protecting stock from bites. These low-cost methods can be applied on a small or a large scale and can be afforded by livestock keepers in Africa – they do however rely on cattle being present and evenly distributed, which is not the case in many areas.

  Though theoretically possible, sustainability of tsetse and trypanosomosis control is confronted with several issues like long term commitment of stakeholders, sustainable funding, policy support, human resources and “biological” issues like potential reinvasion of controlled areas by surrounding tsetse flies.

  Targets and traps are also highly effective in reducing or eliminating tsetse populations and do not rely on the presence of cattle. They are used in some programmes to control g-HAT (recently in Guinea, Uganda, Côte d’Ivoire, Chad, and DRC, see Ndung’u et al., 2020), as well as nagana. Some countries have achieved elimination as a public health problem for g-HAT thanks to combined medical and vector interventions.

  Surra and Nagana caused by mechanically transmitted T. vivax: due to complex populations dynamic of biting flies, the difficulty to control these vectors, and the presence of animal reservoirs for T. evansi and T. vivax, these diseases are difficult to control in endemic areas. Surra has been successfully eliminated only in isolated areas after initial outbreaks, like in America (1906) and Australia (1907) (Hoare, 1972), where the infection was detected very early, during quarantine, and the animals were killed, and also, more recently, in Canary Islands and France where the animals were either killed or treated and followed up for more than 1 year (Desquesnes et al, 2008). However, in the history, once they were durably established, there is no report of eradication of T. evansi, or eradication of T. vivax in areas of mechanical transmission.

  As concerns mechanical vector control, attempts were made to control Tabanids using insecticide (pyrethroids) spray on cattle; observations have shown efficient control of the flies when treating the cattle every 10 days, which is very costly and environmentally unbearable.

  Dourine: Eradication of T. equiperdum is possible by slaughter of all affected, suspected, and serological positive animals (USA, Canada, several European countries). Dourine was eradicated in EU after WWII through treatment, castration, and slaughtering. Since then, some outbreaks were only identified in Italy in mid-1970s, 1980s, 1996 and 2012 (Calistri et al, 2013). These outbreaks have been eradicated by implementing surveillance plans involving euthanasia and castration measures for infected animals.

  g-HAT: At a global level, g-HAT has been eliminated as a public health problem (Franco et al., 2024). The WHO road map for neglected tropical diseases targets elimination of transmission of g-HAT (zero cases detected) and elimination of r-HAT as a public health problem by 2030 (https://www.who.int/publications/i/item/9789240010352 ).

  GAPS

  Nagana: Tailored strategies will be needed to achieve trypanosomosis elimination or control in the most efficient and sustainable way. Scales of control operations vary a lot, from the farmer’s scale using trypanocides to treat his animals, to international control programmes needing multi-years programming and possibly multi-country involvement. Both disease-linked data and economic data should be obtained to identify locally efficient strategies, inform policy makers and

  stakeholders and plan control strategies. A tool to objectively assess spatial and time coverage and the impact of vector control in intervention zones would be helpful.

  g-HAT: Not all g-HAT endemic areas will need vector control, but it can be cost-efficient in certain settings and in active foci it remains the only preventive mean.

  Surra: In a context where the presence of hematophagous flies cannot be regulated, it is necessary to implement strict measures earlier on to limit the risk of an epidemic spreading to other domestic animals or spreading into wildlife and becoming uncontrollable. A strict analysis of the risk of the disease spreading must therefore be carried out to determine the best strategy for limiting the risks of disease expansion (treatment, euthanasia, etc.), depending on the context (Diall et al, 2022).

  Dourine: Given that transmission is predominantly sexual, the risk of uncontrolled spread seems lower. However, the difficulty of definitively concluding that an equine is suffering from dourine and not surra means that we need to analyse the risks of spreading of the disease very precisely before implementing management measures.

  Outbreak control by treatment: in a context where i) treatment failures have already been described for nagana, surra and dourine (parasite relapse or antiparasitic resistance) and ii) animals need to be monitored for several months, or several years, to ensure that the treatment has eliminated parasites from the whole animal, it is necessary to consider the balance between animal welfare benefits and the risk of spreading the disease, to determine whether the use of treatment versus slaughter to stop an outbreak of AT in the EU, or another free area, would be the most appropriate strategy.

  Concurrently, for nagana, dourine and surra, follow-up of different outbreaks, based on treatments, prevention and accurate monitoring and open-data policy do allow capitalizing on outbreaks and help establish guidelines regarding diagnosis, rules and regulations, quarantine, treatment, prevention, follow-up of control measures and their monitoring.

• Costs of above measures

  Note 1: Note 1: all costs are given in 2022 USD unless specified otherwise to aid comparison. Historical

  costs are therefore converted from original literature values using inflation rates.

  Note 2: We cover two different types of costs. Financial costs are those directly paid out by programmes (expenditure) and therefore for HAT do not include donated items such as drugs or Tiny Targets. Full (societal) economic costs include the cost paid by donors as well as programmes. Furthermore, items including a proportion of health care workers’ time spent on HAT and general fixed health facility upkeep will be included in economic costs even if they are paid for by a country’s general ministry of health budget rather than the HAT-specific budget.  Similarly, for vector control and other interventions targeting AT, economic costs include an appropriate proportion of the time of those contributing to the work, and already employed in the veterinary or entomological services, as a well as a share of these services’ overheads.

  Nagana: A wide range of trypanocides of varying quality and composition can be purchased in Africa (see also section “Main means of prevention, detection and control-Therapeutics”). The urban retail prices for a dose for a 250 kg bovine for curative drugs range from USD 0.5 to USD 5. The price of a dose of prophylactic isometamidium chloride is around USD 8 – 10 and is usually expected to have a prophylactic effect lasting about 3 months. In rural areas these costs can easily be double or more. The cost of administration ranges from the cost of needles and syringes, where livestock keepers inject their animals themselves to over USD 20 per animal. Typical fees are in the USD 2.50 - 5 bracket. Large scale curative treatment as part of a research programme cost USD 3.50 per animal (Muhanguzi et al., 2015).

  Surra and Nagana mechanically transmitted: similarly, a large range of varying quality of trypanocide drugs is circulating in Latin America and Asia; with cost of treatment quite close, for example: from about 10 USD for diminazene aceturate treatment, to 24 USD for isometamidium treatment of a horse in 1997 (Seidl et al., 1998).

