Infection by Theileria parasites limits the movement of cattle between countries and can result in production losses and high mortality in susceptible animals. Because these diseases are most severe in recently introduced animals, they are a constraint on the importation of new breeds or improved stock. The two diseases with the greatest economic impact in cattle are East Coast fever (infection with Theileria parva) and tropical theileriosis (infection with Theileria annulata). Theileria lestoquardi, which causes a severe disease with a high morbidity and mortality rate, is the most important species in sheep and goats.

Etiology

Theileriosis results from infection with protozoa in the genus Theileria of the suborder Piroplasmorina. Theileria spp. are obligate intracellular parasites. The two most important species in cattle and buffalo are T. parva, which causes East Coast fever, and T. annulata, which causes tropical theileriosis.
A number of other Theileria species including T mutans, T buffeli, T. velifera, T. taurotragi and T. sergenti can also infect domesticated and wild ruminants. Many of these organisms are carried asymptomatically, but some can cause anemia, and concurrent infections may increase the severity of East Coast fever or tropical theileriosis.
T lestoquardi (formerly T hirci) is the most virulent species in sheep and goats. T. separata and the nonpathogenic species T. ovis also occur in small ruminants.

Transmission

Theileria spp. are transmitted by ticks acting as biological vectors. Rhipicephalus appendiculatus is the most important vector for T. parva,  T. annulata is transmitted by ticks in the genus Hyalomma. Hyalomma spp. are also the vectors for T lestoquardi, T. ovis and T. separata, while T. buffeli and T. sergenti are transmitted by Haemaphysalis spp,. and T mutans and T. velifera are transmitted by Amblyomma spp. Ticks in the genus Rhipicephalus spread T. taurotragi.

Clinical Signs

In East Coast fever, the clinical signs include generalized lymphadenopathy, fever, anorexia and loss of condition with decreased milk yield. Petechiae and ecchymoses may be found on the conjunctiva and oral mucous membranes. Lacrimation, nasal discharge, corneal opacity and diarrhea can also be seen. Terminally ill animals often develop pulmonary edema, severe dyspnea and a frothy nasal discharge. Some cattle have a fatal condition called “turning sickness.” In this form of the disease, infected cells block capillaries in the central nervous system and cause neurological signs. Animals that recover from East Coast fever often become asymptomatic carriers, but some animals have poor productivity and their growth is stunted. Tropical theileriosis generally resembles East Coast fever, but these parasites also destroy red blood cells, causing jaundice, anemia, and in some cases, hemoglobinuria. Hemorrhagic diarrhea may be seen in the late stages. Petechiae are often found on the mucous membranes. Neurological signs have been documented in some terminally ill water buffalo, but “turning sickness” does not seem to be a feature of tropical theileriosis in cattle. Abortions can be seen.

Control

Theileriosis is not transmitted by casual contact. If the infection is newly introduced to an area, it might be eradicated with movement controls, by culling infected animals and by preventing ticks from becoming infected.
In endemic areas, the tick burden can be decreased with acaricides and other methods of tick control such as rotational grazing. The transfer of blood between animals must also be avoided. Antiparasitic drugs are effective in animals with clinical signs, but animals may remain carriers. Treatment is most effective in the early stages of the disease.  Animals can be protected from both East Coast fever and tropical theileriosis by vaccination. Attenuated vaccines are used to control tropical theileriosis in some countries.