Yes. Although the production tends to be centred in a few countries and tests are for the most part expensive.
Less expensive tests would be extremely useful.
Yes. The tools to perform the tests presently in current use, skin test and IGRAs, are available in Europe.
Please see the OIE site http://www.oie.int/fileadmin/Home/eng/Health_standards/tahm/2.04.07_BOVINE_TB.pdf
The skin test does not differentiate between infected and, for example, BCG vaccinated animals. However IGRAs and skin tests based on specific antigens such as ESAT 6 and CFP-10 are already in use for TB diagnosis in humans, are at prototype stages in cattle and have been shown to act as a DIVA tests in the face of BCG vaccination.
Tests based on the detection of the pathogen or products-derived from the pathogen such as LAM are also under development, as are serological assays. Serological assay kits are also commercially available for use in cattle and in other domestic and wildlife species such as deer or South American Camelids.
GAPS: See "Main means of prevention, detection and control - Vaccines"
The priority development areas are (1) to increase the performance of immune diagnostic tests in both sensitivity and specificity, (2) develop better DIVA reagents, (3) develop defined tuberculins, (4) develop diagnostic tests for species other than cattle, and (5) develop tests that are directly detect bacilli or bacillary products from live animals. A more comprehensive list of developmental opportunities is presented below:
At present the only potentially available vaccine against M. bovis infections is BCG, which is a live attenuated strain (various substrains) of M. bovis used for humans. Studies with BCG showed variable efficacy in cattle both at population and individual animal levels. Such variation may be attributable to various factors including vaccine formulation, route of vaccination, and the degree of exposure to environmental mycobacteria.
A large number of field trials have been performed using BCG and the efficacy of BCG in cattle to vary in a similar manner to that reported for humans, although the variability of experiments and trials makes meaningful comparisons of results difficult. Recent field experiments conducted in Ethiopia and Mexico strongly suggest that BCG vaccination can be effective in protecting cattle against BTB.
BCG is also effective against other domestic and wildlife species such as goats, different species of deer, badgers and possums. It is licensed for vaccination of badgers. Large trials of BCG vaccination of cattle are underway for example in Mexico. Safety studies and duration of immunity studies have also been conducted and await assessment by regulators. The main limitation of BCG vaccination is that it compromises the specificities of tuberculin-based tests and thus successful application of BCG vaccination of cattle and other domestic species that are subject to test and slaughter-based control policies will require development of DIVA tests, some of which are now available as prototypes.
BCG Danish has been licensed for use in badgers as injectable vaccine in UK.
A dossier aimed at licensing BCG for cattle in UK is in preparation.
A number of potentially new candidate vaccines are currently being screened with heterologus prime-boost vaccines based on BCG vaccination in combination with viral, protein, or DNA subunits.
The possibility of a vaccine to boost the efficacy of BCG needs investigation. See also "Main means of prevention, detection and control - Vaccines"
Use extremely restricted for animals presently (see above).
BCG is licensed for use in badgers as an injectable vaccine. Oral BCG vaccine is being field tested in Republic of Ireland. Vaccination in cattle is currently prohibited in EU.
BCG can be considered a marker vaccine as genes that were deleted during its attenuation harbour antigens that can be exploited as DIVA antigens (such as ESAT-6) whilst mutations in other BCG antigens result in them not being secreted which further increases the repertoire of potential DIVA antigens (such as Rv3615c). Research efforts are under way to develop additional marker vaccines.
BCG vaccination can prevent disease or substantially reduce M. bovis induced pathology and thus BCG vaccination is expected to reduce onward transmission of infection. However, protective efficacy is variable at both population and individual animal levels. It is important to recognise that the use of BCG will compromise tuberculin skin tests or other immunological tests currently in use. Thus cattle vaccination with BCG should not be used in countries where control or trade measures are based on such testing. BCG vaccination therefore requires the application of DIVA tests. As outlined in other sections, such DIVA prototypes already exist BCG and are referred to in section "Vaccines (inactivated, attenuated, sub-unit, GMO)".
The duration of immunity following BCG vaccination lies between 1 and 2 years, and therefore will be require regular re-vaccinations. This is feasible as recent studies have demonstrated that BCG induced immunity can be extended by BCG revaccination. Several recent GLP safety studies also suggested its safety although they still await assessment by the regulatory authorities.
BCG efficacy has also been demonstrated in domestic species other than cattle such as goats and farmed deer.
BCG vaccines may be considered of potential use to reduce spread of M. bovis in wildlife reservoirs. It is essential before doing so to validate the delivery system for the particular wildlife species. Before embarking on a vaccination programme, the vaccination schedule must be optimised for local conditions.
Although there are still important legal limitations to vaccination against BTB, there is high potential in some countries where BTB remains a problem, particularly where wildlife acts as a reservoir of infection.
At present vaccination of cattle against BTB is not permitted in Europe.
EU acceptance of cattle BTB vaccination.
Provision of data to justify the authorisation of vaccines in term of safety, efficacy and quality may be difficult and expensive to generate. EFSA have published an opinion on what would be required to allow TB vaccination of cattle in EU. (http://www.efsa.europa.eu/en/efsajournal/pub/3475).
Wildlife vaccination may be easier to implement, as no such legal obstacles exist in the EU in relation to wildlife vaccines.
A vaccine could be potentially used in areas of high incidence and areas with very low or free of BTB.Vaccines could be used in a ring vaccination strategy especially to prevent the spread of M. bovis infection in wildlife.
The priorities for vaccine development for BCG are: (1) to establish field performance of vaccine and DIVA prototypes, (2) to improve DIVA performance, (3) to define the correlates of BCG protection and why some individuals fail to be protected by BCG, and (4) to develop vaccination strategies for other species than cattle including wildlife.