  Cyclical vector control: The control of tsetse flies can be aimed at reducing the incidence of AAT and HAT or both. Even when adjusted for inflation and price differences between countries, the costs of tackling tsetse vary greatly (Shaw, 2018, Shaw et al., 2017). Costs differ depending on the method and whether the objective is to maintain tsetse control over several years or to create tsetse free zones and protect them from reinvasion (further details can be found in the following sections: “Mechanical and biological control”; “Past experiences on success (and failures) of prevention, control, eradication in regions outside Europe” and “Vector control Availability. Cost”). Even within a given control method and goal, the cost varies according to tsetse species, ecology, animal and human populations, accessibility, operational scale and organisation. Quoted costs often only refer to a key component (targets, traps, insecticide, flying time) and do not include the full cost of organisation and deployment (Shaw, 2018). Costs are usually quoted for various measures (area treated, area protected, human population protected (in the case of HAT) and these can give quite different results, (Snijders et al., 2024; Courtin et al., 2022; Shaw, 2018). The costs cited below refer to the area protected.

  Ongoing suppression of tsetse numbers to very low levels, using insecticide-treated cattle is likely to cost between USD 40 and USD 100 for treating 10 cattle at monthly intervals (Shaw, 2018; Muhanguzi et al., 2015). If pyrethroid insecticides are used this would also help control ticks (although more frequent applications might be needed) and other biting flies. Application can be using pour-ons, spraying, footbaths or dipping. The use of insecticide-treated targets or screens and traps, with or without insecticide has been discussed (Section “Past experiences on success (and failures) of prevention, control, eradication in regions outside Europe). Costs per km² for insecticide treated traps and standardised targets or screens are similar, in the USD 200 – 250 range. Tiny Targets have been deployed in a few HAT foci. In areas where rivers and marshes need treating, cost per km² protected are in the USD 70 – 100 range, however, in a higher rainfall forest area where the whole area needed treating, costs rose to USD 475 per km² (Courtin et al., 2022, Snijders et al., 2024). Tsetse control by spraying from fixed wing aircraft using the sequential aerosol technique (SAT) usually involves five spaced cycles of spraying and is estimated to cost USD 400 – 1000 per km² protected. SAT would be expected to reduce tsetse numbers to nil in the treated area, although flies would gradually reinvade from neighbouring areas unless another tsetse control technique is deployed full time to create barriers (Shaw et al., 2017, Shaw 2018).

  Costs for local elimination are even more difficult to estimate, and the issues involved and sensitivity to different parameters are discussed in (Shaw et al., 2017). In all cases, larger scale interventions are more cost-effective, and a key issue is reinvasion by tsetse flies, as barriers cease to be maintained or are not fully effective.

  In recent years, the provision by the manufacturers of some key components (pharmaceuticals to treat HAT, Tiny Targets) at zero cost to recipients in Africa, means that there is a divergence between this low or zero financial cost for those items and the full societal economic cost of the resources used in their manufacture.

  HAT: All drugs are available free of charge through WHO. Additional costs that may be charged to the patient may relate to hospital admission charges and costs of additional analyses, medical equipment, consumables, treatment of concomitant diseases etc.

  Treatment costs vary greatly according to which drug regime is used, the duration of hospital stay (if needed) and if fexinidazole is not used, whether patients are diagnosed in the first or second stage of the disease (when the central nervous system has become involved); economic costs for g-HAT for drugs (including drug donations) and possible hospital stay are estimated to range between USD 94 – 564 per patient (Antillon et al., MedRxiv, 2024 and 2022). This does not include out-of-pocket payments made by patients for transport, food and accommodation for accompanying family members which have been estimated to exceed USD 188 in the case of nifurtimox-eflornithine combination therapy (NECT) and can impoverish patients and their families (Sutherland and Tediosi, 2019). Fexinidazole can, in particular cases, avoid the need for hospital stays.

  Treatment for r-HAT has a low-end estimate at around USD 140 or 180 per person if suramin or melarsoprol are used for treatment (requiring inpatient stays) (Matemba et al., 2010). The switch to fexinidazole as the first-line recommended treatment in most cases is expected to reduce the treatment cost to around USD 94 per person (the same as g-HAT treated with fexinidazole).

  Average cost of screening for g-HAT to the health system has been estimated at USD 6.70/person screened and USD 4,464/case diagnosed and treated (based on the old treatment schedules with pentamidine for first stage and NECT for second stage g-HAT) (Snijders et al, 2021). Health economic evaluation of strategies to eliminate g-HAT, suggest that addition of vector control to medical interventions can be cost-effective (Antillon et al., 2023 and 2022).

  Mechanical vector control: as previously indicated, spraying cattle every 10 days is very costly (twice as expensive as a regular tick control) and environmentally unjustifiable. For this reason, there are no recent data on cost of such biting-flies control. New control methods using traps have not yet been validated and their cost efficiency is therefore not estimated.

  GAPS

  Nagana: It is important that cost analyses include the full costs of interventions. Where possible information on what livestock keepers pay for trypanocides, insecticides and their overhead costs are needed. No simple, cheap, and ready field applicable diagnostic test is currently available (see Section “Main means of prevention, detection and control-Diagnostic tools”), particularly for mass screening. Even when done on a large scale, sampling cattle for AAT is expensive, around USD 1 – 2 per animal sampled (Muhanguzi et al., 2015).

  HAT: Analyses are needed to understand how total treatments and programme costs will change should screen-and-treat for g-HAT become available with acoziborole.

  Surra and dourine: major gaps in cost assessment. Cost of control of the outbreaks in Spain and France have not been estimated but should include systematic and regular sampling, complete set of diagnosis (parasitological, molecular and serological), iterative treatments and long-term follow-up (1 year at least after the last positive test), which would certainly go above a mean of 500 USD per head.

• Vector control

  Tsetse control is central to manage cyclically transmitted trypanosomosis because without tsetse, there is no longer cyclical transmission of both animal and human trypanosomes. It should therefore be viewed as a main prevention strategy in the tsetse belt in Africa. Two strategies are used, using different tools: local eradication when applicable, or continuous control.

  Historically, removing the natural habitat of tsetse flies (“bush clearing”, and/or killing the animals on which tsetse feed) has had important impacts in controlling the vector and the disease. Insecticides have also been used as an efficient way to control tsetse flies, either directly sprayed on the vegetation that constitutes tsetse habitat or sprayed using aerial means.

  Attractive toxic systems, being either immobile (traps or their simplified derivatives “targets” or “screens”, based on visual and olfactive attraction applied with or without insecticides), or mobile (insecticide impregnated treated cattle), are efficient and preferred to limit the spread of insecticides in the environment. These tools have been used successfully for long for controlling tsetse flies, but little data are available for the control of other biting flies acting as mechanical vectors. Insecticide treated cattle (ITC) used as different formulations that are effective for tick control can also be used for tsetse, but efficiency has not been tested for all tsetse species, and their efficacy is limited to areas with high density of cattle.

  Biological control of tsetse flies was successfully implemented, and tsetse eradicated using the sterile insect technique (SIT) combined with other control tools in two areas: Zanzibar (Vreysen et al., 2000), and more recently the Niayes in Senegal (Seck et al., 2024). So far, no other biological control method proved to be efficient on the field although a number are being studied, including the promising tsetse microbiome.

