EFFECTS OF ACQUIRED-RESISTANCE ON INFECTION WITH EIMERIA-FALCIFORMIS VAR PRAGENSIS IN MICE

Mice immunized with infections of 500, 5000 or 20,000 oocysts of E. falciformis var. pragensis were reinfected with 20,000 and 100,000 oocysts at 20 and 38 days, respectively, after the initial infection. After the 1st challenge infection, no immunized mice showed clinical signs of coccidiosis; a few mice passed very low numbers of oocysts, and oocyst discharge seemed to correlate negatively with immunizing dose. No mice immunized twice passed oocysts after challenge. Mice immunized with 3 infections were completely immune to challenge for 4 mo. The effect of the immune response on the life cycle of the coccidium was determined by histological examination of the intestines of immune and nonimmune mice infected with the parasite. In the immune and nonimmune groups, sporozoites penetrated absorptive epithelial cells and migrated to crypt epithelial cells during the first 6-24 h post-infection. At 48-72 h post-infection, the sporozoites developed into mature 1st-generation schizonts in the nonimmune mice, whereas the developing 1st-generation schizonts degenerated within the crypt epithelial cells of the immune mice. In nonimmune mice, 3rd-generation merozoites, inoculated intracecally, developed into mature 4th-generation schizonts, whereas in immune mice the developing 4th-generation schizonts degenerated before maturing. A cell-mediated immune mechanism may be responsible for the arrest in schizogony.