EHV—Always a Threat

Equine Herpes Virus comes in many forms, and like many diseases, can be fatal if left unchecked.

Equine Herpes Virus (EHV) is not just one virus. Rather, there are nine types that affect equines, and five of those can be found in domestic horses. Of those five, three (EHV-1, EHV-3, and EHV-4) are commonly known to cause disease. EHV-1 has received the most press recently because it not only causes abortion in mares and respiratory disease and stillbirth in foals, but also causes the potentially deadly neurological form as well as peracute vasculitis, a rare but fatal disease of adult horses. It is a virus that “makes many horse owners nervous,” says Robert Holland, DVM, PhD, a practicing veterinarian in Lexington, Kentucky as well as a director in Pfizer’s equine department.

EHV Movement

It is thought that up to 80 percent of the total horse population may be carriers of EHV, but it remains dormant until a stressful event occurs to activate it. When latent, only the genetic material resides in cells, and the virus doesn’t spread. However, once active, the virus transmits through respiration for up to eight days; a mare can also pass EHV-1 to her foal through the uterus. It is possible that a stallion can shed the virus, but only after an active infection—there is no evidence that they are persistent carriers. Most respiratory forms are caused by EHV-1 and EHV-4, but only laboratory diagnosis of a nasal swab can confirm which type is present.

Neurological EHV

Much is still unknown about the neurological form, but researchers are working every day to learn more. A gene mutation in a strain of EHV-1 causes the neurological type. The last few years have seen an increase in the number of neurological cases, but the cause is unknown. Large outbreaks can occur, partly because it takes weeks to positively identify the mutated form.

Clinical signs usually begin with a spiked fever. Then the horse may seem fine for the six to ten days it takes for the respiratory form to change into the neurological form. Another fever hits and then the neurological symptoms become visible within hours. Some horses exhibit respiratory symptoms such as watery nasal discharge, mild fever, cough and edema in the limbs or abdomen. After several days, the discharge becomes thicker and dries around the horse’s nostrils. The virus spreads via the circulatory system, damaging blood vessels and causing cellular death of neurons. The actual neurological symptoms progress quickly and include no bladder tone, mild ataxia (loss of muscle coordination) and complete recumbency (inability to get up).

Vaccination Strategy

What is the best way to keep EHV at bay? Proper vaccination and overall herd management, says Holland. If the horses are well taken care of in all respects, the overall herd health is improved. He discourages the idea of only vaccinating horses that come and go frequently while not vaccinating animals that always stay at the farm. Vaccinating pregnant mares with the EHV-1 vaccine has been shown to reduce the risk of abortion. The American Association of Equine Practitioners (AAEP) recommends vaccinating at five, seven and nine months of gestation, because the virus causes more abortions in the last two trimesters. Not only does this help keep the mare from aborting, but it also passes immunity to the foal.

In the past, foals were vaccinated at three to four months, but veterinarians now generally agree that is too early, as the foal still has passive immunity from the dam. Instead, the AAEP advises vaccinating foals at five months, with two subsequent boosters at three-week intervals. You will get some cross protection with EHV-1 and EHV-4 vaccines, but unfortunately none of the vaccines protect against the neurological form of EHV-1.

Once a horse is diagnosed with the neurological form, there is conflicting opinion as to giving boosters to other horses. Some vets say boosting is contraindicated while others recommend it. Holland also points out that there are vaccines labeled for abortion and those just for the respiratory form, so it is important to talk with your veterinarian about which is appropriate for your horses.

Management Strategies

When a new horse comes into your facility, Holland recommends a three-week quarantine instead of the traditional two weeks. It’s also wise to monitor the horse’s temperature twice a day. Horses healthy after three weeks can be introduced into the herd, but sick horses should remain isolated. The virus has been shown to travel at least 35 feet through the air. If a horse becomes ill, it is important for your veterinarian to take a nasal swab to determine the exact type of infection.

How to best isolate a sick horse??If possible, have one person deal with it. If that person also has to handle other horses, it is best to treat the sick horse last. There should also be no mixing of buckets, forks, etc., between sick and healthy horses. People should wash thoroughly and change clothes (or wear disposable clothing over regular clothes) after coming into contact with sick horses. And lastly, horses should stay in quarantine for at least three weeks after the last clinical signs, and then test negative for an active infection.

Treatment Methods

There is no cure for EHV. Rather, it is supportive care that gets a horse through the disease. And while vaccination may not completely protect the horse, it usually will decrease the severity of symptoms. Still, it can take weeks of slow and steady progress to recover from the neurological form. Even if the horse survives, it may have residual problems such as weakness in the hind end or a droopy lip or ear, and the bladder may not heal completely, which leads to urine scalding.

A new treatment being used is hyperbaric oxygen therapy. Hyperbaric oxygen therapy increases the amount of oxygen in the blood and is used in combination with other therapies. It has been used to help treat wounds, tendon and ligament injuries and post-colic surgery cases. The idea is that hyperbaric oxygen therapy, in combination with other treatments, will help heal damaged tissue.

In addition, Navigator (an oral paste used to treat horses with EPM) has been affective in people with hepatitis C, so researchers are trying it with neurological horses. Much of their work focuses on finding treatments and vaccines that will affectively battle EHV-1. It is every horse owner’s hope that the research is successful.






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