  Mechanical control of vectors can be implemented in closed spaces using mosquito nets to prevent any contact between hosts and vectors, however Tsetse control is central to manage cyclically transmitted trypanosomosis because without tsetse, there is no longer cyclical transmission of both animal and human trypanosomes. It should therefore be viewed as a main prevention strategy in the tsetse belt in Africa. Two strategies are used, using different tools: local eradication when applicable, or continuous control.

  Historically, removing the natural habitat of tsetse flies (“bush clearing”, and/or killing the animals on which tsetse feed) has had important impacts in controlling the vector and the disease. Insecticides have also been used as an efficient way to control tsetse flies, either directly sprayed on the vegetation that constitutes tsetse habitat or sprayed using aerial means.

  Attractive toxic systems, being either immobile (traps or their simplified derivatives “targets” or “screens”, based on visual and olfactive attraction applied with or without insecticides), or mobile (insecticide impregnated treated cattle), are efficient and preferred to limit the spread of insecticides in the environment. These tools have been used successfully for long for controlling tsetse flies, but little data are available for the control of other biting flies acting as mechanical vectors. Insecticide treated cattle (ITC) used as different formulations that are effective for tick control can also be used for tsetse, but efficiency has not been tested for all tsetse species, and their efficacy is limited to areas with high density of cattle.

  Biological control of tsetse flies was successfully implemented, and tsetse eradicated using the sterile insect technique (SIT) combined with other control tools in two areas: Zanzibar (Vreysen et al., 2000), and more recently the Niayes in Senegal (Seck et al., 2024). So far, no other biological control method proved to be efficient on the field although a number are being studied, including the promising tsetse microbiome.

  Mechanical control of vectors can be implemented in closed spaces using mosquito nets to prevent any contact between hosts and vectors, however maintenance of animals under fly proof conditions is binding.

  Vector control has also proven as a valuable tool, which is complementary to medical interventions for the control and elimination of HAT, including for the g-HAT form.

  Chemical control of mechanical vectors with insecticides has been attempted, but, due to the low remanence of topic insecticides (5-10 days on average), it requires a high treatment frequency which is not compatible with economic and environmental constraints and concern. However, in occasional outbreaks, their use can be made to prevent spreading by mechanical transmission. Control of Stomoxys spp. through the use of insecticide-impregnated polyethylene screens (FlyScreens) was demonstrated in Thailand, but its efficacy was denied in France, in links with the presence of Kdr resistance alleles in local flies (Olafson et al, 2019). GAPS Efficiency of traps on mechanical vectors control should be demonstrated.

  Preliminary research on the biological control of tsetse through the control of their endosymbionts should be pursued.

  Attempts to demonstrate the efficacy of the biological control of Stomoxys through SIT should be made.

  Although tsetse repellents have been published, their use and impact remain to be demonstrated.

  Volatile organic compounds emitted by tsetse larvae may play a role in the aggregation of tsetse flies towards larviposition sites, but this needs further investigations (Gimonneau et al., 2024).

  Regarding other potential biological control tools, such as predatory or parasitic agents used for other fly species (e.g., Spalangia, Macrocheles), their efficacy against tsetse flies should be thoroughly evaluated.

  Priority should be given in the future to the development of environment-friendly tsetse control tools, using no insecticides.

  Given that many tsetse species (in particular those of the morsitans group) are associated with the presence of wildlife, using tsetse as bioindicators of wildlife presence could be interesting.

• Disease information from the WOAH

• Disease notifiable to the WOAH

  Three animal trypanosomoses are listed by the WOAH under the following names:

  - Trypanosomosis (tsetse-transmitted), corresponding to nagana.

  - Surra (T. evansi). Case definition: https://www.woah.org/app/uploads/2021/06/cd-t-evansisurra-20210205-final.pdf

  - Dourine. Case definition. https://www.woah.org/app/uploads/2021/06/cd-t-equiperdumdourine-20210205-final.pdf

  GAPS

  Given that: (i) the designations T. evansi, T. equiperdum and T. brucei (that all belong to Trypanozoon subgenus) correspond to polyphyletic groups that are closely related genetically; (ii) individual cases of trypanosomosis, surra and dourine are sometime and in some hosts impossible to differentiate clinically; (iii) the laboratory differential diagnosis of these infections remains complex; a group of experts has proposed that infections of equids by parasites belonging to the Trypanozoon subgenus should be gather under a specific chapter of the Terrestrial Code. This proposal was not adopted by WOAH, possibly to avoid creating a specific measure for horses that could not apply to other hosts of trypanosomes, so to date, the problems of distinguishing between animal trypanosomoses affecting Equidae persist. Other experts giving priority to the disease (clinical signs / susceptible hosts) and their epidemiology (vectors, mode of transmission, susceptible hosts), versus the phylogeny of the pathogenic agents, support the conventional concept of the three diseases due to trypanosomes. Indeed, even if diagnosis is confusing, once it is established, measures of control are radically different, for example from dourine to surra, or from nagana to surra. Still there is no common and easily usable algorithm for distinction of the three diseases. A gap that hopefully molecular tests could solve in a near future.

  The case definition of nagana (trypanosomosis-tsetse transmitted) is not available.

• WOAH disease card available

  Disease cards are currently available for the three animal trypanosomoses:

  - Dourine (infection with Trypanosoma equiperdum) (https://www.woah.org/app/uploads/2021/03/dourine.pdf). Last updated December 2020.

  - Trypanosoma evansi infections (Surra). (https://www.woah.org/app/uploads/2021/03/trypano-evansi.pdf). Last updated April 2013

  - Animal trypanosomoses (tsetse-transmitted). (https://www.woah.org/app/uploads/2021/03/trypano-tsetse-1.pdf). Last updated March 2021.

  GAPS

  Regular updating of WOAH documents related to nagana, surra and dourine is necessary to maintain consistency between Terrestrial Animal Health Code, Terrestrial Animal Health Manual, case definitions and disease cards.

• WOAH Terrestrial Animal Health Code

  WOAH Terrestrial Animal Health Code chapters are currently available for three animal trypanosomoses:

  - Code Chapter 12.3 : Dourine (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2023/chapitre_dourine.pdf). First adopted in 1968

  - Code chapter 8.19: Infection with Trypanosoma brucei, T. congolense, T. simiae and T. vivax (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2023/chapitre_tryp_brucei_congolense_simiae_vivax.pdf). First adopted in 2021.

  - Code chapter 8.23: Infection with Trypanosoma evansi (Surra) (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_tryp_evansi.htm). First adopted in 2024.

