| Abstract: | Host responses to infections with serially diluted, unirradiated, infective Theileria annulata, tick tissue stabilates and irradiated stabilates, were compared and the resultant immunity tested against lethal homologous challenge. In infections due to unirradiated parasites duration of fever and proportion of schizontinfected cells in regional lymph nodes, and of parasitised erythrocytes in the peripheral circulation were directly related to the dose of the infective inoculum, whereas time responses, that is, times to first onset of fever and swelling of the regional lymph node, and to first appearance of parasites in the lymph node and blood were inversely related to the dose.Irradiation at 50 Gy did not appreciably affect the parasite. But in infections with the parasite irradiated at 100 Gy and 150 Gy, degrees of parasitisation were of reduced intensity and inversely proportional to the level of irradiation, whereas the time responses were unaffected except fever, onset of which was delayed. The minimal effect on time responses suggested that irradiation might have altered virulence rather than reduced the number of infective units in the inoculum.All the calves surviving the immunising infections fully resisted the challenge which produced severe theileriosis in
unimmunised calves killing 3 of them. It was concluded, therefore, that both methods of immunisation conferred comparable levels of immunity on the animals. |
