Increased resistance to Anaplasma marginale infection in cattle chronically infected with Theileria buffeli (syn. T. orientalis)

Calves chronically infected with the benign haemoprotozoan parasite Theileria buffeli (syn. T. orientalis) and T. buffeli-free calves were experimentally infected with virulent Anaplasma marginale. The daily mean maximum parasitaemia in the T. buffeli-carrier calves was lower and delayed relative to that the Theileria-free calves. Anaemia was less marked in the Theileria infected calves, although this difference was not statistically significant. The susceptibility of Theileria-carrier and Theileria-free older cattle to virulent A. marginale infection was also investigated. The mean maximum parasitaemia observed in the Theileria-infected cattle was significantly lower than that of the Theileria-free cattle and the time to maximum parasitaemia was increased significantly in the Theileria-infected relative to the Theileria-free cattle. Of the Theileria-carrier cattle, 33% exhibited maximum parasitaemias of less than 0.1% infected erythrocytes and no clinical anaemia as a result of A. marginale infection. In contrast, the lowest maximum parasitaemia observed in the Theileria-free cattle was 7%. The percentage of cattle requiring treatment to prevent mortality due to anaemia was 50% and 91% in the Theileria-infected and Theileria-free cattle respectively. For the duration of increasing A. marginale parasitaemia, the level of Theileria in carrier cattle was significantly depressed or undetectable. Following the resolution of peak A. marginale parasitaemia, the level of Theileria parasites increased rapidly to become significantly higher than that prior to infection and then decreased gradually a level similar to that prior to infection. The mechanism of the increased resistance to A. marginale infection conferred by T. buffeli-carrier state is unknown, but is likely to involve non-specific cell-mediated immunity, as no serological cross-reactivity exists between these two highly divergent parasite species. The susceptibility of relatively mature cattle to clinical anaplasmosis under field conditions is likely to be significantly affected by the widespread distribution and common occurrence of T. buffeli throughout the range of A. marginale in Australia, Africa and southeast Asia.