This species is morphologically indistinguishable from T. evansi. It causes a disease of horses and asses known as dourine. This is a venereal disease, transmitted by coitus. Dourine is similar to nagana, but runs a more chronic course of 6 months to 2 years. The incubation period is 2 to 12 weeks.
The first sign of the disease is edema of the genitalia and often of the dependent parts of the body. There is slight fever, inappetence, and a mucous discharge from the urethra and vagina. Circumscribed areas of the mucosa of the vulva or penis may become depigmented.
The second stage of the disease, characterized by urticaria, appears after 4 to 6 weeks. Circular, sharply circumscribed, urticarial plaques about 3 cm in diameter arise on the sides of the body, remain 3 or 4 days, and then disappear. They may reappear later. Muscular paralysis later ensues. The muscles of the nostrils and neck are affected first, but the paralysis spreads to the hind limbs and finally to the rest of the body. Incoordination is seen first, and is followed by complete paralysis. Dourine is usually fatal unless treated, altho mild strains of the parasite may occur in some regions.
T. equiperdum is found in Asia, North and South Africa, southern and eastern Europe and the U.S.S.R. It was once common in western Europe and North America, but has been eradicated from these regions. The last place where it was known to occur in North America, the Papago Indian Reservation in Arizona, was released from quarantine in 1949.
Dourine can be diagnosed by finding the parasites in smears of fluid expressed from the urticarial swellings, lymph, the mucous membranes of the genitalia or blood. The signs of the typical disease are characteristic enough to permit diagnosis in endemic areas. Inoculation of mice, rats, rabbits or dogs may also be practiced, but it is often difficult to demonstrate the parasites on the first passage. The complement fixation test is invaluable in detecting early or latent infections, and it was only by its use that dourine was eradicated from North America. All horses imported into the United States must be tested for dourine before they are admitted.
To treat dourine in horses, a single subcutaneous dose of 5 mg/kg antrycide methyl sulfate or two intravenous injections of 2 g suramin each 15 days apart can be used.