Vaccination against tropical theileriosis.

Theileria annulata, the cause of tropical theileriosis is propagated in cattle with stage-to-stage transmission by Hyalomma ticks. Three stages in the life cycle of the parasite--tick-derived sporozoites, intramononuclear schizonts, and erythrocytic merozoites--infect cattle. When cattle are inoculated with schizont-infected cells, the parasite is transferred from the donor cell to the recipient. The main pathological damage in cattle is induced by the schizont stage. Each development stage of T. annulata elicits a specific immune response. Schizont-infected lymphoid cells can be grown indefinitely in culture and prolonged cultivation results in loss of virulence. Blood-derived schizonts induce stronger immunity than culture-derived schizonts, which suggests that restrictions on the parasite population or antigenic variation occur during prolonged cultivation. The duration of immunity following sporozoite or schizont infections has not yet been determined, but does not appear to be lifelong. The attenuated, culture-derived anti-theileria vaccine proved to be safe and effective in prevention of field theileriosis in large enzootic areas.