A comprehensive list of requirements is given below:
Antimicrobial treatment is not considered for cattle.
Unlikely due to the limitations referred to in Section "Main means of detection, prevention and control - Therapeutics".
This has been discussed extensively in Section "Diagnostic availability – Opportunities for new developments". A summary is given below:
- Diagnostics with improved performance: both of sensitivity and specificity.
- Diagnostic assays with objective and reproducible read out: in vitro assays.
- Tuberculin should be standardized in terms of activity for in vivo and in vitro detection assays. Alternative and more standardized methods (in vitro) for potency testing should be developed.
- Improved antigens (in addition to tuberculin, or indeed to replace it) are needed for improved CMI detection assays. Those that have been developed need to be validated.
- DIVA reagents should be developed further and those available need to be validated.
- The sensitivity of serodiagnostic tests need to be increased
- Improved diagnostic tests for wildlife species, which represent a reservoir for bovine TB is essential.
Better understanding of the host pathogen interactions and the cellular immune response to infection by M. bovis might help defining:
- better surrogates of infections; for example multiplexing (cytokines, or cytokines combined with serology);
- further antigens that elicit an important immune response among all infected animals but that are absent from other mycobacteria and from antigens potentially used in vaccines (DIVA tests);
- better diagnosis for live animals applicable to non-bovines (e.g. goats, SAC);
- stage-specific (latency versus active disease) antigens in cattle;
- non-immunological biomarkers;
- biomarkers of vaccine efficacy to aid DIVA.
Please also refer to Section "Vaccines availability - Opportunities for new developments", which gives an extensive list of requirements for the development of better vaccines and main characteristics for improved vaccines and vaccine deployment.
The main problem resides in the difficulty in defining “protective immunity” against M. bovis. An immune response capable of controlling the growth of pathogenic mycobacteria is known but the characteristics of the immune response that allows the elimination of the bacteria from the host is still not understood. In addition, immunological correlates of vaccine efficacy and protection need to be defined and the ones already highlighted need to be validated.
There is a concerted international effort to identify and evaluate candidate vaccines for use in cattle and to define vaccination strategies that will eliminate disease from infected herds. DNA, protein and genetically modified vaccines inoculated in a single dose, given as prime-boost or injected concurrently, which will elicit significant protection against challenge with M. bovis under controlled conditions need to be investigated further. However, vaccines and vaccination strategies require evaluation under field conditions
Non-sensitising vaccines would overcome the problem of skin test sensitisation associated with BCG-based strategies.
DIVA tests for cattle are essential.
1. Detailed investigations into the host pathogen interactions to understand the relationships between M. bovis and the cellular response to infection. This would enable methods to stimulate cellular immunity to be developed through the use of vaccines.
2. Detailed understanding of the epidemiology of M. bovis infections in cattle and cattle herds to enable strategies to be developed for the use of the new vaccines when available.
3. Development of new delivery systems for application to wildlife using new technologies.
4. Correlates of vaccine efficacy and protection.
5. DIVA development and validation.
Bovine tuberculosis (BTB) is an infectious disease that affects cattle, other domesticated animals and certain free or captive wildlife species.
BTB is caused by Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex of the family Mycobacteriaceae.
Other species of the M. tuberculosis complex previously considered to be M. bovis include Mycobacterium caprae (in some countries considered to be a primary pathogen of goats) and Mycobacterium pinnipedii, a pathogen of fur seals and sea lions. These two new species are also known to be zoonotic. In central Europe M. caprae has been identified as a common cause of BTB.
M. tuberculosis, the main agent of human tuberculosis (TB) has been reported to infect cattle, as well other animals, in particular circumstances (e.g. in China, India and Ethiopia).Although clear differences exist between countries the main causative agent of BTB is M. bovis. Except when stated otherwise, BTB will be referred in this document to the disease caused by M. bovis in cattle.
The genome of M. bovis is highly conserved, but molecular analysis has led to the development of methods to identify genotypically distinct strains with local dominance of specific clonal complexes of M. bovis. Clonal complexes have been identified.
See above for different species.
Mycobacterial survival in the environment is still a controversial issue. While general agreement exists on the ability of M. bovis to resist for long periods in quite adverse conditions information is lacking on its ability to resist in the environment and more importantly, to what extent its resistance in the environment represents a clear source of contamination.
Even though cattle are considered the primary target for BTB, other domesticated and several species of wild mammals are susceptible to this disease.
Among domesticated animals goats are particularly susceptible and can maintain infection in the absence of cattle.
Asymptomatic carriers occur and infected animals might transmit the disease with no obvious symptoms.
Wildlife represents a serious problem in several countries worldwide as maintenance and/or spill over species and/or reservoirs of BTB.
See above for more information on wildlife susceptible to BTB and reservoirs.
While M. bovis is a recognised cause of TB in humans, the majority of TB cases in humans are caused by M. tuberculosis. TB screening in humans is mostly targeted at respiratory forms of the disease (most TB cases by M. tuberculosis are pulmonary) while M. bovis in humans occurs mostly as extra-pulmonary TB.
Human disease caused by M. bovis is now rare in countries with successful BTB control and eradication programs, established meat inspection procedures and milk pasteurisation. However, it should be noted that the lack of differentiation between M. tuberculosis and M. bovis may occur even in Europe, leading to under-diagnosis of TB caused by M. bovis in humans. In countries where BTB is poorly controlled in livestock and where consumption of raw milk or unpasteurised dairy products is frequent, BTB may represent an important human health risk.
Reservoirs in wildlife render complete eradication difficult in several countries.
