ABSTRACT
Several factors have contributed to the increased use of live Eimeria oocyst vaccines to control coccidiosis in broiler and layer chickens. Eimeria oocysts are well adapted to withstand a wide range of temperatures, humidities, and ammonia levels that are typically found during growout in a poultry house. The primary and secondary immune responses to Eimeria are unique because the former may have little impact on the parasite completing its life cycle, while acquired immunity is completely effective against challenge infection. One of the drawbacks to coccidiosis vaccines based on live virulent Eimeria is that overall performance is depressed because chicks may ingest high numbers of oocysts that are randomly distributed in litter during growout. The species specificity of the immune response to avian Eimeria has been known for a long time. Intraepithelial lymphocytes are known to transport sporozoites from the epithelial villi to crypts, possibly reflecting a way that Eimeria avoids destruction by harnessing immune cells for transport.
