Surra is caused by infection with Trypanosoma (T.) evansi, a single-celled parasite.
Many domestic animals and some wild species are susceptible: horses, mules, donkeys, cattle, buffaloes, camels, llamas, pigs, sheep, goats, dogs, cats, deer and capybaras. Depending on the geographical region, the main hosts vary: most affected in South America are horses; in China, horses, mules, buffaloes and deer; in Southeast Asia, horses, cattle, buffaloes; and in the Middle East and Africa, camels.
Transmission is by bloodsucking flies including Tabanus spp. (horseflies) and Musca spp. (true flies), with the parasite most commonly transmitted by horseflies. Trypanosomes survive only a short time outside their host. Flies can no longer transmit the pathogen after about 8 hours. Vampire bats (in South and Central America) are hosts, reservoirs, and vectors. They transmit T. evansi with their saliva and can develop high parasitemia. This can also lead to death of the bats. Surviving bats can further transmit the pathogen (carriers). Carnivores can become infected via infected meat.
The incubation period varies between 5-60 days, there is high fever, progressive anaemia, weight loss, jaundice, weakness and lethargy, oedema (especially in the lower part of the body). In horses, it can also cause damage to the nervous system, ataxia, paralysis, especially of the hindquarters, and even death. Trypanosoma evansi can also cause abortion in buffaloes and camels. The disease is often fatal in camels and horses. Death can occur after 2-4 weeks, in the case of chronic infection up to 2 years can pass.