  GAPS

  Not yet available

• WOAH Terrestrial Manual

  WOAH Terrestrial Animal Health Manual chapters are currently available for the three animal trypanosomoses:

  • Nagana: infections with salivarian trypanosomes (excluding Trypanosoma evansi and T. equiperdum). Chapter 3.4.14: https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.04.14_NAGANA.pdf. Version adopted in May 2021.
  • Surra in all species (Trypanosoma evansi infection). Chapter 3.1.21: (https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.01.21_SURRA_TRYPANO.pdf) Last updated 2018.
  • Dourine in horses (Trypanosoma equiperdum infection). Chapter 3.6.3: https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.06.03_DOURINE.pdf. Last updated 2021.

  GAPS

  Given the current geographical distribution of AT, the difficulty to collect, store, share and send a large number of samples to laboratories expert in diagnostics, and in compliance with international regulations related to biosecurity, biosafety and access and benefit sharing, it is currently difficult to obtain a sufficient number of positive field samples to validate test methods and to perform inter-laboratory assays, so the possibilities for updating the diagnostic methods recommended in the WOAH Terrestrial Animal Health Manual chapters for international movements of animals remain limited.

• Socio-economic impact

• Zoonosis: impact on affected individuals and/or aggregated DALY figures

  HAT:

  Untreated, the disease is always fatal in humans and devastating epidemics have occurred over the last century, leading to depopulation of whole settlements. The disease tends to affect the active adult population and sick individuals need a great deal of care. Many are not diagnosed as HAT patients and die as a result. The labour burden on affected households is thus considerable. The financial burden per treated patient is also considerable, variously estimated at the equivalent to 2 to 10 months of an average rural wage. The average DALYs per untreated patient have been estimated at 24 for T. b. rhodesiense and between 27 and 33 for T. b. gambiense. Global annual DALY estimates range from 1.5 – 2 million; this figure understates the impact of the disease, as it is highly focalised so that very heavy burdens are imposed in affected communities. For example, one study showed that, comparing HAT to malaria, there were 133 times as many cases of malaria reported, but these only caused 3 times as high a DALY burden (20).

  With respect to geographic impact, Gambian and Rhodesian forms of the disease occur only in Africa south of the Sahara, where poverty, social instability, insecurity and weak health systems are widespread; The disease is rural, occurring mostly in remote, difficult to access areas, distributed over wide areas of land and affecting a population with poor or no access to health services. As stated in Section “Geographical distribution and spread - speed of spatial spread during an outbreak”, the geographical foci of the two forms are well known and delimited and relatively constant. The high rate of under-reporting and the fact that most diagnosed patients are initially treated for other illnesses, often several times, before being recognised as suffering from HAT reflects both the inherent difficulty in diagnosing the disease and the limitations of under-resourced health services (14).

  GAPS

  HAT:

  Under-reporting is the major unknown. A methodology has been developed for estimating this for T. b. rhodesiense. For T. b. gambiense, the effectiveness of surveillance gives some indications, but more work on this is needed. Associated with its GIS work, WHO is developing tools for the early detection of outbreaks.

  More studies on disease burden need to be added to the handful of studies which have estimated DALYs and financial and labour burden to affected households.

  Critical and quantitative analyses of socio-economic costs and benefits of control are scant.

• Zoonosis: cost of treatment and control of the disease in humans

  HAT: Treatment normally ranges from USD 94 – 564 per person for g-HAT. A much bigger cost than treatment itself is the active and passive screening programme which is needed to confirm cases before treatment and has been calculated to make up almost 99% of costs now that cases are becoming increasingly uncommon (Antillon et al., MedRxiv 2024 and 2022).

  See section “Main means of prevention, detection and control-Therapeutics-Cost of above measures” for more details.

  GAPS

  Analyses are needed to understand how total treatments and programme costs will change should screen-and-treat for g-HAT become available with acoziborole.

• Direct impact (a) on production

  Estimating the effects of disease on livestock on farms rather than in experimental settings is always challenging, especially in the extensive and semi-extensive production systems which predominate in Africa. In such systems animal productivity is not routinely monitored. A number of rigorous studies have investigated the impact of tsetse-transmitted trypanosomosis on a range of cattle productivity parameters, either comparing infected/non-infected animals or comparing population values from high/low trypanosomosis or high/low tsetse challenge situations. Earlier studies have been summarised in Swallow, 2000 and Shaw, 2004.

  Nagana: Milk yields are between 10% and 25% lower. Annual calving rates are generally reduced by between 5 and 10 percentage points due to the effect of the disease. Abortions and stillbirths are linked to AT and where estimated, tsetse control reduces these by half to two thirds. Adult mortality rates in cattle are typically 3 – 6 percentage points higher, with herd death rates for adult cattle often mortality, abortion, productivity losses and treatment costs, was estimated about 4% of the total cow value on affected ranches; in the absence of treatment, the estimated losses would have exceeded 17% of total cow value (Seidl et al., 1998).

  For surra in the Philippines, direct losses due to mortality of livestock was estimated to be USD 7.9 million at 1997 prices over a period of nine years, excluding losses attributable to decreased meat and milk yield, poor reproductive performance, cost of labor and medication (Manuel, 1998).

  For all of the numbers cited above, there are wide variations. There are important interactions between diseases and multiple health and management problems may be implicated in reducing livestock productivity. In eastern Africa, where the tick-borne disease East Coast fever is present and accounts for a high proportion of cattle deaths, trypanosomosis may play an important role in increasing death rates. A study has shown that co-infection with trypanosomes can increase the risk of death due to East Coast fever in calves by a factor of 6 (Thumbi et al., 2014).

  While the individual effects cited above may not seem that dramatic, the economic damage caused by trypanosomosis is due to the fact the disease impacts on all aspects of productivity, in both susceptible and trypanotolerant livestock. When the individual productivity effects are combined in a herd model, the impacts of the disease on productivity reduces livestock keepers’ annual revenue from cattle by between 25 and 50%, with losses per head in moderate to high tsetse challenge areas ranging from USD 15 – 30 (Shaw et al., 2014). Compared to control costs (see section “Main means of prevention, detection and control-Cost of above measures”, for more details), it is very clear that expenditure on tsetse control or targeted use of trypanocides is highly cost-effective.

  Given the range of values for measured inputs described above, making an overall estimate of the cost of AT in Africa is subject to great uncertainties. The most recent estimate calculated that a 50% uptake of a tsetse repellent technology in eighteen tsetse-infested countries could yield USD 900 million of benefits from increased milk and meat production (Abro et al., 2021).

  GAPS

  More information is needed on the impacts of T. evansi and non-tsetse transmitted trypanosomosis, as well as on the effects of trypanosomosis on livestock species other than cattle. Overall, there is a great need for new studies using scientifically robust comparators and measurements to assess productivity impacts on livestock. Such studies need to be linked to prevalence data. On the sociological side, more information on the social impacts of the disease, for example to what extent poorer households are disproportionately affected, is needed.

• Direct impact (b) cost of private and public control measures

  Numerous studies interviewing livestock keepers in trypanosomosis endemic areas have shown that expenditure on trypanocides can account for 30% - 60% of their animal health expenditure.

