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A brief guide to EHV-1 and EHM and controlling the disease
Wednesday, February 07, 2018 - by Kim French, USTA Internet News Editor

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Columbus, OH --- The recent cancellation of competition at The Meadows until Feb. 23, notice from the Ohio Department of Agriculture four horses have tested positive at several locations within the state and the revocation of the Winter Speed Sale that was to be conducted at the Delaware County Fairgrounds on Feb. 12 were all the result of one very contagious condition known as Equine Herpesvirus Myeloencephalopathy (EHM) caused by Equine Herpesvirus-1 (EHV-1), also known as rhinopneumonitis. Therefore, now is the perfect time to discuss background on what the disease consists of and most importantly its prevention and/or control.

What are the clinical signs of EHV-1 and EHM?

Four strains of herpesvirus can infect horses and can cause respiratory disease, abortion, and nervous disease. The most common strain, EHV-1, most often causes a mild respiratory disease with a low-grade fever, lethargy, coughing and nasal discharge. The incubation period is two to ten days; most horses fully recover from this type of infection.

EHV-1 can also cause a neurologic form known as Equine Herpesvirus Myeloencephalopathy (EHM) which affects the nervous system and causes weakness in the hind limbs, loss of tail tone, incoordination (ataxia), urine dribbling and, in severe cases, recumbency with paralysis. The hind legs are usually affected rather than the front limbs and a horse may exhibit respiratory symptoms for ten days to two weeks prior to developing EHM. EHM can have a fatality rate in as high as 30 percent of cases.

EHV-1 can be dormant (latent) in the horses that show no clinical signs and spontaneously cause the respiratory disease or EHM. The disease is very contagious; EHM can occur in just one horse, but because horses are commonly housed together, the disease can cause an outbreak affecting multiple horses. An example of a severe outbreak was at the National Cutting Horse Association Western National championship in the spring of 2011. Ninety horses were identified as being EVH-1 positive after horses returned home with 10 states reporting horses with signs of the disease.

Why are EHV-1 and EHM so tough to control and prevent?

There are three primary problems with this virus that make it so difficult to combat: it hides in a horse’s white blood cells and DNA; is highly contagious and horses carry the virus in its latent form. A horse may never shed EHV-1 its entire life unless it is placed in a stressful situation, but when one horse succumbs, entire barns and groups of horses are immediately at risk of becoming infected. It can also re-emerge when a horse recovers from the initial infection.

“The key thing to remember is EHV-1 is extremely contagious,” said Dr. Nathaniel White II, professor emeritus of equine surgery at the Virginia-Maryland College of Veterinary Medicine’s Marion duPont Scott Equine Medical Center. “It is spread very easily by direct horse-to-horse contact; by aerosol droplets from sneezing and coughing; and contamination of objects such as clothing, using the same feed tubs, and by hands. Fortunately, if proper preventative measures are taken, such as disinfecting all items in the barn and isolating horses, the disease can be controlled. Quarantine is needed to prevent horse movement which risks infection of horses in other locations. Horses should also have their temperatures taken twice daily, be swabbed by veterinarians and quarantined for a period of 21-28 days until all horses test negative for the virus.”

White suggests anyone handling an infected horse or horses, should wear separate clothes to the barn and change immediately after leaving, use rubber boots and exercise rigorous hand hygiene.

To vaccinate or not?

White, a former president of the American Association of Equine Practitioners (AAEP), a chair of the AAEP’s National Equine Health Plan Task Force and responsible for the establishment of the Equine Disease Communication Center (EDCC), cannot stress enough the biosecurity measures that must be made by horse owners to limit infection. He also recommends vaccination as another precaution to reduce a horse’s risk of developing the disease.

“Unfortunately with the latent nature of the disease, a vaccinated horse can still become ill,” he said. “The AAEP vaccination guidelines should be followed closely including initiating vaccination in foals at three months of age then continued regular vaccination at six month intervals throughout their lives.”

What are the main risk factors for development of EHV-1 or EHM?

While any horse can be a carrier or fall prey to EHV-1 or EHM, there are several risk factors which include:

  • Age
  • Confined vs. pasture
  • Higher traffic of people and horses in and out of stables
  • Use of common equipment
  • Training and competition
  • Gender
  • What time of year it may be

Horses that reside in barns are more susceptible to EHV-1 or the development of EHM due to increased stress. Older horses also have a higher incidence of contracting the disease and all horses have an increased risk during winter and spring. Also, horses that are transported regularly have a higher rate of disease.


Where is more information available?

For those wishing to acquire more knowledge on EHV-1 and EHM please visit the AAEP website by clicking here. The AAEP has a large number of resources on many facets of the disease and its prevention/control. The EDCC also provides fact sheets for owners and specific information about the use of biosecurity to decrease a horse’s chance of becoming infected.

The USDA provides background and details on the virus which can be accessed by clicking here and the National Equine Health Plan on the EDCC website has valuable information for horse owners and veterinarians dealing with disease outbreaks.

For the latest updates on EHM outbreaks or more information, please visit the EDCC website by clicking here.


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