Equine coital exanthema is a benign venereal disease of horses caused by equine herpesvirus 3. Clinical signs include multiple, circular, red nodules on the genitalia of both mares and stallions. Diagnosis is suspected based on clinical signs and confirmed by PCR assay or electron microscopy. There are no vaccines or specific treatments. Sexual rest is recommended to allow lesions to heal and to prevent disease transmission.
Equine coital exanthema was first observed in the early 1900s in Ireland. Owing to its highly contagious nature, as well as to the global distribution of its causative agent and lack of a preventive vaccine, it now occurs nearly everywhere breeding horse populations exist. Infection does not lead to systemic illness, infertility, or abortion; however, the disease can negatively affect the equine industry, because infected horses must be temporarily withdrawn from active breeding, which can delay conception and increase expenses.
Etiology and Epidemiology of Equine Coital Exanthema
Equine coital exanthema likely occurs worldwide. It affects both sexes and is caused by equine herpesvirus 3 (EHV-3). This virus has a single antigenic type but also has small- and large-plaque variants in tissue culture, indicating that field outbreaks may vary in severity. Although the primary route of transmission is venereal, outbreaks can occur if the virus is transmitted via contaminated supplies and instruments or by use of a single glove for rectal examination of many mares. Probably for this reason, EHV-3 has been isolated from horses that have not been bred.
Equine coital exanthema is most likely transmitted only in the acute phase; after lesions have healed, horses do not appear to shed the virus. However, the existence of a carrier state is unclear: scars that persist after healing may identify potential carriers, but such subclinical carriers have not been identified. Immunity is short-lived, yet evidence from stallions shows that recurrence is not likely within a single breeding season.
Clinical Findings of Equine Coital Exanthema
Clinical signs of equine coital exanthema in mares develop 4–8 days after sexual contact or cross contamination (eg, during reproductive examination or breeding procedures) and manifest as multiple, circular, red nodules up to 2 mm in diameter on the vulvar and vaginal mucosae, clitoral sinus, and perineal skin. These lesions develop into vesicles and then pustules and eventually rupture, leaving shallow, painful, ulcerated areas that can coalesce into larger lesions (see image). Edema can develop in the perineum and can extend to between the thighs. Occasionally, ulcers develop on the teats, lips, and nasal mucosa. Secondary bacterial infection of ulcers by Streptococcus spp is common, causing ulcers to enlarge and exude mucopurulent discharge. In such cases, the horse can become febrile. Unless secondary bacterial infection occurs, skin heals within 3 weeks; however, clitoral and vaginal ulcers heal more slowly. Skin lesions persist for long periods as unpigmented scars; however, pregnancy rates are not decreased.
Courtesy of Dr. Thomas Lane.
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, ulcers can hemorrhage into the ejaculate, decreasing sperm viability.
Diagnosis of Equine Coital Exanthema
Presumptive: clinical signs
Definitive: PCR assay or electron microscopy
Diagnosis of equine coital exanthema is suspected based on clinical signs but should be confirmed by laboratory analyses. Molecular detection of EHV-3 by PCR assay is the most sensitive, specific, and accurate method for assessing infectivity (1, 2). Samples for laboratory submission can include the following:
0.2 mL whole blood in an EDTA tube
lesion swab
0.2 mL fresh, frozen, or fixed tissue
Additional diagnostic methods, using cells taken from ulcer margins, include 1) electron microscopy for identifying the virus in cells and 2) cytological or histological analysis of preparations for identifying typical intranuclear herpesvirus inclusion bodies.
In acute and convalescent patients, samples for serum neutralization or complement fixation tests can also be diagnostic; however, test results must be interpreted carefully, because both EHV-1 and EHV-4 can also be isolated from genital lesions.
Treatment and Prevention of Equine Coital Exanthema
Sexual rest
To prevent venereal transmission of EHV-3, sexual rest is required until lesions heal. Application of antimicrobial ointments to ulcers to prevent secondary infections is advisable.
Horses with equine coital exanthema should be isolated until all lesions have healed, and disposable equipment should be used for examinations. During the acute phase of disease, mares should be bred only by artificial insemination. All horses should be examined carefully before they are allowed to breed, and the incubation period of up to 10 days should be taken into account.
No vaccine is available.
Key Points
Equine coital exanthema is a venereal disease of horses caused by equine herpesvirus 3; it likely occurs worldwide.
Presumptive diagnosis is based on clinical signs; definitive diagnosis is made by PCR assay or electron microscopy.
Sexual rest is needed to allow ulcers to heal and to prevent transmission.
For More Information
Scott DW, Miller WH. Viral and protozoal skin diseases. In: Equine Dermatology. 2nd ed. Elsevier Saunders; 2010:chap 7.
Also see pet owner content regarding equine coital exanthema.
References
Dynon K, Varrasso A, Ficorilli N, et al. Identification of equine herpesvirus 3 (equine coital exanthema virus), equine gammaherpesviruses 2 and 5, equine adenoviruses 1 and 2, equine arteritis virus and equine rhinitis A virus by polymerase chain reaction. Aust Vet J. 2001;79(10):695-702. doi:10.1111/j.1751-0813.2001.tb10674.x
Kleiboeker SB, Chapman RK. Detection of equine herpesvirus 3 in equine skin lesions by polymerase chain reaction. J Vet Diagn Invest. 2004;16(1):74-79. doi:10.1177/104063870401600113
