RESUMO
A Theileria parasite was isolated from cattle on a ranch in Kenya where it caused mild theileriosis in approximately one third of the cattle exposed to natural tick challenge. The parasite was isolated by inoculation of blood into two experimental cattle. Blood from one of these cattle was used to infect two splenectomised cattle which developed high piroplasm parasitaemias and severe anaemia. A blood stabilate was prepared from one of the splenectomised cattle and produced high parasitaemias in splenectomised cattle. Immunofluorescence tests showed that the unidentified Theileria species was distinct from other African Theileria and Babesia species of cattle. The Theileria species was shown to be antigenically distinct by means of species-specific monoclonal antibodies. The piroplasma stage was relatively large for a Theileria parasite and erythrocyte infections were usually associated with veils and bars. The salivary glands of Rhipicephalus appendiculatus, R pulchellus, R evertsi and Amblyomma variegatum fed on parasitaemic cattle did not become infected and these ticks failed to transmit the parasite. It was concluded that this parasite represented a new species of cattle Theileria in Kenya.