Susceptibility to coccidiosis: contrasting course of primary infections with Eimeria vermiformis in BALB/c and C57/BL/6 mice is based on immune responses.

The effects of three immunosuppressive treatments--whole body irradiation and injections of cortisone acetate or cyclophosphamide, on the course of primary infections with Eimeria vermiformis were investigated in 'resistant' BALB/c and 'susceptible' C57BL/6 mice. Immunosuppression (and the nude athymic mutation in BALB/c mice) resulted in increased reproduction of the parasite in both strains of mice, indicating some immunological control of primary infections. The effect was, however, very much more marked in the BALB/c mice, resulting in an alteration in the relative susceptibilities of the two strains. The findings are discussed and it is suggested that the basis for the differences observed in the course of infection in normal BALB/c and C57BL/6 mice lies in their immune responses to this parasite.