African animal trypanosomiasis (AAT) is a parasitic disease that causes serious economic losses in livestock from anemia, loss of condition and emaciation. Many untreated cases are fatal. AAT is found mainly in those regions of Africa where its biological vector, the tsetse fly, exists. One organism, Trypanosome vivax, has become established in South America, where it is transmitted by biting flies acting as mechanical vectors. Protecting animals from trypanosomiasis is difficult in endemic areas, as bites from tsetse flies and a variety of other insects must be prevented. A tsetse fly eradication program being conducted in Africa may help control this disease, as well as other forms of trypanosomiasis that affect humans.