Theileria Infections other than East Coast Fever

Abstract

Cattle are infected by at least five species of Theileria other than T. parva, of which T. annulata, the cause of Mediterranean or tropical theileriosis, is by far the most important as a disease problem. Other species are of low pathogenicity (T. mutans, T. taurotragi) or avirulent (T. velifera) and cause benign theileriosis while other yet unidentified, mostly non pathogenic theilerias occur on all continents and are possibly related to the T. sergenti/T. orientalis group of the far eastern USSR, the agents of East Asian theileriosis. Hyalomma ticks are the vectors of T. annulata while mainly Amblyomma, Rhipicephalus and Haemaphysalis spp. are involved in the transmission of other species of the organism.

Imported European breeds are far more susceptible to Mediterranean theileriosis than local cattle in endemic regions, which may live with the disease in a state of endemic stability.

Mediterranean theileriosis is an acute febrile disease in susceptible cattle, with a mortality rate which may reach 70%. Involvement of lymphoid tissue is caused by invasion by the schizontic stage, anemia is associated with the erythrocytic piroplasm stage. The course, the post-mortem lesions and the pathogenesis of the various theilerial infections are reviewed.

Recovery from infection by all species concerned is followed by the piroplasm carrier state, but persistence of the parasite is not necessary for immunity, which may last several years. Humoral antibodies are unimportant in protection and immunity is likely to be cell-mediated.

Presumptive diagnosis on clinical signs and post mortem lesions require parasitological confirmation based on demonstrating schizonts and/or a high parasitemia. In addition to morphological characteristics, serological techniques are necessary for species differentiation.

Intensive short-interval acaricidal treatment prevents transmission of theileriosis but there are numerous problems associated with tick control, and effective methods of immunization and treatment are required. Immunization against T. annulata is carried out in some countries, using attenuated schizonts grown in cell culture. Sporozoite vaccine promises to be even more effective. The occurrence of immunologically different strains is a complicating factor in immunization. Certain antimalarial drugs affect the erythrocytic stage of Theileria, while the only schizonticidal drug on the market is the recent coccidiostat halofuginone.