The irradiation of babesia bovis. II. The immunogenicity of irradiated blood parasites for intact cattle and splenectomised calves.

Intraerythrocytic forms of B. bovis were exposed to 350 Grays (Gy) gamma irradiation and were then injected intravenously into intact two and three year old Hereford steers. One of 15 steers died on initial infection and subsequently six steers were given a virulent heterologous challenge three weeks after recovery; all six animals were highly immune. The remaining eight animals were kept under quarantine conditions for 10 months and were then challenged with a different virulent heterologous strain of B. bovis. Seven of eight were highly immune, but one animal died. Subsequently a further 12 steers were injected intravenously with 1 X 10(8) irradiated organisms. All showed only mild transient clinical signs. After 12 months quarantine in a tick-free area these animals were then challenged with a virulent heterologous strain and all 12 were shown to be highly immune. Irradiation reduced the infective dose from 1 X 10(8) to 2.5 X 10(3) parasites. These parasites multiplied at the same rate, and achieved the same maximum parasitaemia as the parent non-irradiated strain, but the disease produced by them was not severe. A dose of 2.5 X 10(3) non-irradiated parasites was lethal to all of the four animals which received it. It was concluded that irradiation had produced a predominantly avirulent parasite population.