The Site of the Immune Reaction against Eimeria bovis in Calves

Ten experiments were conducted using a total of 84 young male Holstein-Friesian calves. In each of seven experiments, 4 to 6 of 10 to 12 calves were immunized with 25,000 to 60,000 oocysts; in three experiments, a total of 8 of 13 calves was immunized with 0.5 to 1.0 million oocysts. About 1 month after the immunizing inoculation, all of the calves were given 0.5 to 1.0 million oocysts. In nine experiments the numbers of merozoites in the cecal contents were determined for the 13th day through the 18th day after inoculation by daily samples taken through cannulas. The mean for 14 nonimmunized calves was 401,300/ml, while for 12 calves immunized with 25,000 to 60,000 oocysts it was 53,000. The corresponding figures for the four nonimmunized calves and the eight immunized with 0.5 or 1.0 million oocysts were 179,900 and 1,000. Most of the other calves were killed 15 to 18 days after inoculation. At necropsy fewer schizonts were found in the small intestine in the immunized than in the nonimmunized calves killed 14 to 16 days after inoculation; however, no marked difference was found in the calves killed 17 and 18 days after inoculation. At examination of histological sections of the large intestine, the immunized calves were found to have a markedly lower percentage of infected epithelial tissue than did nonimmunized calves. In 21 of the 23 immunized calves killed at a mean interval of 27.6 days after the peak oocyst discharge of the immunizing infection, oocysts retained from this infection were found, usually surrounded by macrophages, in the lamina propria of the mucosa. No significant difference in the timing of the schizont, merozoite, or sexual stages was found between immunized and nonimmunized calves. The findings indicate that the immune reaction against Eimeria bovis occurs both in the small and large intestine of the calf, but is of greater importance in the large intestine, and that it affects the numbers but not the timing of the various stages of the life cycle.