Equine Coital Exanthema (Horse Pox) in Horses

Equine coital exanthema is caused by a specific herpes virus, namely herpesvirus – 3.  It is relatively common, occurs worldwide, and in most cases is a self-limited and benign disease.  It affects the genital area of the horse, and does not cause any permanent damage to the reproductive organs.  It is a localized infection that does not spread systemically. Although it is primarily considered a sexually transmitted disease, it can be spread through direct contact with mucous membranes or through contaminated veterinary supplies including equipment, gloves, and breeding materials. In most cases, the disease is spread during sexual contact or breeding, and the incubation period can be less than 2 days. In mares, red bumps develop within the vagina, in the internal and external labia, the clitoris, and perineal area.  Stallions develop red lesions on the penis or on the prepuce.  In both mares and stallions, the bumps develop into fluid filled blisters that then rupture leaving open raw lesions.  If uncomplicated, these lesions typically crust over and heal within three weeks. In some cases, secondary infection develops on the lesions which necessitates topical or systemic antibiotics which prolongs the healing time. While lesions are healing, 3 weeks of sexual rest is recommended.  If the mare is ready for breeding, artificial insemination is recommended.  There appears to be no correlation between infection and abortion.  Prevention is key is limiting contact between affected individuals by isolation, cessation of breeding, and mindful veterinary care.

Coital exanthema, also referred to as horse pox, is a sexually transmissible disease caused by a specific strain of herpes virus. The virus causes inflammation, pain, lesions, and fluid filled blisters of the genital areas of mares and stallions.

• Low grade fever
• Vaginal discharge
• Stallions are reluctant to mate 
• Inflammation of the genital area
• Fluid filled sacs, blisters, or fluid filled vesicles
• Lumps or pustules on the female genitalia including the inner and outer labia and the clitoris 
• Lumps or pustules on the penis, foreskin, and perineum of the stallion
• Tenderness in the genital area
• Loss of libido in stallions
• White penile discharge
• Frequent urination in the stallion
• Arching of the back while urinating in the stallion
• Vulvar discharge in mares
• Swelling of the vulvar area that can extend down into the thighs
• Lesions can also be found on the teats, lips, or in the nose

Type 

Coital exanthema is caused by a specific herpes virus, and its clinical presentation is consistent amongst affected horses. In the case in which a secondary infection develops on the lesions, treatment and healing are more complicated.

• Equine Herpes Virus 3 
• Sexual contact with an infected horse is the most common means of acquiring the disease
• Infection can also be spread via the use of contaminated veterinary equipment, breeding tools, examination sleeves, gloves, or hands
• Direct contact of the lips or nose of an infected horse

Diagnosis can often be made by clinical examination of the genital area of the horse and recent history.  Horses that have recently been bred, been examined, or have been in contact with an infected horse further support the diagnosis of coital exanthema. There are two laboratory methods of confirming the diagnosis: sequential serology testing and isolation of the virus from cultures of the genital lesions.  

Mares usually develop small red bumps in the vulva within 4-8 days after breeding. These bumps then develop into fluid filled vesicles that will rupture leaving excoriated lesions. If secondary infection develops, the lesions become purulent and drain pus.  At times, the mares will develop fever with secondary infection. Uncomplicated lesions will usually heal within three weeks, although lesions of the vagina and clitoris may heal somewhat slower. Scarring may remain at the site of the lesions, but they will not interfere with future pregnancies or future fertility.

Lesions on the stallions are similar in appearance, but are very painful. The stallion may refuse to copulate when he has active lesions, and sperm count may be affected by the bleeding of the vesicles.

Three weeks of sexual rest is recommended. If the mare is ready to be bred, only artificial insemination should be used. Antibiotic ointment should be applied to prevent secondary infections from developing on lesions.

Antiseptics should be used for cleansing and to encourage drying of the lesions. Anti-inflammatory agents can be used to decrease discomfort. In severe cases, antibiotics may be needed. While corticosteroids can be used to decrease inflammation, their use may delay healing of the lesions.

Healing and recovery from horse pox is usually uncomplicated, and vesicles heal within three weeks. Healing can be delayed if secondary infection develops on the lesion.  The disease does not impact fertility of the mare or stallion.  Given that there is no vaccine for this relatively common infection, prevention is of utmost importance.  

Prevention Recommendations

• Careful examination for the presence of lesions or inflammation of the genital area prior to breeding given that incubation period is approximately 10 days after exposure to the virus
• Heightened awareness and strict sanitary methods used by those examining the horses.  The use of disposable gloves and sleeves are encouraged
• Mare preparation chutes should be thoroughly cleaned between mares to prevent possible infection
• The penis and prepuce of the stallion should be rinsed with plain warm water between each mating
• All buckets, instruments, and tools should be sterilized between breeding procedures 
• Isolation of mares and stallions with active lesions must be in place