Dourine is caused by the protozoan parasite, Trypanosoma equiperdum, which unlike other trypanosomal infections, is sexually transmitted during natural mating or by artificial insemination (AI) with infected semen. Transmission from stallions to mares is more common, but mares can also transmit the disease to stallions. T. equiperdum can be found in the vaginal secretions of infected mares and the seminal fluid, mucous exudate of the penis, and sheath of stallions. Periodically, the parasites disappear from the genital tract and the animal becomes noninfectious for weeks to months. Transmission is most likely early in the disease process as non-infectious periods are more common late in the disease. Male (jack) donkeys can become asymptomatic carriers and sexually immature jacks that become infected can transmit the organism when they mature.
Rarely, infected mares pass the infection to their foals, possibly before birth or through colostrum and milk, and infections may also be acquired through mucous membranes such as the conjunctivae. There is currently no evidence that arthropod vectors play a significant role in transmission of dourine, but this possibility cannot be ruled out.
