Veterinarians Review Equine Herpesvirus

Rectal temperatures can indicate infection, so Aceto recommends owners take the temperature of exposed horses at least twice a day.

Photo: Stephanie L. Church, Editor-in-Chief

An outbreak of equine herpes myeloencephalopathy (EHM), an unusual and potentially life-threatening form of equine herpesvirus (EHV-1) infection, is very big news for every horse owner, barn manager, and veterinarian.

Extremely contagious and easily spread, EHV-1 can cause a variety of problems, including respiratory infection, abortion, and in rare cases, paralysis and other neurologic complications. When EHV-1 causes neurological impairment, it is referred to as EHM.

Infected horses must be isolated, everything associated with those horses disinfected, and biosecurity protocols established as soon as possible, say University of Pennsylvania School of Veterinary Medicine (Penn Vet) experts. State authorities will assist with testing and must be notified, and quarantines should be put in place.

Because the horse and veterinary communities are so concerned about EHV-1, experts from Penn Vet’s New Bolton Center held an information session about the disease in June, attended by about 30 local veterinarians and others in the horse industry. Presenters included:

Drs. Helen Aceto (left) and Amy Johnson

Photo: University of Pennsylvania School of Veterinary Medicine's New Bolton Center

  • Helen Aceto, VMD, PhD, New Bolton Center associate professor of epidemiology and director of biosecurity;
  • Amy Johnson, DVM, Dipl. ACVIM, New Bolton Center assistant professor of large animal medicine and neurology; and
  • Aliza Simeone, VMD, veterinary medical field officer with the Pennsylvania Department of Agriculture’s Bureau of Animal Health and Diagnostic Services.

Southeast Pennsylvania, like many other parts of the country, saw a significant increase in diagnosed cases of EHM in the past year, said Simeone, a Penn Vet alumna. The state managed several quarantines in southeast Pennsylvania between October and May, including barns at local horse racetracks. The state has been regulating EHM since 2008.

This virus affects all sectors of the horse industry worldwide, and has serious animal welfare and economic effects, impacting horse movement, breeding, and competition, the speakers said.

What is EHV-1?

Equine herpesvirus-1 is a common virus that infects horses. In most cases, infection with this virus results in a mild respiratory infection or even no disease at all, but in some cases it can cause potentially serious disease. The virus is easily spread by direct horse-to-horse contact; contaminated hands, equipment, and tack; and even through the air from nasal secretions.

The virus—like all herpesviruses—can establish life-long infections that go into remission but then reactivate periodically, resulting in shedding of the virus in nasal secretions.

A horse can also be infected and pass on the virus to others, but show no clinical signs. “Sometimes it can be hard to pin down where they got infected,” Aceto said.

The incubation period is variable. Aceto recommends watching a horse carefully for at least 14 days after going to a competition. Most horses show clinical respiratory signs within an average of four to seven days. Neurologic signs typically develop eight to 12 days after infection.

How to Recognize the Virus

Horse owners should try to recognize the condition early and know what to do to contain the infection. Johnson said clinical signs can include one or more of the following:

  • Fever 102°F or above;
  • Respiratory signs, nasal discharge, and cough;
  • Neurologic signs including paresis (weakness) and ataxia (incoordination), or lack of muscle control, rear to front;
  • Recumbency, or lying down and being unable to rise without assistance;
  • Difficulty urinating and defecating (dribbling urine is a classic sign of EHM); and
  • Brain dysfunction and seizures are possible, but rare.

What causes reactivation from latency? Many situations, mostly involving stress on the horse, including transportation, change of housing, or weaning, Aceto said.

“Reactivation is generally silent,” Aceto said. “Anything shed in respiratory secretions can cause infection in other animals. It is constantly cycling in the population.”

Horse owners and barn managers should have a plan in place to manage the virus, she said.

“There is nothing that can be done to eliminate future outbreaks, unless we get an effective vaccine,” Aceto said. “So you must be prepared. You need to have a plan.”

An owner should call a veterinarian immediately if an infection is suspected. Veterinarians might ask these questions:

  • What are the horse’s current clinical signs? Look for fever, ataxia, cranial nerve signs, urination difficulties, any signs of respiratory disease.
  • Are any of the other horses on the farm ill? Look for abortion, fever, respiratory disease, neurologic disease.
  • Has the horse traveled recently? Has the horse been to a show, event, or on commercial transport, or has it had a known possible exposure to an EHM case?
  • What is the current vaccination status? (Vaccination does not prevent EHM, but appropriate vaccination of the group of horses might provide “herd immunity” and limit the spread of the disease.)

What to Expect if Infection is Suspected

If an infection could be present, samples must be sent in for diagnostic tests. In Pennsylvania, the lab that runs the test for EHV is in Harrisburg. Nasal swabs and a blood sample are required.

The horse must be isolated and the area made as biosecure as possible. Do not wait for the test results if it is likely that the horse is infected.