The potential host range for the disease comprises any of the free-ranging mammal species but the host status of these species is variable. Some are too restricted in numbers or distribution to have any significant role in disease dynamics; others exhibit limited susceptibility, or are dead end hosts that become infected but not infectious. Controversy has arisen over the distinction between two categories of infectious host: spill over hosts that require an external source of re-infection to maintain the disease within their population, and reservoir hosts where the disease persists by cycling within the population.
An understanding of the potential of each wild animal species as a reservoir of infection for domestic animals is reached by determining the nature of the disease in each wild animal species, the routes of infection for domestic species and the risk of domestic animals encountering an infectious dose. The mere presence of infection in a wild animal population does not by itself provide evidence of a significant wildlife reservoir. Relevant factors include: infection routes, anatomical position of lesions and infection, level and routes of elimination, routes of potential transmission to domestic animals, and knowledge of minimal dose for infection.
Species considered maintenance hosts are: brush–tailed possums (and possibly ferrets) in New Zealand; badgers in the UK and Ireland; bison and elk in Canada; kudu and African buffalo in southern Africa; wild boar and red deer in the Iberian Peninsula.
White-tailed deer in the USA (Michigan) has been classified as maintenance host. However, some authors now believe this species may be a spill over host that maintains the organism only when its population density is high. Probably the same applies to red deer and wild boar in the Iberian Peninsula.
Species reported to be spill over hosts include sheep, horse, pig, dog, cat, ferret, camel, llama, alpaca, many species of wild ruminants including deer and elk; elephant, rhinoceros, fox, coyote, mink, primate, opossum, otters, seal, sea lion, hare, raccoon, bear, warthog, large cats (including lion, tiger, leopard, cheetah and lynx), wolves and several species of rodents.
Little is known about the susceptibility of birds to M. bovis. Although they are generally thought to be resistant, cases of psittacines infected with M. bovis or M. tuberculosis have been reported.
In South American camelids (SAC) BTB burden in natural habitat is largely unknown. Infection occurs mainly via respiratory route. Infection more common when animals held under intensive management.
Close contact and high density of animals seems to be the most important factors to promote transmission of BTB.
In countries without disease control, cattle-to-cattle transmission rate can be high, particularly when animals are kept under high-intensity/density husbandry.
Mathematical models of intra or interspecies transmission have been developed for few host-pathogen systems (possum-cattle in New Zealand, badger-cattle in the UK, deer-cattle in North America and wild boar-red deer-cattle in France).
BTB signs and morbidity are known to vary greatly between distinct mammal species.
In cattle BTB is usually a chronic slowly progressive condition with no obvious signs of disease. In countries with eradication programs, most infected cattle are identified early and symptomatic infections are uncommon. In late stages of BTB, common symptoms include progressive emaciation, a low–grade fluctuating fever, weakness and lack of appetite. Animals with pulmonary involvement usually have a moist cough that is worse in the morning, during cold weather or exercise, and may have dyspnoea or tachypnoea. In the terminal stages, animals may become extremely emaciated and develop obvious respiratory distress. In some animals, the retropharyngeal or other lymph nodes enlarge and may rupture and drain. Greatly enlarged lymph nodes can also obstruct blood vessels, airways, or the digestive tract. If the digestive tract is involved, intermittent diarrhoea and constipation may be present.
In cervids, BTB may be a sub-acute or chronic disease, and the rate of progression is variable. In some animals, the only symptom may be abscesses in isolated lymph nodes, and symptoms may not develop for several years. In other cases, the disease may be disseminated, with a rapid, fulminating course.
In cats, disease seems to evolve more rapidly than cattle and the symptoms may include weight loss, a persistent or fluctuating low-grade fever, dehydration, decreased appetite and possibly episodes of vomiting or diarrhoea.
In brush-tailed opossums, BTB is usually a lethal pulmonary disease that typically lasts two to six months. In the final stage of the disease, animals become disoriented, cannot climb, and may be seen wandering about in daylight.
A significant proportion of infected badgers have no visible lesions and can survive for many years. In symptomatic badgers, BTB is primarily a respiratory disease.
Primates develop necrogranulomatous pneumonia: these animals may show behaviour signs like depressive-like behaviour and the animals tend to be solitary.
Lesions in brushtail possum and badgers have little or no fibrosis, and mineralisation is very rarely observed. The macroscopic appearance may resemble pyogenic abscesses. Aditionally to the respiratory lesions, the badgers also can show open wounds and behaviour changes. Elbow hygromata lesions with associated lameness may be observed in lions. They might also show chorneal opacity, skin wounds, and poor cicatrization. Swollen head nodes with draining fistulae are almost pathognomonic in greater kudu.
Information is still scarce on disease progression in South American camelids (SAC) in natural habitat (severe disease but can be asymptomatic shortly before animals die).
The symptoms of BTB usually take months to develop although, as referred to above; information is lacking on the disease progression in several mammal species.
Infections can also remain persistent for years in cattle and reactivate during periods of weaker immune system or in old age. However, it is known that in experimental infection, pathology (but not clinical symptoms) can develop within weeks upon infection (known for several species that were experimentally infected and even for young calves).
For the other species precise information is missing to a large extent but it is accepted that upon infection, animals, depending on the species, might develop or not signs of disease.
In countries with BTB control programs, BTB is often, but not always, confined to a very limited number of animals per herd and most reactors are detected during routine testing. Consequently, mortality from BTB is extremely uncommon in cattle in these countries.
Mortality due to BTB has been clearly reported in a few wildlife species like for example buffalos and lions in the Kruger National Park in South Africa, but the overall picture is clearly incomplete.
