Bovine immunity - a driver for diversity in Theileria parasites?

Theileria parvar and Theileria annulata are tick-borne parasites of cattle that infect and transform leukocytes, causing severe and often fatal parasitic leukoses. Both species provoke strong immunity against subsequent infection. However, considerable diversity is observed in field populations of each parasite and protection is only assured against homologous challenge. The life cycles of these parasites are complex and involve prolonged exposure to host and vector defence mechanisms. Although the relevant vector mechanisms are poorly defined, protective responses of cattle seem to be tightly focused and variable in their specificity between individuals. This review considers whether bovine immunity acts as a driver for diversity in T. parva and T. annulata and explores other factors that might underlie genetic variation in these parasites.