Immunity to Eimeria separata (Apicomplexa: Eimeriina): expose-and-challenge studies in rats.

Oocyst production of Eimeria separata of the rat was determined in initial and challenge infections. The total number of oocysts produced was no higher at 100 oocysts than at 1,000, 10,000, or 100,000 in initial infections. In 2 experiments, the reproductive index with 100 oocysts was 17,000 and 34,000, respectively. On challenge with 10,000 oocysts, there was 91% protection at 100 oocysts. There was slightly lower protection at 10,000 and 100,000 oocysts, 72% and 81%, respectively, and t-values suggest lower protection at these higher inocula.