Shedding of M. bovis by cattle has been mostly associated with respiratory secretions, although bacteria might also be present in faeces and milk and to a lesser extent shedding has been described in urine, vaginal secretions or semen. Large numbers of organisms may be shed in the late stages of infection.
Under experimental infection conditions, shedding is intermittent and with low bacillary loads (nasal secretions). However, shedding can occur early post-infection and in the absence of visible pathology and before acquired immunity has developed.
Badgers with advanced generalised disease excrete from the respiratory tract, in urine if renal lesions have developed, in faeces if infected respiratory excretions are swallowed, and from wound exudates. Experimentally infected white tailed deer shed by the oral, nasal and rectal routes. In both species shedding can occur intermittently few weeks after experimental infection.
M. bovis is a facultative intracellular pathogen. Within the host it infects essentially macrophages and dendritic cells. The most relevant virulence factor is its ability to resist bactericidal mechanism of these cells.
For most animals, even a good immune response may lead only to a control of bacterial growth but it is believed to be unable to eradicate the bacteria from infected animals.
Pathology is to a great extent caused by the immune responses towards the pathogen. In cattle and several other species, the immune response against M. bovis leads to the formation of granulomas. The progressive evolution of granulomas with central necrosis, caseous and surrounded by fibrosis leads to the progressive destruction of the affected organs.
More knowledge is needed about the disease progression in different species to understand the mechanisms of pathogenicity. The evaluation of the location and structure of the lesions might provide important information. Studies on the immune response against mycobacteria in different species should also provide important knowledge on the disease pathogenicity, which is an essential component for the development of better diagnostic tools and control measures such as vaccines.
While human infections with M. bovis are now very rare in developed nations, due to BTB control/eradication programs and reduced consumption of unpasteurized dairy products, little is known about what is happening in the developing world. From these countries, where information is available, M. bovis accounts for a percentage of TB cases ranging from 0.4 to 8% percent, showing that M. bovis is an important factor in human disease. The global prevalence of human TB caused by M. bovis was estimated to be 3.1% of all human TB cases accounting for 2.1% and 9.4% of pulmonary and extra pulmonary TB cases, respectively.
It is important to note, as referred previously in 1.1., that ongoing studies have been identified M. tuberculosis as a cause of TB in cattle (e.g. in China, India and Ethiopia). How this contributes to human disease is still unknown.
TB caused by M. bovis in humans tends to be extra-pulmonary, and most studies of TB in humans are based on sputum analysis. As referred in 2.2., if special care on the microbiological culture conditions is not taken, it is possible to miss infection by M. bovis or misdiagnose a M. bovis infection as a M. tuberculosis infection (the best culture media are not the same for M. bovis and M. tuberculosis).
M. bovis can infect humans, primarily by the ingestion of unpasteurized milk and dairy products but also by aerosols and through breaks in the skin.
People living in developing countries, where no or weak BTB control/eradication programs exist, are considered at risk, especially when consumption of unpasteurised milk and dairy products is common.
Infection with HIV clearly increases the susceptibility to mycobacterial infections.
In Africa, 80% of the population is estimated to be rural and to depend solely on livestock for food and wealth and 85% of the cattle as well as 82% of the people live where BTB is only partially or not controlled.
A specific risk factor for children is the fact that 90% of the total milk produced in Africa is consumed raw or soured.
Thus BTB is an important issue in countries where no BTB control programs are implemented and is potentially even more relevant in countries where the rural populations represent a high percentage of the total population.
Humans can also acquire tuberculosis by direct contact with infected livestock and wild animals. Specific groups at risk of this occupational zoonosis are farmers, hunters, abattoirs workers and veterinarians. Infection can occur through respiratory route or through punctures in the skin. Some cases have been reported but there is limited information. Usually there is a lack of coordination between animal health and public health institutions. Also, handling animals for skin test may result in injuries.
Scattered information on professional risk groups should be compiled to better define surveillance strategies adapted to these professions.
In humans TB due to M. bovis is indistinguishable from that due to M. tuberculosis, considering clinical, radiological and pathological features. The main difference being that TB due to M. tuberculosis is mostly pulmonary (aerosol infection) and in contrast, TB caused by M. bovis is for the most part extra-pulmonary (cervical lymphadenopathy, gastro-intestinal). Pulmonary TB due to M. bovis is considered rare (although it is possibly under-diagnosed in developing countries), with most cases confined to rural areas and probably as a result of airborne infection from diseased cattle. Before the advent of pasteurisation M. bovis infection was common in children. Tuberculosis in children is often difficult to diagnose – especially in developing counties.
Symptoms depend to a great extent on the affected organ. The symptoms of TB are for the most cases unspecific like moderate fever, fatigue, weight loss and night sweats. Depending on the organ mostly affected people might present cough (for pulmonary TB) or lymphadenopathy (for BTB localized in specific lymph nodes) and diverse digestive problems (for gastro-intestinal TB). Gastro-intestinal TB is a major health problem in many developing countries. A recent significant increase has occurred in developed countries, especially in association with HIV infection.
Under reporting is potentially present in some European countries with high incidence of HIV infection, although increased awareness of the specific microbiological culture conditions to grow M. bovis was obvious for the last years. Tuberculosis in children is often difficult to diagnose.
Under reporting is most probably very high for most developing countries in which BTB is present in animals.
Person-to-person transmission is rare. However, M. bovis has occasionally been transmitted within small clusters of immunosuppressed individuals.
BTB poses a serious wildlife conservation and welfare problem.
