Coccidiosis is regarded as one of the most important economic diseases of poultry industries in the world. Chemotheray is the most important method of it’s control and prevention. Recently, some attemps have been directed to induce the immunity against avian coccidiosis and some live-attenuated vaccines have been introduced commerically. The present study was carried out to investigate the immnunogenecity of sporozoite and merozoite of E. tenella by intra-rectal inoculation. One hundred day-old chickens were divided to five groups which including 20 birds. Four groups of birds were inoculated by low and high dose of sporozoite and merozoite, remainder by 20 p1 of sterile Hank’s medium as control group. All immunized and nonimmunized chickens were challenged by 10,000 sporulated oocysts of homologous strain at 3-week-old age. The bird faeces were examined daily 8 days after prepatent period and the mean of oocyst per gram (OPG) was calculated in each group. Th results showed significant difference between birds immunized by sporozoite and control group. Furthermore, there was significant difference between birds immunized by high dose of merozoite and control group. The results indicated no significant difference between birds immunized by low dose of merozoite and control group. There was higher protection in chickens immunized by sporozoite. The finding revealed that the formation and maturation of first generation schizont of 1imeria tenella has an important role in induction of immune response. Field study was carried out to show long-term protection by low dose of sporozoite, it seems that litter rearing cause contineous ingestion of oocysts which introduce antigen presentation contineously.
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