Surra is caused by infection with Trypanosoma (T.) evansi, an insect-borne unicellular parasite.
Surra is a disease mainly found in horses and camels. T. evansi is closely related to other members of the Trypanozoon subgenus, T. brucei (T. brucei brucei, T. b. gambiense and T. b. rhodesiense) and T. equiperdum (the causative agent of dourine).
Affected animals develop high fever, progressive anemia, weight loss, jaundice, weakness and lethargy, and edema, which is seen primarily in distal areas of the body. In horses, damage to the nervous system, ataxia, and paralysis, especially of the hindquarters, may also occur. Trypanosoma evansi can cause abortions in buffaloes and camels. The disease is often fatal in camels and horses. In infected animals, death can occur after only 2-4 weeks; in a chronic course, up to 2 years can pass before the animal dies.
Trypanosoma evansi occurs in Africa, Asia, Central and South America. In addition to horses and camels, donkeys, mules, llamas, buffaloes, cattle, sheep, goats, dogs, cats and other animal species (e.g. bats in South America) are also affected. In very rare cases, humans can also become infected.
Transmission occurs mechanically by blood-sucking insects and vampire bats. Insects that can transmit the infection include mainly Tabanus spp. (horseflies) but also Musca spp. (true flies). Carnivorous animals can become infected by feeding on infected meat. Vertical transmission (transplacental or via colostrum and milk) to offspring appears possible.