Babesia divergens: combination of dead and live parasites in an irradiated vaccine.

Two experiments were carried out to investigate the hypothesis that the protective effect of an irradiated Babesia divergens vaccine was due to the combined inoculation of large numbers of dead parasites and small number of live ones. In the first experiment it was demonstrated that animals receiving 10(8) parasites irradiated at 25 kilorads had more severe B divergens reactions than animals in a previous experiment which had received 10(10) parasites irradiated at 24, 28 or 32 kilorads. In the second experiment it was shown that the simultaneous inoculation of 10(10) parasites killed by irradiation at 50 kilorads and 10(8) parasites irradiated at 25 kilorads resulted in the suppression of the relatively severe reactions induced by inoculation of the 25 kilorad-irradiated parasites alone. It was also shown that the simultaneous inoculation of 10(10) parasites irradiated at 50 kilorads and 10(4) live parasites similarly suppressed the anticipated reactions of the calves. These results indicated that the hypothesis was correct under the conditions of the experiment.