Bovine babesiosis: pathogenicity and heterologous species immunity of tick-borne Babesia bovis and B bigemina infections.

A total of 22 nonsplenectomized Hereford, Holstein-Friesian, and cross-bred 18-month-old cattle were used to assess the degree of protection conferred by previous infection with Babesia bovis or B bigemina against infection and disease from tick-borne challenge exposure with the heterologous species. Prior infection with B bigemina or B bovis did not significantly (P less than 0.05) reduce the susceptibility of cattle to tick-borne infection and disease caused by the heterologous species. Carrier infections were not activated during heterologous species challenge exposure, nor was the severity of the challenge-exposure infection aggravated. Immunologic cross reactivity in the indirect fluorescent antibody test was restricted to the period during and shortly after recovery. Homologous indirect fluorescent antibody titers persisted long after primary infections with B bovis, but gradually declined in B bigemina-immune cattle. Babesia bovis infections caused severe reactions characterized by high fever, pancytopenia, and death of 9 of 15 infected cattle. Total serum bilirubin and urea nitrogen values increased markedly within 24 hours preceding death and were often accompanied by clinical signs of CNS involvement (incoordination, opisthotonos, and paddling). Microscopic lesions were restricted largely to kidney, CNS, and liver parenchyma and were characterized by vascular congestion and aggregation of infected erythrocytes in small vessels. Babesia bigemina, in contrast, caused only mild fever and anemia despite prolonged parasitemia and marked thrombocytopenia. The absence of significant (P less than 0.05) heterologous species immunity indicated that protection of cattle from these 2 babesial parasites will require specific immunization against each species.