Dourine is a chronic or acute contagious disease of breeding equids that is transmitted directly from animal to animal during coitus. The causal organism is Trypanosoma (Trypanozoon) equiperdum. Trypanosoma equiperdum is primarily a tissue parasite that is rarely detected in the blood. There is no known natural reservoir of the parasite other than infected equids. It is present in the genital secretions of both infected males and females. The incubation period, severity, and duration of the disease vary considerably; it is often fatal, however spontaneous recoveries do occur as do latent carriers and subclinical infections. Donkeys and mules are more resistant than horses and may remain inapparent carriers. Although adaptation to other hosts is not always possible, dogs, rabbits, rats and mice can be infected experimentally and be used to isolate and maintain strains of the parasite indefinitely. The clinical signs are marked by periodic exacerbation and relapse, ending in death, sometimes after paraplegia, or, possibly, recovery. Moderate fever, local oedema of the genitalia and mammary glands, cutaneous eruptions, incoordination, facial and lip paralysis, ocular lesions, anaemia, and emaciation may all be observed. Oedematous cutaneous plaques, 5–8 cm in diameter and 1 cm thick, are still considered as pathognomonic, although they are also occasionally found in equids infected with T. evansi. There are no vaccines available for this parasite. The only effective control is through the slaughter of infected animals. Good hygiene is essential during assisted mating because infection may be transmitted through contaminated fomites.