BTB is a particularly serious problem for wildlife conservation in the Kruger National Park in South Africa, where the geographical spread of the disease has been reported in buffaloes, with the incidental spread to other animal species living in the parks, including kudu, baboons, lions, cheetah and leopards. In Uganda, BTB reported in the Queen Elizabeth National Park since the late sixties has been confirmed in buffaloes in the Kadepo Valley National Park. In Zambia, BTB has been identified for several years in Red Lechwe on the Kafue flats. Transmission of the infection to herds of wildebeest was confirmed for the first time in 1998.
In Europe, BTB is one of the major infectious causes of death in the critically endangered Iberian Lynx.
In the USA the outbreak of BTB that had struck white-tailed deer in the state of Michigan continues to pose problems as it has been found that the disease has been transmitted to coyotes, red foxes, racoons, black bears and bobcat. Attempts are under way to combat the disease in cervids by reducing the density of their populations and by prohibiting the feeding of deer by humans. In Minnesota an outbreak of BTB in cattle and white-tailed deer was controlled by aggressive efforts to reduce deer densities including liberalized hunting and sharpshooting and a ban on feeding of deer.
In Canada, BTB is endemic in a sub-population of bison, elk and white-tailed deer.
Many wildlife species are affected but whether this has an impact on endangered species depends on the definition of endangered species (see list at IUCN). The problems in lions in Africa (e.g. Kruger National Park in South Africa) should be highlighted as well as the critically endangered Iberian lynx for which BTB is considered one of the most important deadly diseases (an endangered species with extreme low numbers of individuals and patched population).
Control of BTB based on test-and-slaughter principles has been very successful for cattle and for some particular cases in wildlife. It is however expected to have a limited role for the control BTB in endangered free-ranging wildlife species.
In Minnesota the measures taken to control an outbreak of BTB included >50% reduction in white-tailed deer densities by liberalized hunting and sharpshooting. In Australia BTB was eradicated from feral water buffalo by large-scale stamping out of hundreds of thousands of animals.
In New Zealand, culling of brushtail possums is a central component of the campaign to control BTB in cattle and farmed deer. This strategy reduced the number of infected cattle and deer by over 60% between 1994 and 2001. In some parts of New Zealand the culling of ferrets was included in the program. Recently, a randomized trial in the UK showed that culling badgers to reduce BTB incidence in cattle is still a controversial issue.
Information on the effectiveness of strategies based on culling is missing for several wildlife species.
BTB is known to be widespread across the world (http://www.oie.int). Although complete eradication is exceedingly difficult to achieve, control programs have eliminated or nearly eliminated this disease from domesticated animals in many countries. However, in the developing countries where surveillance and control activities are inadequate or unavailable, the levels of disease remain mostly unknown.
Distribution of the disease in cattle and wildlife is mostly unknown in most developing countries.
Seasonality is not applicable.
Information to clearly determine the speed of spread of BTB is missing. Within an area or country, it is known to depend on networks and the degree of contact between uninfected and infected animals. M. bovis spread after the introduction in the sixties through the entire Kruger Park (400 km) took about 50 years.
Determination of the speed of spatial spread during outbreaks in distinct conditions is needed.
BTB presents high transboundary potential, due to the chronic nature of the disease and the vast range of host, combined with insufficient border control and understaffed veterinary services, in combination with wildlife population movements and trade of animals between countries.
Translocation of wildlife for conservation purposes can also be a big threat. The Wood Buffalo National Park, in Canada, became an infected area after plains bison were translocated. The disease spread out contaminating cattle in ranches around the park.
In cattle M. bovis is believed to be predominantly transmitted between animals by the inhalation of aerosols during close contacts.
Some animals become infected when they ingest the pathogen; this route may be particularly important in calves that nurse from infected cows (by drinking milk and colostrum).
Transcutaneous (through breaks in the skin), genital, and congenital infections have been reported but are rare in cattle.
The importance of the routes varies greatly between species. Ingestion appears to be the primary route of transmission for carnivores and species that feed on dead animals like pigs, ferrets and cats cats but also probably for non-carnivorous animals such as deer. In addition, cats might be infected by the respiratory route or via transcutaneous transmission in bites and scratches. Aerosol transmission also seems to be the main route of spread in badgers, but transmission through bite wounds can be significant, especially in high-density badger populations, as it is in the South-West of England. Badgers with advanced disease can shed M. bovis in the urine, and organisms have been found in the faeces. Due to behavioural changes, badgers and possums are most likely to transmit M. bovis to cattle during the late stages of disease.
See also Section "Description of infection & disease in natural hosts- Transmissibility".
See also Section "Description of infection & disease in natural hosts- Transmissibility".
Transmission is known to increase with increasing intensification. Some studies indicate that different breeds have difference resistance and transmissibility profiles.
Some wild species might become at risk when population increase dramatically.
Supplementary feeding of wildlife causes animals to congregate around feed dumps that may facilitate aerosol and oral transmission.
Differences on genetic resistance and transmissibility are largely unknown.
Neither the nature of the protective immune responses that control natural infection nor the immune processes that cause pathology (immune pathology) are completely understood. Whilst it is clear that Th-1 responses and in particular interferon-gamma and TNF are necessary for both processes, they are not sufficient to account for fully protective immunity or progressive immune pathology. Over recent years a number of other cytokines have been implicated in protective responses such as IL-17A and IL-22. These cytokines have also been associated with disease and disease severity and are therefore also linked to pathology. Apart from CD4+ T cells, other T-cell subsets have also been identified as components in the very diverse and multi-faceted immune responses to tuberculosis. These subsets include CD8+ and gamma-delta TCT+ T-cells as well as innate T-cells such as NKT cells. However, the relative contribution of these T-cell subsets remains to be elucidated. Whilst it is generally accepted that the initial interaction of bacilli with the host is with pulmonary macrophages, the precise relation of the initial host-pathogen encounter is not fully understood especially at the sites of infection and disease. Therefore studies to better define the nature of the host-pathogen interaction, innate and adaptive immunity to infection are essential.
