a genus of parasitic protozoans of the family Theileriidae that parasitize cells of the reticuloendothelial system and the erythrocytes in animals.

Several species have been described as causing the disease theileriasis: T. annulata, T. parva, T. mutans, T. dispar, and T. sergenti in cattle; T. ovis, T. hirci, and T. recondita in sheep and goats; T. tarandirangiferis in reindeer; and T. cervi in other deer. T. annulata, T. sergenti, and T. tarandirangiferis are responsible for much economic damage in the USSR. Ixodids have been shown to be vectors of Theileria; the vectors of Theileria in deer have not yet been identified.

Theileria can survive in warm-blooded animals for years; in ticks, they survive for a single generation. The protozoans enter the body of a warm-blooded host with the saliva of the tick vector when the tick feeds. After reaching the nearest lymph nodes, they penetrate the reticuloendothelial cells. There they multiply by schizogony and form macroschizonts and microschizonts (Koch’s blue bodies) up to 30 micrometers in diameter and sometimes larger. They then penetrate other lymph nodes, parenchymatous organs, and the peripheral blood. Schizonts are also found outside of cells in cases of mass infection. On disintegration, a schizont produces a larger number of parasites that penetrate the erythrocytes. A single erythrocyte may contain one to four or, sometimes, more parasites. The parasites may be round (0.5–2 micrometers in diameter) or elongated (up to 4–5 micrometers long); other shapes have also been observed. The cytoplasm and nucleus of the parasite can be readily discerned in stained blood smears. The parasites reproduce in erythrocytes by dividing into two or four individuals. Theileria present in the blood of animals are the source of infection of ticks.