Relatively benign or mildly pathogenic strains of bovine Theileria, an intraerythrocytic protozoan parasite, have been reported in many parts of the world including T. mutans in Africa, T. buffeli in Australia, T. orientalis in Britain, and a Theileria sp. from the Southwestern United States (US). Although the Theileria found in the US has not been specifically speciated, it has been referred to as T. mutans-like and as T. orientalis (USA). Although cattle in the US infected with these Theileria spp. usually have a circulating parasitemia of less than 1% and are asymptomatic, recently an animal suffering from severe anemia was admitted to the Large Animal Clinic at Texas A&M University with a circulating parasitemia of over 50%.

Parasites in Giemsa stained blood films were pleomorphic in shape, with some parasites apparently connected to the erythrocyte membrane (Fig. 1) by structures which appeared similar to the bar structure found within eland (Taurotragus oryx) erythrocytes parasitized by T. taurotragi. Examination by TEM revealed that in addition to the main body of the parasite, parasitized erythrocytes contained parasites which possessed cytoplasmic extensions and an interaction between the parasite and the erythrocyte membrane which involved invaginations of the erythrocyte membrane which appeared to be surrounded by parasite cytoplasm (Fig. 2).