Members of this family are relatively small, round, ovoid, irregular or bacilliform parasites. They occur in the erythrocytes and lymphocytes or histiocytes. Schizogony takes place in the lymphocytes or histiocytes, and is followed by invasion of the erythrocytes. The forms in the erythrocytes may or may not reproduce; in the latter case they divide into 2 or 4 daughter cells. Reichenow (1940, 1953) maintained that schizogony does not occur in the vertebrate host but is simulated by repeated binary fissions. However, observations on the protozoa in tissue culture (Tsur-Tchernomoretz, 1945; Brocklesby and Hawking, 1958) indicate that schizogony does occur.
The vectors are ixodid ticks. Binary fission, schizogony and sexual reproduction have been said to occur in the tick, but the existence of sexual reproduction is dubious, and Reichenow (1940, 1953) believed that schizogony is simulated by repeated binary fissions.
Members of this family cause an important group of diseases, known collectively as theilerioses, in cattle, sheep and goats. These have caused heavy losses in Africa, southern Europe and Asia.
This group has been reviewed by Reichenow (1953), Poisson (1953), and most comprehensively by Neitz (1956, 1957, 1959). Most authors place all members of the family in the genus Theileria (e.g., Poisson, 1953), while some accept the genus Cytauxzoon as well (e.g., Reichenow, 1953). However, Neitz and Jansen (1956) divided the group into 3 genera on the basis of biological characteristics. They even placed them in 2 families in a new suborder Leucosporidea, but this latter treatment does not seem justified.
In the genus Theileria as redefined by Neitz and Jansen, the forms in the erythrocytes do not divide, the parasites cannot be transmitted by blood inoculation, and recovered animals do not remain carriers (i.e., there is no premunition). In the redefined genus Gonderia, the forms in the erythrocytes do divide, the parasites can be transmitted by blood inoculation, and recovered animals remain carriers for life (i.e., premunition is present). In the genus Cytauxzoon, schizogony takes place in the histiocytes rather than in the lymphocytes as in the other 2 genera, and the forms in the erythrocytes reproduce by binary fission.
Neitz (1959) recognized 1 species of Theileria and 5 of Gonderia in domestic animals, and 10 named and 36 unnamed species of Theileria, 1 of Gonderia and 2 of Cytauxzoon in wild animals. The great majority occur in African ruminants. They are all tabulated by Neitz (1957). Since the forms in the erythrocytes of all 3 genera look alike, and since practically all the species in wild animals are known only from these forms, their assignment to the genus Theileria is clearly provisional.