Response of Immunized and Nonimmunized Calves to Cecal Inoculation of First-Generation Merozoites of Eimeria bovis

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Abstract
Four experiments were conducted, in each of which 0.4 to 1.9 billion first-generation merozoites of Eimeria bovis were inoculated by laparotomy into the ligated cecum of a nonimmunized calf and a calf immunized with 25,000 or 50,000 oocysts. These mero-zoites were obtained from calves killed 15 days after inoculation with 2.0 million oocysts. An initial biopsy was performed at the time mero-zoites were inoculated, and three or four additional biopsies were done in each calf at intervals of 3 to 24 hr during the following 2 or 3 days. The ligature was removed 1 to 2 days after inoculation, and fecal samples collected daily were examined for oocysts. Approximately 1 week after the last biopsy in each experiment, the calves were challenged with an oral inoculation of 1.0 million oocysts and were killed 19 days later. Moderate infection occurred after the inoculation of mero-zoites in the nonimmunized calves of Experiments 1 and 2, but only slight infections occurred in the nonimmunized calves of Experiments 3 and 4. No infection could be detected in any of the immunized calves, except for a few parasites found in one calf at 24 hr. These findings suggest that the penetration of host cells by first-generation merozoites is inhibited in immunized calves. All of the immunized calves and two of the four nonimmunized calves showed little or no infection after challenge, but the nonimmunized calves of Experiments 3 and 4 became moderately infected. These results indicate that infection caused by inoculation into the cecum of first-generation merozoites stimulates the development of immunity.