  The amounts spent probably average between USD 2 and USD 5 per head of cattle per annum. Adult animals are those most often treated. Some livestock keepers in some areas spend far higher amounts as well as spending money on insecticides to deal with both tsetse and tick-borne diseases, notably East Coast fever where it is present.

  For AAT, there have been many programmes and projects over the years, but government involvements and expenditures have varied greatly. There are rarely clear and ongoing commitments to controlling the vector and, while veterinary and livestock extension services do provide access to curative or prophylactic trypanocides, this is done in response to individual requests from livestock producers.

  For HAT in the DRC (with 64% of the reported global g-HAT cases in the last 5 years) the economic costing has been estimated to be around USD 15M per year with financing from the Ministry of Health and donors funding specific intervention programmes, drug and Tiny Target donations and some shipping costs (Antillon et al, MedRxiv, 2024). This estimate does not include funding for recent or on-going research activities such as clinical trials for new drugs. Forecasting for elimination, modelling suggests that an uplift in funding in the DRC would be needed initially (up to around USD 23M per year), although strategies with a high probability of elimination by 2030 would be able to scale back sooner compared to continuation of current activities. Currently it appears that, even if the additional funding became immediately available, it would be a very large challenge for the national programme to operationalise the needed scale up rapidly across the whole country, particularly in areas which are geographically isolated or have political unrest.

  For NTTAT, in a study carried out in The Philippines, the total net-benefit from effective surra control for a typical village in a moderate/high risk area was estimated at USD 158,000 per year, and the value added to buffaloes, cattle, horses, goats/sheep and pigs as a result of a trypanocidal control was estimated at USD 88, 84, 151, 7, 114 per animal/year, respectively (Dobson et al., 2009).

  GAPS

  More knowledge on the costs and types of trypanocides being used is needed.

• Indirect impact

  AAT: As a chronic disease affecting livestock throughout the sub-Saharan region, trypanosomosis is not associated with epidemics and therefore not with sudden changes in price levels, disruption of food supplies or food security. Instead, it undermines people’s livelihoods, food supplies and wealth, year in, year out. Infected livestock are sold or slaughtered at low weights, or in poor condition and thus fetch low prices. The effects on livestock keepers’ incomes are discussed above, in the “Zoonosis: Cost of treatment and control of the disease in humans” section.

  HAT: in some areas, tourists are warned to avoid being bitten by tsetse. Overall, there is a very low probability that tourists would be infected due to the very low prevalence, although occasionally rHAT cases have occurred in tourists (Franco et al., 2022).

  The presence of tsetse has long been considered to be a barrier to the expansion of grazing grounds, thus limiting livestock populations. Over the last decades, growing human population and resulting pressure on land resources have meant that livestock keepers in search of grazing have increasingly colonised areas that they previously avoided, and their animals are becoming more used to living with tsetse. Lastly, it has often been hoped that better tsetse control would enable an expansion of livestock production using exotic species or crossbreds. Kenya and Tanzania’s thriving smallholder dairy industries with crossbred Holstein/Friesian stock do extend into the tsetse-infested areas. In these areas zero-grazing can reduce contact with tsetse and cattle are often treated with insecticides, to protect them from tsetse, other biting flies and ticks. Currently, following elimination of tsetse flies in the Niayes region of Senegal, exotic breeds are being imported to increase milk production, and first milk production figures do indeed show an increase. In addition to trypanosomosis, raising exotic breeds or cross-bred cattle between local and exotic breeds faces additional challenges (food and water supply, heat, other diseases, value chain) that need to be globally addressed.

  NTTAT is a huge constraint on horse management; in Asia, horse breeding is not possible in the close vicinity of T. evansi reservoir constituted by cattle and buffaloes, therefore strict separation of these livestock is necessary. In Latin America, working with horses for livestock breeding is only made possible by permanent chemoprophylactic treatment of working horses.

  GAPS

  For Nagana and HAT there are no important gaps in this area: for NTTAT more knowledge is needed.

  In places where trypanosomosis is properly controlled or even eliminated, and where exotic breeds or cross-bred cattle are being reared, the impacts should be clearly assessed, whether they are socio-economic or environmental, positive or negative.

• Trade implications

• Impact on international trade/exports from the EU

  The list of diagnostic tests that must be carried out to allow an animal to enter the EU is described in the animal health law (https://eur-lex.europa.eu/EN/legal-content/summary/the-eu-animal-health-law.html). For example, it will not be possible for a horse with a positive dourine complement fixation test to enter the European Union. In parallel, detailed standards for trade/exports are described in the WOAH Terrestrial Animal Health code.

  Besides, as concerns surra, regulation of importations is presented in the Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): Trypanosoma evansi infections (including Surra) (https://efsa.onlinelibrary.wiley.com/doi/full/10.2903/j.efsa.2017.4892), briefly summarized below:

  Import of bovines, sheep, goats and pigs are only allowed into the European Union (EU) if the animals originate from authorised third countries: Andorra, Chile, Canada, Greenland, Island, Mayotte, New Zealand and Saint Pierre et Miquelon. For camelids originating from non-authorised countries, a quarantine should be made in Saint Pierre et Miquelon, and documents and negative blood smear examination are requested at 2 and 42 days after their arrival (Annex I to Regulation (EC) No 206/2010 1).

  For live Equidae and their products, temporary or permanent importations are strictly regulated for each type and third country according to a Commission Decision of 6 January 2004 (2004/211/EC). 2

  However, in this list at least six countries are authorised to export all types of equines to the EU, although they are known to be enzootic for Surra; namely: Argentina, Algeria, Morocco, Tunisia, Israel and Paraguay.

  GAPS

  Tests recommended for international trade for the control of dourine and surra are based on serological methods. Given that serological tests are by nature never 100% specific nor 100% sensitive, establishing a threshold that avoids the risk of false negatives while limiting the risk of false positives remains a challenge.

  Regulations are strictly necessary, however, because of regulations, sanitary authorities tend to hide some information to avoid the consequences.

• Impact on EU intra-community trade

  To date, mainlands of European Union countries are considered free of nagana, surra and dourine and to our knowledge, there are no regulations in place for the control of AT during EU intra-community trade. However, such regulations do exist for Overseas Departments from which importations are submitted to quarantine in St Pierre & Miquelon. Surveillance of the potential emergence of these AT relies on the existence of veterinary services within the States, which, during the various veterinary controls, could detect the appearance of clinical signs suggestive of early-stage AT and thus signal the emergence of an epizootic in order to implement appropriate management measures, as it was the case during the dourine epizootic in Italy in 2011 as an example.

  GAPS

  In the absence of EU intra-community regulations related to AT, we have not identified Gap(s) in availability of products/knowledge having an impact on EU intra-community trade.

