Theileria spp. are apicomplexan protozoa in the order Piroplasmida, family Theileridae, and genus Theileria. These are obligate intracellular tick-borne pathogens. Novel, unnamed species of Theileria are frequently being discovered, especially in wild African mammals. Wildlife serve as reservoir hosts for several Theileria species that can ultimately spread to livestock. Of particular concern to domestic species are T. parva (which causes East Coast fever), T. annulata (which causes tropical theileriosis), and T. orientalis (which causes Theileria-associated bovine anaemia or Oriental theileriosis).
For the purpose of voluntary reporting on non OIE-notifiable disease in wildlife, “theileriosis in wildlife” refers to theileriosis in all non-domestic species. Information on infections of theileriosis in domestic species is discussed individually in a separate technical card.