Theileria parva: the nature of the immune response and its significance for immunoprophylaxis.

Theileria parva is a tick-borne haemoprotozoan parasite of cattle and buffalo which is responsible for considerable economic losses to cattle farming in Eastern, Central and Southern Africa. Infection with the parasite results in an acute lymphoproliferative disorder with high mortality, but animals which survive infection are solidly immune to homologous challenge. Such immunity can be reproduced by infecting cattle with the parasite and treating them with tetracyclines or theilericidal drugs, but the widespread use of this technique as a method of control is hindered by its dependence on live parasites which require cryopreservation for maintenance and can give rise to carrier states. In addition, cross-protection between different strains of the parasite is not absolute. These problems have prompted a search for methods of immunisation based on the use of inactivated parasites or their derivatives. Such efforts have met with consistent failure, however, and in recent years scientists have adopted the more rational approach of defining the immune responses of cattle to the parasite with a view to identifying the parasite components which provoke them. These studies have revealed that protection is likely to be mediated by parasite-specific cytotoxic T cells which are restricted by class I major histocompatibility complex (MHC) products. The significance of this observation to the development of an effective subunit vaccine is discussed in the light of current knowledge of the inductive requirements of cytotoxic T cells.