• Impact on national trade

  To date, mainlands of European Union countries are considered free of nagana, surra and dourine and to our knowledge, there are no regulations in place for the control of AT during national trade. Surveillance of the potential emergence of these AT relies on the existence of veterinary services within the states, which, during the various veterinary controls, could detect the appearance of clinical signs suggestive of early-stage AT and thus signal the emergence of an epizootic to implement appropriate management measures, as it was the case during the dourine epizootic in Italy in 2011 as an example.

  GAPS In the absence of national regulations related to AT, no gap(s) in availability of products/knowledge having an impact on national trade have been identified.

• Links to climate

  Seasonal cycle linked to climate

  For nagana in Africa, transmission roughly occurs all year round, but seasonal variations are observed in links with i) tsetse population dynamic and ii) contact between livestock and tsetse. In fact, transhumance and local movements of cattle in search of food and water can temporally increase tsetse-cattle contacts and thus disease incidence (Dayo et al, 2010). Besides, the geographical spreading of tsetse flies during wet periods seasonally favours contact with their hosts. This variation depends on local factors.

  Although tsetse flies are active all year long, which is essential given the slow female reproduction rate, seasonal variation in population density and geographical distribution are known to occur as well as change in physiological condition or feeding behaviour. All these seasonal variations may impact transmission dynamics for both nagana and HAT. The seasonal effects in tsetse have for example shown well-marked annual cycles in wing length for female tsetse in Zimbabwe, length being maximal in the cool season, and minimal in the hot dry season. As temperatures increase, females produce progressively smaller and less viable offspring emerging with low reserves of fat. There are very clear temperature dependent relationships for both pupation time and adult survival. The viability of tsetse pupae is confined to 16–31°C, with drastic increases in mortality outside this range.

  The transmission of T. evansi and T. vivax is even more impacted by season and climate as tabanids and Stomoxys have a seasonal life cycle with population bursts during or after wet seasons. Hence transmission is a lot more seasonal.

  HAT: For g-HAT the long infection period means any impact of seasonality of tsetse is not apparent in case data.

  GAPS

  The exact nature of disease risk and transmission variation with seasons for the different diseases and how that will be impacted by climate change is poorly understood and warrant some further studies.

  An improved prediction of the peaks of density of the main mechanical vectors in links with climate changes is highly needed.

• Distribution of disease or vector linked to climate

  There is some local evidence that tsetse flies may have disappeared/gone extinct in some parts of Africa, not because of vector control but because of global change (e.g. in Burkina Faso, Courtin et al., 2010; in Chad, Guihini Mollo et al., 2024), but the reverse is not impossible, as suggested by Longbottom et al., 2020 in Zimbabwe, where high elevation areas may become suitable for the expansion of some Glossina species due to global warming.

  In terms of global change drivers, cropland expansion and intensification of cropping systems due to human growth represent the main strategies to boost agricultural production, but are also major drivers of biodiversity decline, including insects and especially tsetse flies. Land use changes are driven by agricultural expansion (at the expense of grassland, bushland, and woodland), simplification of landscapes, cutting trees for various purposes (such as firewood, charcoal, and construction material), overgrazing, and the expansion of settlements. Such changes lead to the fragmentation of wild mammalian hosts and tsetse and other biting flies’ habitats. Besides, pesticides are also a major concern as they are increasingly used to protect crops from damage by pests and diseases; while no insecticide resistance has been observed in tsetse, it is very common in stable flies, whose control is therefore seriously impacted. Such changes may also increase or modify tsetse interactions with humans and domestic animals, which may be favourable to some tsetse species like the riverine Palpalis group with close connection to human settlements and pig production (currently under study, for instance in Côte d’Ivoire).

  GAPS

  There is a need to study how global changes may impact global tsetse and trypanosomosis distribution from reduction in some areas to increases in some other areas, for example higher altitudes that now become more favourable. There is a large modelling effort needed to forecast future tsetse and mechanical vector distribution under optimistic and pessimistic global change scenarios, plus a need of updated field data to validate the outputs of modelling.

  The data on mechanical vectors is globally lacking to enable similar understanding and modelling efforts. There is a need for more surveillance and production of such atlases.

  There is no clear understanding how climate driven effect on vector distribution may impact disease risk and that once again would benefit from modelling efforts.

• Outbreaks linked to extreme weather

  Extreme weather events are known to impact animal transhumance and migration both for wildlife and domestic animals impacting also their interaction with vectors and disease. For example, pastoralists in East Africa during the last 5 years of drought were allowed to bring cattle to graze nearby and even within parks and reserves. It increased wildlife/domestic animal Extreme weather events are known to impact animal transhumance and migration both for wildlife and domestic animals impacting also their interaction with vectors and disease. For example, pastoralists in East Africa during the last 5 years of drought were allowed to bring cattle to graze nearby and even within parks and reserves. It increased wildlife/domestic animal interactions and exposure to tsetse bites with a risk known to be higher within close vicinity of protected areas.

  Besides, drought use to stress animals and enhance the passage of healthy carriers to sick and parasitaemic status, therefore, such animals favour mechanical transmission and may be at the origin of outbreaks, such as observed for T. vivax in Latin America.

  GAPS

  Extreme weather events are expected to increase in frequency in Africa with namely extreme droughts and rainfalls events but the exact link to AAT and HAT risks and vector population dynamics is not understood.

  Effect of extreme weather events on biting fly dynamics, and their impact on parasites transmission, are unknown.

  Effect of extreme weather events on livestock and human displacement, and how this could interact with trypanosomes risk transmission is unknown.

• Sensitivity of disease or vectors to the effects of global climate change (climate/environment/land use)

  Tsetse are highly sensitive to variation in their external conditions, including change to the environment and climate, and nutritional stress, and hence are highly impacted by global change. For example, land use changes like deforestation for agricultural intensification to ensure food security have been shown to impact tsetse abundance and distribution but also tsetse physiological condition and susceptibility to infection.

  Environmental stressors can cause mortality or affect fecundity and susceptibility to infection (Akoda et al., 2009a, Lord et al., 2021). In laboratory conditions, nutritional stress can increase the death rate up to 5 times and decreases the birth rate up to 90%. There is also a two-times increase in infection susceptibility for starved Glossina morsitans morsitans infected with T.b. brucei or T. congolense. While in the wild, a decrease in distance from human settlements increases the likelihood of trypanosome infection for G. m. morsitans with T. vivax, T. congolense, and T.b. brucei, because of tsetse habitat degradation and reduced host availability, decreasing levels of nutrition and fat reserves and hence tsetse immunity against trypanosome infection (Chilongo et al., 2021a and b).

  Tsetse are highly sensitive to variation in their external conditions, including change to the environment and climate, and nutritional stress, and hence are highly impacted by global change. For example, land use changes like deforestation for agricultural intensification to ensure food security have been shown to impact tsetse abundance and distribution but also tsetse physiological condition and susceptibility to infection.

