Virulence and heterologous strain immunity of South African and Australian Babesia bovis strains with reduced pathogenicity.

A South African Babesia bovis strain showed loss of virulence after 10 rapid passages in splenectomized calves. The virulence was comparable with that of an attenuated Australian vaccine strain. Vaccination of cross-bred Bos indicus cattle with the local strain resulted in a solid immunity to heterologous challenge. The degree of protection afforded by the Australian vaccine strain was adequate for controlling challenge with a virulent South African strain, but somewhat less than the degree conferred by the local vaccine strain. Serological observations with the indirect fluorescent antibody test confirmed a close relationship between the 2 modified strains.