Exposure of cattle immunised against redwater to tick-induced challenge in the field: challenge by a heterologous strain of Babesia divergens.

Eighteen, one-and-a-half-year-old Friesian heifers and a three-year-old Friesian bull were immunised against bovine redwater by inoculation with blood infected with a known strain of Babesia divergens which had been irradiated at 30 kilorads. Three weeks later these cattle plus 10 Friesian bullocks of the same age which served as controls were introduced to a grazing area infested with ticks able to transmit infections of a heterologous strain of B divergens. The control cattle all became infected with the heterologous strain, two severely. None of the inoculated cattle became clinically affected.