Progress towards a vaccine against Theileria parva: relevance for heartwater research

Such progress has been made in the last decade in the characterization of bovine immunity to Theileria parva. The evidence is overwhelming that cattle that become immune through infection can clear subsequent challenge infections by deploying parasite-specific cytotoxic T lymphocytes (CTL). Furthermore, high titres of neutralising antibodies are generated against the sporozoite surface after multiple exposure to the parasite, and these can neutralise infection in vitro. Although unlikely to be of relevance under natural circumstances, the latter observation has been exploited to generate a promising candidate neutralising vaccine based on a recombinant form of the major surface antigen of the T. parva sporozoites. Efforts are under way to identify the target antigen(s) of T. parva-specific CTL, and when this has been achieved, an improved vaccine targeted on both infective and pathogenic stages of the parasite will be within reach. The elucidation of the basis of immunity of ruminants to Cowdria ruminantium, the causal agent of heartwater, is at a comparatively early stage. However, many of the principles and techniques that have led to the current understanding of the immunology of T. parva are already being applied to heartwater, and these should enable rapid progress to be made in the development of a vaccine against C. ruminantium.