Immunity Patterns during Acute Infection by Eimeria bovis

Cellular and humoral responses were investigated following gavage inoculation of 6-wk-old bull calves with 35,000-40,000 oocysts of Eimeria bovis. At 3-4-day intervals for 40 days after inoculation (DAI), blood was taken and assessed for serum IgG against merozoites and sporozoites of E. bovis. Proliferative responses of peripheral blood lymphocytes were measured following stimulation with either concanavalin A (Con A) or a soluble antigen derived from E. bovis oocysts (EbAg). Serum IgG against merozoites and sporozoites reached a peak of activity between 10 and 20 DAI, coinciding with oocyst shedding on days 17 to 24. Serum antibody titers had dropped to base levels by 40 DAI, although anti-merozoite titers remained elevated for the duration of the study (i.e., from days 12 and 20 to day 40). Con A stimulation of lymphocytes was not affected by infection; there was no evidence of suppressed or augmented responsiveness. Lymphocyte responses to EbAg had reached a maximum by day 20 and remained elevated throughout the study. These results indicate (a) that sporozoites and merozoites share antigens recognized by serum IgG, (b) that there is no episode of marked immunosuppression during acute infection, and (c) that cellular immunity is probably more important in resistance against reinfection than humoral immunity.