Theileria is a genus of parasites that belongs to the phylum Apicomplexa, and is closely related to Plasmodium. Two Theileria species, T. annulata and T. parva, are important cattle parasites. Theileria equi is a known cause of equine piroplasmosis. T. annulata causes tropical theileriosis and T. parva causes East Coast fever. Theileria species are transmitted by ticks.