Protection of cattle against Babesia major by the inoculation of irradiated piroplasms.

Two experiments were carried out on Babesia major in splenectomised calves. In the first one, the parasite was titrated and a linear relationship was observed between infective dose (10(2)--10(7) parasites) and onset of disease. In the second experiment aliquots of 1.2 X 10(10) parasites were irradiated at doses of 24, 28, 32, 36 and 40 krads and inoculated into groups of calves, while control calves received 1.2 X 10(7) or 1.2 X 10(4) parasites. Animals which received parasites irradiated at 24 or 28 krads had mild reactions and were immune to homologous challenge. Animals receiving parasites irradiated at the higher dose levels had limited ability to resist the challenge.