theileriasis, any of a group of livestock diseases caused by protozoan parasites of the genus Theileria (Gonderia), transmitted by tick bites. The most serious is East Coast fever of cattle, caused by T. parva; it has 90–100 percent mortality in Africa. Tropical theileriasis, from T. annulata (T. dispar), is a milder disease of cattle along the Mediterranean and in the Middle East. Theileriases of sheep and goats are mild diseases that occur in Africa, the Balkans, and the Caucasus. The elimination of ticks reduces the incidence of the disease. Infected animals are given drug therapy.