The irradiation of Babesia bovis. 1. The difference in pathogenicity between irradiated and non-irradiated populations.

Babesia bovis parasites attenuated by 35 krads gamma irradiation and parasites not exposed to irradiation, were injected into intact 2-year-old Hereford steers. All five animals receiving non-irradiated blood died but the five animals which received irradiated blood were only mildly affected. Highly significant differences were observed in changes to plasma fibrinogen, serum fibrinogen-like proteins, packed cell volume, partial thromboplastin time, prothrombin time, blood kinins, and plasma kininogen levels in the control animals but non-significant changes in these parameters occurred in the group receiving iradiated blood. Significant changes in the antiplasmin alpha 2M, and the antithrombin levels occurred in control cattle but not in the group receiving irradiated blood. Parasite multiplications rates and maximum parasitaemias were similar in both groups. Irradiation reduced the dose of living parasites from 1 x 10(8) to 2.5 x 10(3), but this was not the reason for the mild reactions. It was concluded that irradiation had selected an avirulent parasite population.