It is believed that the elimination of mycobacteria depends on the microbicidal activities of the infected macrophages. The ability of macrophages to kill mycobacteria is enhanced after the development of a cell-mediated immune response involving activated T cells. These cells produce cytokines, especially interferon-gamma (IFN-γ), which activate microbicidal properties of macrophages. The role of antibody responses has been quite controversial and it seems to vary among different species. It is now also evident that direct contact between CD4+ T-cells and infected cells facilitates bacillary control within such infected cells.
After infection, mycobacteria are phagocytosed by local macrophages and/or dendritic cells. Infected dendritic cells migrate to the draining lymph nodes. T cell activation is initiated at the level of the draining lymph nodes. Activated T cells migrate to the site of infection attracted by the infected macrophages, activate and increase the number of mononuclear cells at the site of infection, and contribute to cell death with subsequent development of caseous necrosis.
Calf vaccination studies suggest that the subcutaneous injection of the vaccine Bacille-Calmette-Guerin (BCG) induces significant protection against experimental and natural challenge with M. bovis. This protection was associated with strong whole blood IFN-γ responses to bovine PPD 2-4 weeks after vaccination, but within the BCG-vaccinated groups, these responses were not correlated with protection. Use of a variety of vaccination strategies has shown that IFN-γ responses is essential for protection but not sufficient and in isolation is not necessarily associated with protection. Other components of the immune response against mycobacteria that complement IFN-γ production have been found to be associated with protective immunity such as IL-17 and IL-22.
Essential information on the establishment of a protective immune response, essential to understand how a vaccine could be protective, is missing. This includes:
The diagnosis based on the host immune response might use one of three different components of the immune response:
- Delayed-type hypersensitivity skin reaction (DTH) .The predominant method for diagnosis of BTB in cattle, the skin test, consists of an intradermal injection of PPD (a purified protein derivative from a culture of M. bovis - bovine PPD) or more specific antigens like ESAT-6 and CFP-10 (investigation is being performed on its applicability), which reflects the delayed type hypersensitivity (DTH) reaction. By using bovine and avian PPD (comparative cervical intradermal test) the specificity of the test is improved.
- IFN-γ release assays (IGRAs). In vitro stimulation of blood cells with specific antigens (PPD or other more specific antigens like ESAT-6 and CFP-10) leads to the production of IFN-γ by antigen-specific T cells from infected animals. IFN-γ is often detected detected by ELISA.
- Production of antibodies. Although the role of the antibodies in the context of a protective immune response is quite controversial, it is evident that most animals produce antibodies upon M. bovis infection. Interestingly, the production of antibodies seems to vary not just between animal species but also between animals from the same species. As a consequence, serological tests to detect antibodies present moderate to high specificity rates but still low to moderate sensitivity. Several new serological tests have yielded promising preliminary results, however, these tests have not been tested extensively under field conditions in domestic animals and even less in wildlife species.
Regarding DIVA (differentiating infected from vaccinated) see Section "Main means of prevention, detection and control - Vaccines".
Sanitation and disinfection may reduce the spread of the agent within the herd. M. bovis is relatively resistant to disinfectants and requires long contact times for inactivation. On infected properties, mechanical, physical and chemical agents are used to render rooms, materials, fluids and other substances non-infectious. The most effective methods for destroying mycobacteria are those that are based on the use of heat, such as hot air, burning, cooking, pasteurization, and running or pressurized steam. Ultraviolet rays are biologically active at 21 to 33 °C and bacilli are killed within 20 minutes after they have been exposed to direct sunlight. Effective disinfectants include 5% phenol, iodine solutions with a high concentration of available iodine, glutaraldehyde and formaldehyde. In environments with low concentrations of organic material, 1% sodium hypochlorite with a long contact time is also effective. M. bovis is also susceptible to moist heat of 121 °C (250 °F) for a minimum of 15 minutes.
In countries that have established control and eradication programs these are based on test-and-slaughter. When test-and-slaughter measures are applied, affected herds are re-tested periodically to eliminate newly-infected animals. Infected herds are usually quarantined, and animals that have been in contact with reactors are traced.
When test-and-slaughter programs are not possible, or during the early stages of control and eradication programs, some countries use test-and-segregation programs, and switch to test-and-slaughter methods in a more advanced and final stage.
The enforcement of test and slaughter is difficult in the absence of compensation funds and consequently very seldom used in developing countries. The use of surplus payments on milk and other animal products can be an alternative to straightforward financial compensation, although it tends to be more effective to higher yielding intensive dairy systems.
Treatment is not applicable for cattle.
The potential applicability of BCG is currently being investigated. Other vaccines are under development. None is presently licensed for cattle.
Information on the efficacy of test and segregation in different contexts could be extremely informative. At least two distinct situations should be considered:
Diagnostic tools are used in two distinct situations:
A) to diagnose BTB post-mortem;
B) to identify live infected animals.
A) BTB diagnosis post-mortem is based in one or a combination of the following methods:
· Detection of tuberculosis-like lesion macroscopically;
· Histopathology and/or microscopic demonstration of acid-fast bacilli;
· Isolation of M. bovis on selective culture media and confirmation by PCR;
· PCR used to detect M. bovis directly in samples;
· Genetic fingerprinting techniques (e.g. spoligotyping) to distinguish different strains of M. bovis.
B) BTB diagnosis in live animals – see Section "Detection and Immune response to infection".
