X-irradiation of Eimeria tenella oocysts provides direct evidence that sporozoite invasion and early schizont development induce a protective immune response(s).

Sporulated oocysts of the protozoan parasite Eimeria tenella were attenuated by exposure to various doses of X-radiation to inhibit intracellular replication and thus determine whether sporozoites alone can induce a protective immune response. Exposure to doses greater than 15-kilorads had a significant effect on development, as indicated by the absence of oocyst production in chickens infected with parasites treated with 20 or 30 kilorads of radiation. Infection with nonirradiated or 15-kilorad-exposed parasites led to either normal or reduced oocyst shedding. Equivalent protection was afforded chickens inoculated with a minimum immunizing dose of either nonirradiated or 20-kilorad-irradiated E. tenella oocysts. Immunofluorescence staining of cecal tissue from chickens inoculated with 10(7) nonirradiated or 20- or 30-kilorad-irradiated oocysts with stage-specific monoclonal antibodies showed no significant difference in sporozoite invasion between treatment groups. Normal merogonic development was observed at appropriate times (48, 60, 72, and 96 h) postinfection in chickens inoculated with nonirradiated oocysts. In contrast, irradiated parasites exhibited minimal merogonic development at 48 h postinfection. Furthermore, no merogonic stages were observed at times of otherwise peak merozoite development (60, 72, and 96 h) in cecal tissue from chickens inoculated with irradiated parasites. Infection of chicken cells with irradiated or nonirradiated parasites in vitro corroborated these findings and indicate that events early after sporozoite invasion induce a protective immune response against this parasite.