When managing an EHV-1 case, clean and disinfect all equipment, clothing, footwear, and feed and water buckets.

Photo: University of Pennsylvania School of Veterinary Medicine's New Bolton Center

“Anything that comes in contact with the horse can spread the virus,” Aceto said, noting that it can be spread in the air, as far as 25 feet. “It’s pretty easy to clean up the environment around the infected horse and keep it clean. Good hygiene is key to control,” she said, and includes:

  • Isolating horses. Move the horse at least 10 to 12 feet away from other horses, if possible. Put a barrier around the front of the stall, even if it is just with cones and tape. To limit aerosol spread, a tarpaulin or sheet can be hung across the front of the stall.
  • Disinfecting surfaces. All equipment, clothing, footwear, and feed and water buckets have to be cleaned with a detergent and disinfected where appropriate (e.g. dilute bleach for feed and water buckets), then rinsed and allowed to dry.
  • Washing or sanitizing (using an alcohol-based hand sanitizer) hands between horses.
  • Using disposable or washable boots, clothing covers, and gloves when dealing with infected horses.
  • Washing and disinfecting all equipment, including bits and tack, twitches, rope, water hoses, and anything that has been used on an infected horse.
  • Maintaining separate tools for infected and exposed stalls: separate brooms, shovels, pitchforks, and wheel barrows. Individual feed and water buckets are essential. Do not use a hose from one bucket to another.
  • Disposing properly of manure and used bedding. Composting is an effective method. Abortions, fetal tissues, and placenta and uterine fluids need to be examined and disposed of in a biosecure manner.
  • Separating the horses in the barn with the virus separate from other horses on the farm, and, if possible, use different personnel for each group.
  • Limiting human visitors to a minimum.

“Feed, clean, and otherwise attend to the horses in this order: unexposed to exposed to infected,” Aceto said. “If you go back in the order, be sure to change clothing and boots and completely clean your hands.”

Rectal temperatures can indicate infection, so Aceto recommends owners take the temperature of exposed horses at least twice a day. An early, mild fever (about 101.5°F) indicates possible infection, and those horses should be isolated. Within three to five days they will start to shed virus as a higher fever develops.

“That time gives you the opportunity to get those horses away from the other horses,” she said.


The first treatment is supportive care, making sure the horses are comfortable and have plenty of food and water, Johnson said. They might need bladder care, including a catheter, to prevent permanent bladder problems.

Anti-inflammatory medications might help them eat and drink better. Steroids are also a possibility, but are usually used only in the most severe cases. Antiviral medications—such as intravenous acyclovir or oral valacyclovir—might also be prescribed.

The prognosis varies depending on the clinical signs. “If they are mildly or moderately affected, they have a fair to good chance of recovery,” Johnson said. “Once recumbent, the prognosis is poor.”

Vaccines are available for herpesvirus, Johnson said. The vaccination to prevent abortion is highly recommended for at-risk mares, those on breeding farms, and race and show horses. The available vaccines for the respiratory form can reduce viral shedding. There are no vaccines available to prevent the neurologic form of herpesvirus infection.

Horses that survive EHM often show improvement within weeks, but total neurologic recovery can take as long as six to 12 months, and more severe cases often do not show complete neurologic recovery, Johnson said. Those with milder signs of disease have a good chance of returning to athletic performance. Johnson said she does not know of any horse that has gotten the neurologic form twice.


Racetracks are particularly challenging to manage, since there are many different owners and veterinary practices, and the horses are often under stress, Simeone said.

“This is a reportable disease, based on suspicion,” she said. “We can help with testing.”

The state is most concerned about the form of the disease that causes neurologic signs, and that is the only form of the disease that is currently reportable to the state, she said. The EHM cases she has handled in the past six months were characterized with very high fevers, topping 104°F, and even with medication, the fevers did not subside.

Not all of the infected horses have clinical signs. In one barn, five of 32 horses that tested positive did not show any signs of illness, Simeone said.

Quarantines are used to assist in management of affected premises.

Photo: University of Pennsylvania School of Veterinary Medicine's New Bolton Center

State-mandated quarantines are used to assist in management of affected premises and are especially helpful when there are multiple owners involved. Quarantine stays in place until there are no new cases and no signs of illness for at least 21 days. The state can use blood and nasal swab tests to document the progress. Temperatures must be taken at least twice a day and recorded, as well.

Simeone suggests that veterinarians have the state-mandated blood collection tubes and nasal swabs on hand, and their state department of agriculture phone numbers handy.

She also recommends that veterinarians, horse owners, and barn managers think about possible places to house sick horses, if necessary. In one case, she spent seven hours calling around before someone identified an empty barn in which to place an infected racehorse.

“People are so afraid of dealing with this,” Simeone said. “But we finally found a beautiful farm to place the horse, and he recovered very well.”


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