Protective immunity against coccidiosis elicited by radiation-attenuated Eimeria maxima sporozoites that are incapable of asexual development.

Eimeria maxima oocysts were exposed to various doses of gamma radiation that did not affect sporozoite invasion of intestinal epithelium but did prevent subsequent merogonic development therein. Although merogony and oocyst formation were inhibited, parasites exposed to 12 kRad radiation induced a level of immunity against E. maxima challenge equivalent to that induced by non-irradiated oocysts. Chickens immunized per os with 20 kRad-treated E. maxima oocysts were not protected against coccidial challenge. Immunization of chickens with a single low dose (five oocysts) of non-irradiated (0 kRad) or irradiated (12 kRad) E. maxima was effective in preventing weight depression after coccidial challenge. Immunofluorescence staining of intestinal tissue from chickens infected with irradiated (12 or 20 kRad) or non-irradiated (0 kRad) E. maxima oocysts with developmental stage-specific monoclonal antibodies showed that sporozoite invasion was similar in all groups. However, merogonic development was not observed at any time postinfection in chickens infected with irradiated oocysts, unlike the case with chickens infected with non-irradiated parasites. These results suggest that sporozoite-infected host cells are capable of eliciting complete protection against E. maxima challenge.