  Environmental stressors can cause mortality or affect fecundity and susceptibility to infection (Akoda et al., 2009a, Lord et al., 2021). In laboratory conditions, nutritional stress can increase the death rate up to 5 times and decreases the birth rate up to 90%. There is also a two-times increase in infection susceptibility for starved Glossina morsitans morsitans infected with T.b. brucei or T. congolense. While in the wild, a decrease in distance from human settlements increases the likelihood of trypanosome infection for G. m. morsitans with T. vivax, T. congolense, and T.b. brucei, because of tsetse habitat degradation and reduced host availability, decreasing levels of nutrition and fat reserves and hence tsetse immunity against trypanosome infection (Chilongo et al., 2021a and b). Besides, tsetse susceptibility to infection is high in newly emerged adult flies but decreases strongly past the first blood meal, which is called the teneral phenomenon, at least in some species. However, in the laboratory, nutritional stress cancels this teneral phenomenon as starved non-teneral flies become as susceptible as teneral flies (Kubi et al. 2006).

  Current stressors can also affect later generations through transgenerational processes, extending the period over which they impact. Whilst transgenerational effects are relatively well resolved in some animal and even insect systems, they are not resolved in disease vectors. In the laboratory, offspring from nutritionally stressed tsetse females have an increased mortality rate and a lower tolerance for starvation. Teneral offspring from parents starved in the laboratory have an increased susceptibility to infection for G. m. morsitans infected with T.b. brucei or T. congolense (Akoda et al., 2009b).

  GAPS

  There is a need to better understand the impact of climate, environmental, and land use changes on tsetse and biting flies biology and how they may further impact disease risk.

  Current models have shown that tsetse’s ability to transmit trypanosomes is influenced by the density of the vector population, the biting rate, the longevity, and the extrinsic incubation period of the parasite (time it takes for a tsetse to become infectious after feeding on an infected host). Any variation in tsetse birth and death rates, and infection susceptibility with environmental change will influence the transmission risk to vertebrate hosts. However, most models use simplified representations of tsetse biology and fail to account for how environmental factors influence tsetse behaviour and disease transmission. This is particularly problematic when considering the broader implications of global climate change.

  The effect of global changes on biting flies and the associated risk of transmission is unknown.

Risk

• AAT (including human infective trypanosomes):

  Sustained tsetse control on various scales is feasible (in large parts of sub-Saharan Africa) and low-cost effective options exist. Creation of tsetse free zones is also possible, but preventing reinvasion is in important issue in all but a limited number of isolated populations. The development of drug resistant trypanosomes will lead to potential increased levels of disease in animals and humans. Whilst the use of insecticides to kill tsetse, applied from aircraft, to cattle (so as to control both tsetse and ticks) or to stationary targets and traps, is effective there is the possibility of development of insecticide resistant tsetse flies (risk is probably low but behavioural resistance could become an issue).

  NTTAT: Indirect risk for surra is the immunosuppressive effects which can enhance inter-current diseases and interfere with vaccination campaigns. Eradication is impossible.

  T. evansi: possible risk for introduction into EU.

  T. equipderum: idem, need for more vigilance.

  GAPS:

  NTTAT: CODE, test validation, rules and regulations on EU level for Surra.

Main critical gaps

• Control: diagnostics, vaccines and therapeutics

  Absence of point of care diagnostic, although it is mandatory to make relevant treatment decision. Such a diagnostic should be inexpensive and amenable to routine use in multiple specimens and samples. Accurate and timely diagnosis would not only be the basis for proper treatment decisions and follow-up but will also support epidemiological studies and will allow a better control of international movements.

  The insufficient spread, standardisation and actual use of the diagnostic tools that already exist for surveillance and monitoring (parasitology with concentration techniques, PCR and ELISA). Routine use of PCR and ELISA should be made available at regional level in all enzootic areas, to increase diagnosis availability; this would be considerably helpful in the implementation of the PCP. Hopefully, sensitive sub-species-specific DNA detection methods should be developed to detect and identify taxa of the Trypanozoon subgenus (T.b. evansi / T.b. equiperdum / T. b. brucei / T. b. gambiense / T.b. rhodesiense...).

  Absence of vaccine is a critical gap. Even if it was so far unsuccessful, vaccine development should still be explored based on new approaches.

  Treatment failures, which can have a variety of causes (fake drugs, incorrect use and wrong dosage, chemo-resistance) and need to be well documented before they can be resolved. This requires a global improvement of the supply chains, the support from veterinary service, and training. The arrival of a new drug would provide a therapeutic alternative.

  Control or elimination of the tsetse fly is successful in some areas, but it requires a sustainable commitment of many stakeholders. Optimal sustainable and cost-effective intervention strategies need to be established and adapted to various epidemiological contexts and agro-ecological settings. New tools to control tsetse flies without insecticide are needed. Control methods for mechanical vectors are not considered efficient so far, due to the very high prolificity of these oviparous biting flies

  r-HAT: Significant underreporting remains. The lack of rapid diagnostic tests for r-HAT, its focality and rareness, result in limited knowledge about the disease and experience in its diagnosis and treatment at the level of the health staff. More treatment options for r-HAT should become available, in particular for children. A new, single dose oral drug which has proven effective for treatment of g-HAT in clinical trials would, theoretically, also be effective for r-HAT. The rarity of cases however leads to practical problems in developing diagnostics and organising clinical trials. This problem is even accentuated in children.

  Pathogenicity, host immune response and vector competency

  Significant knowledge gaps remain concerning the complex interplay of factors that determine the course and spread of trypanosomoses across different host species. Lack of fundamental knowledge about livestock immune response to trypanosomes and its role in protection or pathogenesis, lack of fundamental understanding of cattle trypanotolerance and more globally variation in host species susceptibility and parasite pathogenicity. Such knowledge could benefit the creation of a vaccine.

  Knowledge of parasites and vector interactions, including vector microbiota, and of the bases of vector competency, is lacking. Identification of microbes promoting or inhibiting parasite installation in the vectors could provide new innovative control tools.

  Epidemiology and impact

  It is unknown how long mammals can remain healthy carriers of trypanosomes in the absence of treatment, and what is the epidemiological role and potential risk of animals carrying T.b. gambiense. These questions are key to address in order to propose an appropriate care.

  The impact of global change, including climate change, on the vectors (both tsetse flies and biting flies), on AT and HAT is largely unknown. Positive side effects of global changes could make AAT and HAT control easier through a strong negative impact on tsetse flies’ habitat. However, there could also be new opportunities for tsetse to colonize areas where they were not occurring in the past, or, for biting flies, to increase their range and activities.

  Optimal cost-effective intervention strategies are not established for all tsetse species and agro-ecological settings.

