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What is Dourine?

Dourine, also known as covering sickness, is a serious and often fatal infestation by the protozoa Trypanosoma equiperdum. This disease gets its name of covering sickness from the fact that it is spread during breeding, or covering. It affects only horses, donkeys, and mules, and donkeys and mules appear to be more resistant to the parasite than horses. It is a slow and progressive disease that is marked by periods of apparent recovery followed by relapse, usually ending in death. This infestation has been eradicated in many parts of the world and should be promptly reported to authorities if it manifests.

Dourine is an often fatal venereal disease caused by the protozoa Trypanosoma equiperdum. It has been eradicated in many parts of the world but remains a threat in developing countries.

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Symptoms of Dourine in Horses

Symptoms from dourine, also known as covering sickness, occur long after infection, usually several months to years after being exposed and tend to disappear and then reappear. 

  • Anemia
  • Conjunctivitis
  • Elevated tail
  • Facial paralysis
  • Fever
  • Knuckling of the joints
  • Lesions on the eyes
  • Lying dow
  • Paralysis
  • Paraphimosis
  • Progressive weakness
  • Restlessness
  • Shifting of weight from leg to leg
  • Skin plaques
  • Swelling of the vagina
  • Swelling of the vulva
  • Ulcers or vesicles on the genitalia that leave white scars when healed
  • Vaginal discharge in mares
  • Variable edema of the prepuce and glans penis in stallions


There are three known strains of protozoa in the Trypanosoma family. 

Trypanosoma equiperdum - This is the protozoa responsible for dourine, also known as covering sickness.

Trypanosoma evansi - This protozoa is responsible for a disease known as surra, which can affect not only horses, donkeys, and mules, but also cattle, buffalo, deer, camels, llamas, dogs, and cats. It is transmitted by horsefly and vampire bat, and unless treatment is sought, it is usually fatal.

Trypanosoma brucei - This is the only member of the Trypanosoma family that can affect humans as well as other vertebrates. Animals who are infested by this protozoa are afflicted with a disease called nagana, but when people are infected with this organism, it is known as sleeping sickness.

Causes of Dourine in Horses

This protozoan is transmitted almost exclusively through mating as it has a poor survival rating outside of its host. It is more common for the stallion to pass it to the mare, but it is also transferrable from the mare to the stallion. Although rare, infected mares can pass it on to their offspring through milk. Covering sickness has been eliminated in many places but still occurs in Asia, Africa, Russia, South America, the Middle East, and parts of southeastern Europe.

Diagnosis of Dourine in Horses

Where lesions are present, skin scrapings may be taken for microscopic examination, however, the trypanosomes are usually sparsely scattered and difficult to locate. The presence of T. equiperdum is generally diagnosed by clinical signs and a serology test. When this parasite is found in the equine’s blood serum, additional testing may be requested to confirm the diagnosis. The complement fixation assay can be used to confirm an active infection or to uncover latent infections.

If the results of the assay test are returned as inconclusive or nonspecific, an indirect fluorescent antibody test will be requested, although cross-reactions can occur if the animal is infested with one of the other varieties of Trypanosoma, particularly in areas where T. evansi is found. Results from both donkeys and mules are more likely to inconsistent responses than results for horses. The structure of T. equiperdum and T. evansi is remarkably similar and makes it difficult to differentiate between the two, even by microscopic evaluation.

Treatment of Dourine in Horses

In countries where this disease has been eradicated, such as the United States, euthanization is recommended to avoid a resurgence of this deadly disorder. There are drugs on the market that can help treat the signs and symptoms of dourine but their effectiveness against T. equiperdum is often inadequate and relapses frequently occur even when treatment is attempted. The mortality rate for horses that contract dourine is high, although there have been reported cases of spontaneous recovery.

In some cases, infected stallions that have recovered have been gelded in an attempt to prevent the spread of this disease, but it has been proven futile because even after gelding they can pass on the parasite if they display copulatory behavior. This parasite is very sturdy within its host, and vaccines against this disease have been unsuccessful thus far. It is this inability to create either a cure or an effective vaccine that leads The World Organization for Animal Health to recommend the euthanization of any horses, donkeys, or mules that test positive for T. equiperdum.

Recovery of Dourine in Horses

Prognosis for horses that contract this venereal disease is poor, although the severity and duration of the illness are variable depending on the particular strain that is introduced, as well as the horse’s current health. Many animals seem to progress to the end stages of dourine infestation within about one to two months once signs are apparent, however, horses who were infected experimentally have been known to live up to a decade.  Animals that overcome this illness remain infectious even when asymptomatic and reactivation of the symptoms is common when physical or emotional stressors are introduced into the patient’s environment.