Study of virulence and vector transmission of Babesia bovis by use of cloned parasite lines.

Cloned lines of Babesia bovis were prepared from the avirulent vaccine strain, Ka, by an in vivo limiting dilution procedure. The virulence of these clones for adult Bos taurus cattle varied from completely avirulent to highly virulent. This suggests that the parent strain, Ka, is composed of a mixture of subpopulations of varied virulence. Passage of the avirulent clone K-19-47 in intact (nonsplenectomized) cattle resulted in its full reversion to virulence. This suggests that two mechanisms are operating to enable virulence to be a readily modified characteristic in this parasite: differential gene expression and phenotypic selection of subpopulations. A series of experiments demonstrated that all clones were non-tick transmissible. This lack of vector transmission was a stable characteristic and could not be altered by the passage of K-19-47 in intact cattle, despite the fact that passage in intact animals caused this clone to revert to the fully virulent phenotype. A mechanism is suggested for the virulence and vector transmission variations observed in B. bovis.