Theileria is a genus comprising tick-transmitted parasitic protozoa in the family Theileriidae, order Piroplasmida, subclass Piroplasmia, phylum Apicomplexa. It includes parasites previously allocated to the genera Cytauxzoon, Gonderia and Haematoxenus. The taxonomy of the genus at species level is controversial.
The disease caused by Theileria species in wildlife was initially referred to as cytauxzoonosis caused by various Cytauxzoon species . The Cytauxzoon species (C. strepsicerosi, C. sylvicaprae, and C. taurotragi) has been reclassified as Theileria and the disease is currently referred to as theileriosis in the various species of wildlife that become infected and manifest clinical disease. Various species of wildlife (including African buffaloes, impala, blue wildebeest, tsessebe, eland, lechwe, kudu, and sable antelope)  may be asymptomatic carriers of the organism, even though some of them, under different circumstances may succumb of the infection
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