Epidemiological aspects of the use of live anticoccidial vaccines for chickens.

This review address the epidemiology (epizootiology) of coccidiosis in commercial chickens with emphasis on the effects on the use of live vaccines. Surveys suggest that all seven valid species of chicken coccidia (Eimeria acervulina, Eimeria brunetti, Eimeria maxima, Eimeria mitis, Eimeria necatrix, Eimeria praecox and Eimeria tenella) are ubiquitous. All species are pathogenic to various extents. New results are presented on the pathogeneicities of E. acervulina, E. mitis and E. Praecox. Unless ingested by chickens, oocysts in poultry-house litter may die after about 3 weeks. Oocyst sporulation may be better in drier, rather than wetter, litter. Whether sporulated or not, up to 20% of ingested oocysts may pass undamaged through a chicken's intestine. The excreted, sporulated oocysts can be immediately reingested to initiate an infection; the unsporulated oocysts can still sporulate after passing through the intestine. The seven species differ in their times of appearance in commercial flock; hence particular vaccines may be designed for rearing standard broilers for up to about 6 weeks or for breeding stock. Attenuated, precocious lines of Eimeria in vaccines have low reproductive potentials, thus avoiding crowding, developing optimally, and stimulating immune response with minimal tissue damage. Cross-immunity between Eimeria species is probably minimal. There is reciprocity between the immune status of chicken and their excretion of oocysts for each species, ensuring continual stimulation of immune responses in birds on litter. Paracox vaccine, comprising all seven Eimeria species, is shown here to stimulate immunity to each of them independently. Total oocyst accumulation in litter following Paracox vaccination at 1 week comprises a small peak of vaccinal oocysts at 2-4 weeks, then a higher peak of the local virulent population at 4-7 weeks, which rapidly wanes. The attenuated drug-sensitive vaccinal oocysts probably interbreed with the corresponding wild species, reducing both virulence and drug-resistance in the local population. Anticoccidial vaccines may not induce complete immunity in chickens with lowered immunocompetence due to stressors, including certain viral disease. Future development of live vaccines for standard broilers may be expected in the relatively short term. The useful lives of anticoccidial drugs might be extended by rotating them with live vaccines.