Trypanosomiasis is a protozoal disease of horses and other mammals that is primarily caused by the blood parasite Trypanosoma evansi. The disease takes different forms in different regions and involves different Trypanosome species. It is primarily found in tropical and subtropical regions. However, it is considered by some to be an emerging disease at this time, because the geographic range in which it has been found is spreading.

Surra is transmitted by biting flies and reused needles, and may also be transmitted by ticks and bats.

Common signs of disease include fever, weight loss or muscle wasting, lethargy, swollen lymph nodes, edema and neurologic signs. This disease may also cause infertility and abortion in horses. Acute disease can cause death in a few days, while horses with the chronic form of this disease can live for months.

Mal de Caderas is Trypanosomiasis seen in Latin America. The signs are somewhat different. There are other variations on Trypanosomiasis depending on location and specific species of parasite.

In the United States, Trypanosomiasis is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

Diagnosis involves blood tests (direct blood exam for parasites and serology). No technique is entirely reliable.

Treatment includes the use of anti-protozoal drugs, but is not consistently effective.