SPECIFIC IMMUNE-RESPONSES ARE REQUIRED TO CONTROL PARASITEMIA IN BABESIA-EQUI INFECTION

Horses possessing a normal immune system and spleen often control infe ction caused by Babesia equi. However, splenectomized horses are unabl e to control B. equi infection and usually succumb to the infection. T o investigate the role of the spleen in the control of B. equi infecti on in the absence of specific immune responses, two 1-month-old foals with severe combined immunodeficiency (SCID) and two age-matched norma l foals were inoculated with B. equi. The SCID foals became febrile se ven days postinoculation and developed terminal parasitemias of 41 and 29%. The SCID foals had greater than 50% decreases in indices of tota l erythrocytes, packed-cell volumes, and hemoglobin concentrations. Bo th SCID foals were euthanized in extremis at 10 days postinoculation. As expected, the serum of the SCID foals lacked detectable antibodies to B. equi antigens. In contrast, the normal foals inoculated with B. equi produced detectable anti-erythrocyte-stage parasite antibodies by 7 days and controlled clinical disease by 12 days postinoculation. Al though SCID foals lack functional T and B lymphocytes, they do possess complement, macrophages, granulocytes, and natural killer cells, as w ell as a spleen. Therefore, the data indicate that specific immune res ponses are required to control B. equi parasitemia but are not require d for erythrocyte lysis in infected horses. Furthermore, the spleen is not able to control B. equi parasitemia in the absence of specific im mune responses to parasite antigens.