Reduction in pathogenicity of Babesia bovis for its tick vector, Boophilus microplus, after rapid blood passage in splenectomized calves.

Two strains of Babesia bovis that had been serially blood passaged in splenectomized calves 27 to 33 times, a procedure known to have reduced their virulence for normal cattle, were shown to have low pathogenicity for replete, female Boophilus microplus. In comparison with a strain of B. bovis unmodified by repeated blood passage, the two modified strains infected higher proportions of ticks and produced comparable numbers of morphologically similar parasites in their haemolymph, but killed significantly fewer of them. Red discolouration of haemolymph was observed in many ticks infected with the unmodified strain, but in none of those infected with the modified strains. It is suggested that the modified strains have lost a quality causing pathological effects on the gut cells of infected ticks.