How herpes virus affects breeding stock

HV-3 strain has no long-term effects, but precautions must be taken to prevent secondary infectionIn recent weeks, at least six cases of equine genital herpes have been reported at no fewer than three Kentucky farms, and the outbreak has prompted concerns over the diseases short-term and long-term effects on stallions and broodmares. The good news is that, unlike human genital herpes, the equine version is not a lifelong condition. It does, however, require immediate attention to prevent its spread and secondary infections.

Equine genital herpesvirus is a sexually transmitted disease that can be identified by the appearance of lesions on the shaft of the stallions penis. In the mare, it is commonly observed as a vaginal discharge. The disease is contracted by direct genital contact with an infected animal or contaminated breeding equipment.

Because it is a virus and not a bacteria, the disease is not detectable through a culture, so even a mare who comes to the breeding shed bearing a health certificate and clean culture may be carrying the virus.

Coital exanthema, its veterinary name, is classified as equine herpesvirus-3 (EHV-3). Unlike other strains of equine herpesvirus, genital herpes does not cause abortion or produce respiratory or neurological problems. Incubation period for the virus is five to seven days, with symptoms appearing about a week after exposure.

Small blisters first appear on the penis, which generally go unnoticed. The blisters progress to lesions, which then erode as sores, much like human cold sores, with a diameter of 2 millimeters (mm) to 3 mm. The lesions rapidly advance to ulcers or pustules between 5 mm to 10 mm in diameter, which are painful to the touch.

Contagious period
The stallion is contagious during the period of time when the lesions are visible, and the disease cannot be transmitted after the ulcers heal. In mares, lesions are obscure, and the vaginal discharge they produce is often presumed to be caused by a bacteria or fungus. When a culture comes back negative, the discharge is usually dismissed as inconsequential. This is typically the point when transmission occurs, with the mare sent to the breeding shed presumed to be clean.

As the virus progresses, the horses immune system develops a resistance to the disease, and symptoms begin to dissipate.

From the first exposure, the disease should fully run its course in two-to-three-weeks. However, secondary bacterial infections in the stallion can produce mucopurulent exudate (running sores) from the lesions, which complicates treatment and prolongs recovery time.

Veterinarians recommend application of a topical antibiotic to the emerging sores to prevent secondary bacterial infections in the stallion from setting in, but, because the disease is self-limiting and a virus, no other treatment is generally prescribed. Once called horse pox or spots, genital herpes leaves the stallion with telltale, unpigmented spots where the lesions occurred.

Laboratory testing is available to confirm the diagnosis of EHV-3 through cytological smears of affected skin cells of the penis but, because the virus is short-lived and inconsequential, the time and expense involved in testing is rarely warranted.

EHV-3 is different from human genital herpesvirus in that, once it runs its course, it leaves no lasting effects, nor is it a disease that infects the animal for life. Other than suspending the horses breeding activity during the period of contagion, the condition is little more than an inconvenience.

Nevertheless, barring a popular stallion from the breeding shed even for ten days at the height of the breeding season may have a considerable economic impact on the stallion station as well as the broodmare owner, who either has to short-cycle his mares estrus or wait another month to breed.

Equine genital herpesvirus is not a reportable disease, which means that veterinarians diagnosing the disease are not required by law to report it to the United States Department of Agriculture. For this reason, it is difficult to estimate how many cases of EHV-3 actually occur. News of the recent central Kentucky outbreak spread by word of mouth.
Typically, mares are only booked to one stallion per breeding season. It is conceivable that one infected stallion could have passed the virus on to numerous mares, but the question remains how it was spread among six stallions.

A different virus

Rhinopneumonitis, another form of equine herpesvirus classified as EHV-1 and EHV-4, can be deadly for the unborn foal. Rhinopneumonitis is a disease of the upper respiratory tract in young and adult horses that is characterized by a nasal discharge, fever, and coughing.

The virus tends to run its course the same as a human cold and, unless complications arise, is not cause for alarm. However, if a pregnant mare is exposed to the disease after her fifth month of gestation, rhino will attack the placenta at the point where it attaches to the mares uterus. Spontaneous abortion results when the placenta finally dislodges from the uterus, causing fetal death.
Abortion may occur months after exposure, with no symptoms present in the mare. Numerous vaccines have been developed to guard against EHV-1 and EHV-4, and are recommended for the fifth, seventh, and ninth months of gestation. Foals should be vaccinated every two months beginning at 16 weeks of age.

In adult horses, EHV-1 sometimes affects the nervous system producing incoordination, paralysis, and incontinence. Researchers theorize that the immune system in certain horses may overreact to the virus and congest the blood vessels nourishing the nerves to the hindquarters. Serious complications may develop from the horses inability to stand.