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Equine HerpesvirusBy Kentucky Equine Research Staff · November 2, 2002

Equine herpesvirus (EHV) refers to a group of closely related viruses that infect equines. Five have been identified in domestic horses, while four others are limited to zebras, donkeys, and kiangs (Asian wild asses).

How common are these viruses in the general equine population?

Because exposure usually occurs very early in life, it's safe to say that virtually all horses harbor one or more herpesviruses. EHV has been found in foals as young as 25 days, suggesting that transmission can hardly be prevented whenever two or more horses come in contact.

What problems can EHV cause?

Five types of EHV affect horses, and some of these types have several strains. Evidence of infection varies depending on what type and strain is causing the illness. For instance, a common strain of EHV-1 causes rhinopneumonitis, an inflammatory disease of the lungs and upper airways. This infection is common in young horses and is indicated by a runny nose, cough, fever, and diminished appetite. The virus is transmitted when a healthy horse comes in contact with nasal secretions from an infected animal. This can be by direct contact or through aerosol transmission when a horse coughs or snorts. Outward signs disappear in a few weeks, but it may take that long again for the horse's respiratory system to recover completely.

Another manifestation of EHV-1 occurs when the virus moves from the respiratory system to set up an inflammation of the uterine tissues of pregnant mares. Infected mares may or may not show respiratory signs, and abortion can take place weeks or months after the mares come in contact with the virus.

In some cases, EHV-1 targets blood vessels that supply the brain and spinal cord. Horses that fall prey to this particularly deadly strain develop neurologic signs such as uncoordinated gaits, stumbling, and urine leakage. Animals that are mildly affected and can remain standing have a good chance for recovery, but the prognosis is not as positive for those that collapse or need to be supported in slings. Severely affected horses that do not die often suffer permanent neurologic damage.

EHV-2, also known as equine cytomegalovirus, is found in up to 97% of horses over the age of four weeks. Although it is very common, there is no certain evidence that it has any negative effect on health. Some studies link EHV-2 and mild respiratory or eye infections in foals, and some experts think a connection may eventually be found between EHV-2 and lowered immune response.

The only disease-causing equine herpesvirus that does not affect the respiratory tract is EHV-3, also known as equine coital exanthema virus. This virus is passed from horse to horse by skin contact, such as between a stallion and mare during breeding. It can also be spread to uninfected horses by tools, grooming instruments, or human hands that have been in contact with an infected animal. The virus causes painful sores on the external genitals and occasionally on the udder, muzzle, or eyelids. Discomfort makes infected stallions reluctant to breed, and mares may not allow their foals to nurse because of lesions on the udder. This type of herpesvirus does not cause abortion or make horses sterile, but its presence can seriously disrupt a farm's breeding program.

Still another type, EHV-4, is so closely related to EHV-1 that it was determined to be a separate virus less than 20 years ago. EHV-4 tends to produce a milder respiratory illness than its near-twin. Because it replicates more slowly and is usually confined to the lungs, EHV-4 almost never causes abortion and has not been linked to neurologic disease.

EHV-5 is similar to EHV-2. It has been found in the respiratory tracts of almost 100% of tested domestic horses as well as zoo-kept zebras and Przewalski's horses. It has never been known to cause any type of illness.

Can humans catch herpesvirus from horses?

No. Of more than 100 known viruses in the herpes family, most affect only one species (cats and dogs have been infected with equine herpesvirus under experimental conditions, but it is not known whether such transmission could occur naturally). Humans are susceptible to five types that are distinct from equine forms; still other herpesviruses are specific to goats, elephants, pigs, chickens, baboons, cats, and other animals. Chicken pox, shingles, mononucleosis, cold sores, and genital herpes are among the human maladies caused by herpesviruses.

Can EHV be cured?

No cure is known. When a horse comes into contact with a particular type or strain, he may or may not develop signs of illness as the virus becomes established in the tissues. The virus has the ability to lie dormant for many years, often causing no discernible problems and somehow managing to hide from the horse's immune system. Months or years later, periods of stress can cause the virus to flare up into an active stage. At these times the horse can pass the infection to his pasturemates, even though he still may not show obvious signs of illness.

Can EHV be treated?

As a virus, EHV cannot be killed by antibiotics. Horses that are mildly ill may not need any treatment, recovering with rest and good nutrition. Seriously ill horses may require supportive care such as anti-inflammatory medications, fluids to maintain hydration, and slings to prevent collapse. Acyclovir, a drug used to treat herpesvirus in humans, has been used to reduce the severity of infections, but little research has been done on its effectiveness in equines.

Studies in humans and some other species have shown that high levels of lysine, an amino acid, interfere with the replication of herpesviruses. On this basis, some owners have supplemented their horses in an attempt to prevent flare-ups of EHV during stressful periods (weaning, travel, heavy training or showing, and so on). Although problems with toxicity would not be anticipated, there is some concern that long-term lysine supplementation could disrupt the lysine-arginine balance. Both these amino acids play important roles in immune system function.

Can EHV be prevented?

Short of long-term isolation, it is probably unrealistic to try to prevent all EHV infection. Quarantining to eliminate contact between sick and healthy horses can help control the spread of infection, as can disinfection of stalls, trailers, and equipment that have been in contact with an infected horse. A solution of one part bleach to ten parts water is a suitable disinfectant.

Vaccination at an early age, and then periodically depending on management factors, is recommended to reduce the incidence and severity of disease. No vaccine provides complete, permanent protection, although research is in progress to produce a more effective vaccine. Owners should contact a veterinarian for advice on vaccinating young horses, pregnant mares, and horses that may have been exposed to EHV.