The Occurrence of a Second Asexual Generation in the Life Cycle of Eimeria bovis in Calves

To determine whether Eimeria bovis has a second asexual generation occurring in the large intestine of calves, three biopsy experiments were conducted. In each, 0.5 to 1.0 billion E. bovis merozoites were introduced into the ligated cecum of a young calf. The merozoites were in mucosal scrapings obtained from the small intestine of a calf killed 15 days after inoculation with 2.0 million oocysts, 95% E. bovis. During the 6 to 72 hr after the inoculation of merozoites, three or four cecal biopsies were made in each calf. One calf was killed 80 hr after inoculation. The other two discharged only E. bovis oocysts on the 5th and 6th and 5th to 7th day after inoculation, respectively. In the histological sections of the cecum of each calf, many immature schizonts were found in the tissues taken at 6 to 36 hr, numerous mature schizonts were seen at 42 to 48 hr, many very early gametocytes at 48 hr, and progressively more advanced gametocytes at 60, 72, and 80 hr. Mature schizonts in the sections had a mean size of 8.9 by 10.0 µ they contained 30 to 36 merozoites, averaging 1.2 by 3.5 µ. Fresh merozoites were 5.7 to 6.8 µ long, with a mean of 6.2. The findings demonstrate that a second asexual generation is included in the life cycle of E. bovis and occurs in the epithelial cells of the cecum and colon.