Past and future: vaccination against Eimeria.

Eimeria spp. are the causative agents of coccidiosis, a major disease affecting many intensively-reared livestock, especially poultry. The chicken is host to 7 species of Eimeria that develop within intestinal epithelial cells and produce varying degrees of morbidity and mortality. Control of coccidiosis by the poultry industry is dominated by prophylactic chemotherapy but drug resistance is a serious problem. Strongly protective but species-specific immunity can be induced in chickens by infection with any of the Eimeria spp. At the Institute of Animal Health in Houghton, UK in the 1980s we showed that all 7 Eimeria spp. could be stably attenuated by serial passage in chickens of the earliest oocysts produced (i.e. the first parasites to complete their endogenous development) and this process resulted in the depletion of asexual development. Despite being highly attenuated, the precocious lines retained their immunizing capacity. Subsequent work led to the commercial introduction of the first live attenuated vaccine, Paracox, that has now been in use for 20 years. As much work still remains to be done before the development of recombinant vaccines becomes a reality, it is likely that reliance upon live, attenuated vaccines will increase in years to come.