  Critical and quantitative analyses of the productivity impacts and socioeconomic costs of animal trypanosomoses, and of the costs and benefits of control have been undertaken for AA, but vary greatly between locations, study quality and protocols. Such studies are scant for surra, mechanically transmitted T. vivax and dourine.

  A global lack of financial resources to fight trypanosomoses that are considered as neglected diseases because they mainly impact poor and remote populations in developing countries is a real problem.

Conclusion

• African trypanosomoses can be split into at least six different entities: nagana, g-HAT, r-HAT, mechanically transmitted nagana, surra and the sexually transmitted dourine in equids. They exhibit common points, while there are some peculiarities that make the global picture quite complex.

  Animal trypanosomoses act as a constant drain on livestock productivity and livestock keepers’ livelihoods, contributing to poverty, food insecurity and inequality. Those affecting cattle are the most important economically since they are a major cause of reduced meat and milk production and limit the use of draught power for agricultural production.

  Tsetse transmitted trypanosomosis (nagana or AAT) is geographically delimited, which can help to control it, however it also exhibits a large range of hosts, including wild animals, which hampers its control. Within the tsetse belt, tsetse transmitted trypanosomoses hinder land use and livestock farming and production. For example, horses which are very susceptible to nagana can hardly survive inside the tsetse belt, and this has had important consequences on the human history on the African continent. Similarly, livestock breeds that are highly susceptible to AAT cannot be bred inside this area, which limits improvement of livestock, notably for dairy production. In the absence of vaccine, alleviating the negative impact of AT on livestock production is based on three pillars: diagnostic, treatment and vector control. Gaps in products and knowledge remain to be filled in these three fields to ensure a sustainable, integrated and cost-effective AT control. Moreover, significant knowledge gaps remain about livestock immune response to trypanosomes, its interaction with the pathogenic factors of trypanosomes, and its role in protection or pathogenesis. Improved knowledge and advances in biotechnology could lead to the creation of a vaccine. Non-tsetse transmitted animal trypanosomoses (NTTAT) have long been neglected, especially in Africa, due to the very high pathogenicity of the nagana complex, and the difficulty to have a specific diagnosis. However, outside Africa, mechanically transmitted trypanosomoses have largely spread and, contrary to tsetse transmitted trypanosomoses, they continue to extend. Indeed, NTTAT have still a large potential for geographical extension, and they are probably currently extending, as was recently demonstrated in Iran, where T. vivax was detected all over the country although it had always been claimed that only surra is present. While surra has already largely spread, both surra and mechanically transmitted nagana have a very large geographical extension potential; they may for example invade North America, Europe and Australia where they would find suitable vectors. For this reason, NTTAT should be taken seriously. Surveillance and control are needed especially in the trading of living animals, since NTTAT may be introduced, the same way they were introduced into Latin America four centuries ago by the Spanish conquistadores, and nearly two decades ago from Canary Island to the Spanish and French mainland, where it hopefully did not permanently establish itself. As with AAT, there is a need to improve diagnosis and medication, and to establish, evaluate and implement innovative vector control tools.

  HAT: At a global level, g-HAT has been eliminated as a public health problem (Franco et al., 2024). The WHO road map for neglected tropical diseases targets elimination of transmission of g-HAT (zero cases detected) and elimination of r-HAT as a public health problem by 2030 (https://www.who.int/publications/i/item/9789240010352). Tools and strategies for their implementation need to be adapted to the low HAT prevalence that is actually observed. It is obvious that with fewer HAT cases detected, the disease risks to be neglected again. However, to avoid re-emergence as observed in the past, interest in the HAT needs to be maintained at all levels involved: the population at risk, health care staff, ministries, research and funders.

  In conclusion, improved diagnostics, treatments and vector control tools are required to sustainably control trypanosomoses, and in some places even eliminate locally the disease. Proper tools usage must be integrated in a larger framework of control policies and planning, with the involvement of stakeholders. Investments in AT control have to spread over five main areas: (i) human resource development; (ii) improved technology for diagnosis and disease treatment, global surveillance and monitoring; (iii) improved vector control; (iv) increased exchange of information and (v) international, regional, national and local institutional support, commitment and awareness. To support the development of new tools and policies, a better understanding of complex interactions between main hosts, parasites, vectors in different agro-ecological settings is needed.

  Alleviate the negative impact of these diseases would be a major contribution to sustainable development goals.

Sources of information

• Expert group composition

  Antoine Barreaux. French Agricultural Research Centre for International Development (CIRAD)-UMR Intertryp UM, IRD, CIRAD, Montpellier FRANCE and Visiting Scientist at the International Center for Insect Physiology and Ecology (ICIPE)- KENYA.

  Marjorie Bouchier. CEVA Santé Animale- FRANCE.

  Alain Boulangé. French Agricultural Research Centre for International Development (CIRAD)- UMR Intertryp UM, IRD, CIRAD, Montpellier FRANCE. Institut Pierre Richet (IPR-INSP) - CÔTE D’IVOIRE.

  Giuliano Cecchi. Food and Agricultural Organization of the United Nations (FAO), Rome-ITALY.

  Marc Desquesnes. French Agricultural Research Centre for International Development (CIRAD)- UMR Intertryp UM, IRD, CIRAD. National Veterinary School of Toulouse (ENVT)- FRANCE.

  Geoffrey Gimonneau. French Agricultural Research Centre for International Development (CIRAD)-UMR Intertryp UM, IRD, CIRAD- FRANCE. Institut Sénégalais de Recherche Agricole (ISRA) Dakar- SENEGAL

  Marisa Gonzatti. Department of Cell Biology. Universidad Simón Bolívar, Caracas- VENEZUELA.

  Laurent Hébert. French Agency for Food, Environmental and Occupational Health & Safety (ANSES)- FRANCE.

  Veerle Lejon. French Institute for Sustainable Development (IRD)- UMR Intertryp UM, IRD, CIRAD, Montpellier- FRANCE.

  Liam Morrison. University of Edinburgh, Roslin Institute- UNITED KINGDOM

  Luís Neves. University of Pretoria- SOUTH AFRICA. Universidade Eduardo Mondlane, Centro de Biotecnologia, Maputo- MOZAMBIQUE

  Alexandra Shaw. University of Edinburgh/AP Consultants-UNITED KINGDOM

  Kat Rock. University of Warwick, Coventry- UNITED KINGDOM

  Alireza Sazmand. Bu-Ali Sina University, Hamedan- IRAN.

  Philippe Solano. French Institute for Sustainable Development (IRD)- UMR Intertryp UM, IRD, CIRAD, Montpellier- FRANCE

  Sophie Thévenon. French Agricultural Research Centre for International Development (CIRAD) - UMR Intertryp UM, IRD, CIRAD, Montpellier- FRANCE.

• Reviewed by

  Project Management Board.

• Date of submission by expert group

  March 2025

• References

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