Species differentiation methods have classically relied on biological features, including: oocyst shape and size, prepatent period, minimum sporulation time, intestinal site of colonization and lesion characteristics.
PCR assays are available for the seven Eimeria species of domestic fowl, mainly targeted to the ITS rRNA sequences and Sequence-Characterized Amplified Region (SCAR) markers. The seven chicken Eimeria species can be simultaneously detected and discriminated in a single-tube multiplex PCR assay. Quantitative PCR assays have also been developed for the seven species.
PCR diagnostic assays targeted to ITS1 rDNA have recently been reported for four turkey Eimeria species but these are not yet reported to be in general lab use.
There are no rapid tests to differentiate drug sensitive from drug resistant parasites
There are no rapid or pen side tests to confirm identification of infecting species, or to identify presence of new variant genotypes.
Medium – potential increase if new genotypes currently restricted to the southern hemisphere spread further northwards.
No DIVA tests are available.
GAP: DIVA tests that differentiate live vaccine strains from field strains would be useful to determine the efficacy of live vaccines in replacing pathogenic/drug-resistant field strains.
For chickens, live vaccines (attenuated or virulent) comprising sporulated oocysts of different species /strain combinations depending upon the target market (i.e. broilers or breeders and layers) are available. Vaccines are applied orally, on feed, in water, in eye-drops, by gel administration or by hatchery spray, in chicks up to approximately 9 days old. In the US live vaccine for in ovo administration is also available.
A sub-unit vaccine formulated as an oil emulsion for maternal immunisation of broiler breeders to provide passive protection to broiler chicks was licensed in selected countries but is no longer available.
For turkeys, virulent live vaccines containing selected species of turkey Eimeria oocysts for application in water, by gel administration or by hatchery spray, are available.
Vaccine effectiveness using current live vaccines could be increased by improvements in vaccine delivery. This is particularly important when using virulent vaccines, which in many parts of the world including the USA, is the favoured product.
MSD Animal Health produces both layer/breeder and broiler vaccines for chickens with market authorization throughout Europe. Biopharm and Huvepharma produce layer/breeder and broiler vaccines, while Hipra and ABIC produce broiler vaccines for chickens all with market authorization in selected EU member states. All of the live oocysts vaccines are attenuated; the ABIC vaccine is a sub-unit preparation.
There are no anticoccidial vaccines for turkeys currently licensed in Europe.
Current production capacity (in chickens) limits availability of live vaccines.
Gaps or breakdown of commercial production can pose big risks in the vaccine supply chain.
There is a need for an EU licensed anticoccidial vaccine for turkeys.
None. Live attenuated vaccine lines possess traits that have the potential to differentiate them from wild type lines. It is however not currently practical to do this on a routine basis.
GAP: Genetic characterization of live attenuated lines may allow the development of molecular methods for the differentiation of attenuated versus wild type field isolates.
Current live vaccines (virulent or attenuated) are efficient in stimulating protective immunity. These vaccines however rely on recycling of oocysts via the litter to build up immunity and are therefore susceptible to problems where environmental conditions that are not conducive to oocyst survival may occur.
Live (virulent) vaccines are associated with risks of direct vaccine related effects on production and may also predispose the birds to other diseases.
Current subunit vaccination, based on the provision of passive protection to chicks, is reliant on the development of active immunity following a limited natural infection over the first couple of weeks of life whilst maternal antibody is present at a sufficient level to protect the chick. Vaccine efficacy is therefore dependent upon the level of challenge present in the environment as well as the problems associated with oocyst recycling outlined above.
In general protection is species specific and in consequence vaccines need to include all species for which protection is being claimed.
Vaccine lines that can be distinguished easily from wild type isolates
Vaccines that provide cross species protection
Vaccines with reduced dependency on oocyst recycling in litter
Vaccines that have long shelf life with no cold chain.
Vaccines that do not require production in chickens.
Bearing in mind the size of the European broiler market and the proposal to ban anticoccidial drugs there is significant commercial potential for anticoccidial vaccines, in particular if the shortcomings identified above can be overcome.
Whilst the use of anticoccidial drugs remains acceptable, cost of vaccine manufacture relative to anticoccidial drug production remains a significant issue.
There is a need to develop processes for the in vitro production of vaccines
Eimeria are ubiquitous in the environment and in consequence barrier protection is not considered a feasible option.
A number of research groups globally are actively involved in the identification of protective antigens. If suitable candidates providing acceptable levels of efficacy, can be found, delivery methods capable of overcoming the shortcomings identified above may be developed within a 5 to 10 year timescale.
There is potential to improve the manufacturing process should a reliable in vitro method for the production of oocysts be developed.
GAP: New delivery methods for vaccines.
Anticoccidial drugs (ionophores and synthetic compounds) are available and widely used as additives in the feed for the prevention of coccidiosis. These drugs vary in their efficacy depending upon several factors of which acquired resistance by the parasites is the most problematic.
In order to retain drug efficacy most poultry producers employ rotation programs involving the use of drugs that differ in their mode of action. Rotation programs with live vaccines are also becoming popular.
Most drugs have a withdrawal period of several days before slaughter during which they must not be employed. In practice poultry companies use substantially longer withdrawal periods than those prescribed.
A few anticoccidial drugs are available to treat birds that are suffering from coccidiosis usually by including a soluble compound in the drinking water. The results can be equivocal.
GAP: Better therapeutic drugs to treat outbreaks of clinical disease
Fundamental research to discover unique biochemical pathways in Eimeria is desirable. An example is the recent elucidation of the mannitol pathway that has identified several enzymes that could be potential targets for drug inhibition.
Such research is long term but could, if pursued, eventually lead to better treatments for coccidiosis.
GAP: Need to more fully understand parasite specific biochemistry that could be exploited for novel anticoccidial drug development.
Anticoccidial drug discovery is undertaken by private companies whose activities are confidential. However, there is no indication that such research is presently underway. Many companies that previously were active in this field no longer conduct the screening necessary for new drug discovery. No new drugs have been introduced in the last twenty years. It has been suggested that modern approaches using molecular methods might help identify novel targets in the parasite susceptible to drug inhibition but there is little evidence that this is being pursued.
New developments for diagnostic tests
Strain differentiation methods are required to discriminate vaccine and field strains and to differentiate drug-resistant from drug-susceptible strains. Tests that can rapidly identify new genetic variants would be very useful especially in global studies of parasite epidemiology.
GAP: Development of intra-specific diagnostic tools based on microsatellite data or other potential genomic markers.