Equine coital exanthema is a benign (noncancerous) sexually transmitted disease of horses that probably occurs worldwide. It affects both sexes and is caused by equine herpesvirus type 3. Although the primary route of transmission is through sexual activity, outbreaks have been documented in which transmission occurred through contaminated supplies and instruments or by the use of a single glove for rectal examination of numerous mares. The infection is probably transmitted only in the early phase of the disease; after the sores have healed, horses do not appear to transmit the virus. Immunity against a second infection is very short, but there is little evidence of recurrence within a single breeding season.
Signs in mares develop 4 to 8 days after sexual contact or veterinary examination. Signs include the appearance of multiple, circular, red spots on the vulva and vagina. These may become infected by bacteria and develop into abscesses. The abscesses eventually rupture, leaving shallow, painful sores. Occasionally these spots may be found on the teats, lips, and nasal tissue (mucosa). Unless another bacterial infection occurs, the skin heals within 3 weeks, but spots in the vagina heal more slowly. Affected areas in stallions are similar to those in mares and are found on both the penis and prepuce. As a result, the stallion may be reluctant to copulate. Diagnosis is based on physical examination and blood tests.
Sexual rest is essential to allow affected spots to heal and prevent the spread of the disease. The use of antibiotic ointments to prevent secondary bacterial infections is also recommended. Affected horses should be isolated until healed, and disposable equipment should be used for examinations. During the early phase of the disease, mares should be bred only by artificial insemination. No vaccine is available. All horses should be examined carefully before they are allowed to breed, keeping in mind that the signs of the infection may not appear for up to 10 days.