Assays that measure the cellular immunity
· Intradermal tuberculin test - Single Intradermal tuberculin test using bovine tuberculin and comparative intradermal tuberculin test using bovine and avian tuberculin. The single tuberculin test is widely used for BTB diagnosis although the comparative intradermal tuberculin test presents higher specificity. The comparative cervical test is used for the initial screening of cattle in Europe. The USA uses the caudal fold (bovine tuberculin) test for the preliminary screening of cattle; reactors are re-tested with the comparative cervical test. The single cervical test is used for preliminary screening of cervids. Intradermal tests are used for also for non-human primates and lions in Kruger National Park.
· IGRAs have been reported to detect infected animals that are negative for the tuberculin skin tests. It is use routinely in certain European countries, USA, New Zealand and others. When used in combination with skin test (parallel testing) overall sensitivity is increased. It is also useful in animals that are difficult to capture or handle, as they must be captured only once, rather than twice as it is the case for the tuberculin test. An adaptation of this assay has been shown to be useful to test wild African buffaloes in South Africa.
· Defined antigens: For use in both skin and IGRA test defined and more specific antigens have been developed based on either synthetic peptides or recombinant proteins. These antigens can also be used to differentitate BCG vaccinataed from infected animals (DIVA principle). Example prototype antigens are ESAT-6, CFP-10, Rv3615c and others.
Assays that measure humoral immunity
· Serodiagnosis assays – several tests are presently being evaluated. Applicability varies considerably between tests and between animal species. Potential advantages of these tests might be that they are quite simple, rapid and only one visit is needed. The most relevant potential disadvantage is still the lack of sensitivity.
The tuberculin skin test is widely used and assays based on IGRAs are increasingly applied in most European countries as well as several American countries and others.
BCG is the only potentially available vaccine. Studies are ongoing to test BCG for cattle vaccination in BTB high prevalence regions. As well as for other domestic and wild species (BCG is licenced for use as an injectable vaccine for use in badgers in the UK).
Vaccines improving BCG-induced protection are under active development for cattle based on DNA, protein or virally vectored subunits, used in conjunction with BCG (heterologous prime-boosting).
Future alternatives of vaccines and/or route of vaccination for wildlife would include aerosol to be dispersed by air or a spray connected to baits. Oral baits could also work well specially for small mammals.
A major limitation to the use of BCG is that tuberculin-based diagnostic tests are compromised in their specificity by BCG vaccination. Thus BCG vaccination interferes with traditional test and slaughter TB control strategies. This limitation has, to some degree, been overcome by the development of tests that allow the detection of infection in vaccinated animals (so called DIVA tests) by using defined antigens that are not encoded by the genome of BCG but that are present in M. bovid.
Antimicrobial treatment is not applicable for BTB control.
It has been attempted in some particular situations, but it must be performed for long periods, and clinical improvement can occur without bacteriological cure. The risk of shedding organisms, hazards to humans and potential for drug resistance make treatment extremely controversial.
Discussed in the context of other topics.
Specific biosecurity measures are needed to prevent transmission between livestock and wildlife (and vice-versa) in areas where habitats overlap; however, these are difficult to implement and are costly, and may not be compatible with ecological requirements. Some measures that have been evaluated on pilot studies are fencing and limited access to water holes using specific devices according to target species.
Effective control and/or eradication on a national scale can only be achieved if there is full control over all cattle movements, compulsory identification of all cattle, payment of incentive to owners for the slaughter of positive reactors, compulsory testing of all cattle within specified intervals as well as the establishment and maintenance of disease-free areas, with the eventual aim of incorporating the whole country. The success of a campaign will depend largely upon the political will and stability within a country as these affect the availability of funding and manpower resources.
Prevention relies mostly on the early identification of infected animals, reduction of the contact between infected and non-infected by culling or isolating the infected animals.
Measures to prevent contact between wildlife reservoirs and cattle might also be of relevance (barriers around hay storage areas, security of feed stores etc). See "Biosecurity measures".
Culling to reduce the population density can decrease transmission. However, each situation must be assessed individually; culling may have unanticipated effects such as increasing the dispersal of the remaining members of a species. Prohibition of supplemental feeding and baiting (feeding of wild ruminants by hunters) may decrease transmission at feeding areas.
Description of the usefulness of pre-movement tests to prevent translocation of affected animals varies depending on reports. This could varies based on prevalence of infection, frequency of compulsory test/s, type of skin test in use and interpretation, reagents, and skill of veterinarians applying the test/s. Pre-movement test in cattle is compulsory in some countries. Pre-movement test in wildlife is affected by the limitations of existing diagnostic tests.
Surveillance programs exist, mainly through routine BTB testing and surveillance through abattoirs. Finding of lesions plus adequate recognition requires a level of training. Close link between abattoir system and animal health systems is needed.
After the successful eradication of the disease and the termination of regular testing regimes, meat inspection forms the corner stone to ensure the disease free status.
Training for diagnostic tools application for live animals and correct identification of lesions post-morten is necessary.
A better surveillance of wildlife is required to reduce the risk of transmission to cattle, at national but also international levels since wildlife epidemio-surveillance is poorly implemented in an international context. Few countries have established an efficient wildlife surveillance system.
The breakthrough in the eradication of BTB was achieved through mandated tuberculin testing, compulsory slaughter of reactors, meat inspection and pasteurisation of milk. Eradication programs have been quite effective in several European countries, USA, Australia, Cuba and other countries.
The rapid success in combating BTB was, however, not immediately paralleled by a decline in M. bovis TB cases in humans. Possible explanations include long periods of latency in adults with reactivation of previous foci of M. bovis infection acquired before the control programs were fully established.
The main failures of eradication programs based on test-and-slaughter have been associated with the existence of large numbers of infected wildlife in the same areas as susceptible cattle such as in the UK, Republic or Ireland, Spain, USA and New Zealand.
