The aetiology, pathogenesis and control of theileriosis in domestic animals

The Theileria genus includes a large number of species of tick-borne parasites that infect domestic animals and wildlife species, predominantly ruminants. These range from species, such as T. parva and T. annulata, which cause acute lymphoproliferative diseases in cattle resulting in high levels of mortality, to others that are non-pathogenic. In the last decade, several new pathogenic species of Theileria have been identified and pathogenic strains of other previously low-pathogenic species have emerged. Theileria parasites are characterised by developmental stages within leukocytes and erythrocytes. The capacity of the most pathogenic species to undergo extensive multiplication during intra-leukocyte development is central to their ability to cause disease. However, this is not the sole property responsible for disease, as illustrated by T parva, which grows in a similar manner in buffalo cells but does not cause disease in this species. Because of the highly pathogenic nature of these parasites in livestock and the susceptibility of young animals to disease, control of the diseases is challenging. Control by chemotherapy and prevention of tick infestation has proved expensive and difficult to sustain. Vaccines using live parasites are available for T parva and T annulata and have been used with some success in the field. However, their widespread use has been hampered by practical constraints in production and distribution of the vaccines. Studies of the immune responses in immune cattle have helped to elucidate the protective immune responses and identified a number of parasite antigens that are currently being explored for development of alternative vaccines.