Babesia divergens in splenectomised calves: titration of the infective dose.

Two experiments were carried out in which Babesia divergens was titrated in splenectomised calves. In the first experiment, a linear relationship was observed between infective dose (10(5)--10(9)) parasites) and onset of disease. In the second experiment, a similar trend was observed for infective doses of 10(2)--10(6) parasites, but 10(3) parasites appeared to be the minimum dose required to produce overt disease or resistance to homologous challenge in inoculated animals.