In other situations failure to regularly test cattle and remove infected animals and/or herds allows the disease to be maintained and spread to neighbouring farms and over longer distances by cattle movements.
Reports from many countries.
Important information can be found at the OIE site (http://www.oie.int/wahis/public.php) and specific information about EU countries can be found in the EFSA report on zoonotic agents (http://www.efsa.europa.eu/en/supporting/pub/135e.htm).
BTB is considered by the WHO a neglected zoonosis. The impact of M. bovis in humans in European countries with established control programs is considered negligible.
In developing countries information is very scarce. Existing data suggest the situation varies greatly among different developing countries.
Information on DALY is very scarce.
The cost of treatment is very high due essentially to the long antibiotic treatment required and necessary close follow up of the patients during this period.
Short course treatments would be extremely useful.
Losses at slaughter, culling at youngest ages, replacement losses due to the need to buy additional cattle. Milk yields and draft power are reduced and infected carcasses are rejected for sale and consumption.
GAP: Very scarce information exists on the direct impact on production in countries where the disease is not under control.
One of the major economic consequences is the issue of compensation.
Impact strongly depends on the level of BTB prevalence/incidence.
Effects on the market discussed below in Section "Trade implications".
The possible loss of genetic value (when it affects selected animals of breeding programmes or protected autochthonous breeds) might be of relevance.
The risk of exporting infected cattle to Officially Tuberculosis-Free status (OTF) regions from a region with BTB. Pre-movement testing may not resolve this problem due to the weaknesses of the test in detecting all the infected cattle.
The same argument applies to other traded species such as for example alpacas and goats.
Controls on movements include pre-movement testing where appropriate and may have impact on the intra-community trade (for more information see http://ec.europa.eu/food/committees/regulatory/scfcah/animal_health/report_working_group_bovine_tuberculosis.pdf).
M. bovis has multiple reservoirs, and easily crosses species.
The failure in BTB eradication in countries with established control programs is mostly associated with the existence of wildlife reservoirs.
Lack of cheap and accurate diagnostics.
Lack of applicable vaccination strategies for cattle useful in combination with the presently available diagnostic tests.
Lack of economical conditions to establish test-and-slaughter control programs in most developing countries as control measures result in significant economic losses.
Availability of resources (human and economical).
Commitment of all sectors involved in the programme, including policy-makers/government, veterinarians applying the diagnostic tools and abattoir surveillance, and (the most important) farmers.
Generate the legal and societal alternative strategies such as vaccination and programs based on slaughterhouse surveillance and trace back of animals with BTB to herds of origin.
Trained public health personnel and public education help on the implementation of BTB control programs taking into consideration existing cultural and societal constrains.
Not directly applicable, however seasonality in cattle movement might play a role.
No vectors associated with BTB.
The role of climate variables in the disease distribution of BTB is modest but it might be of relevance in what concerns disease transmission. Temperature and moisture related climatic indicators, especially their timing and variability, appear to be more important predictors of BTB in England and Wales than do variables related to vegetation or land-use. The transmission of BTB is therefore likely to be linked to the seasonality and sequence of ecological events during the year, and probably sensitive to changes in this seasonality from one year to the next. This aspect of epizootiology is rarely incorporated into statistical models of disease distribution and points to the type of process-based biological model that will be most appropriate for BTB.
It should also be considered that when animals are kept under extensive management system, and an extreme dry season limits water resources, an increase of contact between animals might occur and facilitate BTB transmission.
No outbreak reports linked to extreme weather in BTB.
The impact of adverse weather on food availability may result in alterations on BTB transmission, that might occur in opposite directions: a) a reduction in the population density of the reservoir hosts with a consequential reduction in disease transmission between cattle; b) a reduction on the immune response might increase M. bovis transmission.
To allow a better identification of areas at risk, the World Animal Health Information Database has been created, providing access to all data held within the OIE’s new “World Animal Health Information System”. Not all countries are represented and the quality of data is variable and could be improved. Data are available for domestic animals but also for wild species: a yearly report compiling data about wildlife is available.
Control measures must be applied not only to domestic cattle, but also to wildlife in areas where the incidence of BTB is high. These measures require the implementation of positive as well as preventive measures. Trade measures recommended by the OIE must also be applied
A better understanding of transmission pathways and risk factors for TB transmission is required and there is also a need to improve diagnosis tests and test methodologies. There are knowledge gaps around the role of the environment in BTB transmission and in many countries the exact role of wildlife species implicated in the transmission of BTB is not known. Further studies are needed to better understand the transmission mechanisms between probable wild maintenance hosts and cattle. The role of environmental persistence of M. bovis needs further investigation in order to assess its role in the epidemiology and transmission of BTB to cattle. Genetic resistance, the possibility of latency of M. bovis in cattle and the host-pathogen relation in cattle and wildlife species are areas that could be targeted by researchers.
The role of M. tuberculosis infection in cattle as a zoonosis especially in developing countries should be further investigated.
Expert group members are included where permission has been given
Glyn Hewinson, AHVLA, UK - [Leader]
Martin Vordermeier, AHVLA, UK
Kevin Kenny, Research Officer Ireland, Ireland
Tiny Hlokwe, ARC-Onderstepoort Veterinary Institute, South-Africa
Gunilla Källenius, Karolinska Institutet, Sweden
Margarida Correia-Neves, University of Minho, Portugal
Mitesh Mittal, Central Military Veterinary Laboratory, India
BCG - Bacille-Calmette-Guerin
BTB – Bovine tuberculosis
IGRA - IFN-γ release assays
IFN-γ - Interferon-gamma
TB - TuberculosisFURTHER READING
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