Equine coital exanthema is a benign venereal disease of horses that likely occurs worldwide. It affects both sexes and is caused by equine herpesvirus type 3 (EHV-3). This virus has a single antigenic type but also has small and large plaque variants in tissue culture, indicating that variation may occur in the severity of field outbreaks. Although the primary route of transmission is venereal, outbreaks have been documented in which transmission occurred via contaminated supplies and instruments or by the use of a single glove for rectal examination of many mares. It is probably for this reason that EHV-3 has also been isolated from horses that have not been bred.
Equine coital exanthema is probably transmitted only in the acute phase of the disease; after the lesions have healed, horses do not appear to shed the virus. However, the existence of a carrier state is unclear: the scars that persist after healing may identify potential carriers, but such asymptomatic carriers have not been identified. Immunity is short-lived, but evidence from stallions shows that recurrence is not likely within a single breeding season.
Clinical signs of equine coital exanthema in mares develop 4–8 days after sexual contact or veterinary examination and are manifest by the appearance of multiple, circular, red nodules up to 2 mm in diameter on the vulvar and vaginal mucosa, the clitoral sinus, and perineal skin. These lesions develop into vesicles and then pustules and eventually rupture, leaving shallow, painful, ulcerated areas that may coalesce into larger lesions. Edema can develop in the perineum and may extend to between the thighs. Occasionally, ulcers will be found on the teats, lips, and nasal mucosa. Secondary bacterial infection of the ulcers by Streptococcus spp is common, causing the ulcers to enlarge and exude a mucopurulent discharge. In such cases, the horse may become febrile. Unless secondary bacterial infection occurs, skin healing is complete within 3 weeks, but clitoral and vaginal ulcers heal more slowly. Skin lesions persist for long periods as unpigmented scars. However, pregnancy rates are not reduced.
Lesions in stallions are similar to those in mares and are found on both the penis and prepuce. As a result, intromission is painful, and the stallion may be reluctant to copulate. If copulation does occur during the ulcerative stage, the ulcers may hemorrhage into the ejaculate, reducing sperm viability.
A tentative diagnosis of equine coital exanthema is based on clinical signs and confirmed by identifying (using electron microscopy) the virus in cells from the margin of ulcers. Typical intranuclear herpesvirus inclusion bodies can also be seen in cytologic or histologic preparations. Acute and convalescent samples for serum neutralization or complement fixation tests can also be diagnostic, but these tests must be interpreted carefully because both EHV-1 and EHV-4 have also been isolated from genital lesions.
Sexual rest is essential to allow ulcers to heal and prevent spread of equine coital exanthema. The use of antibiotic ointments to prevent secondary infections is also advisable. Affected horses should be isolated until all lesions have healed, and disposable equipment should be used for examinations. During the acute 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 incubation period is up to 10 days.