Deadliest Catch: Equine Herpes Myeloencephalopathy

EHM presents a significant danger to horses. Here’s what you need to know about it.


Owning a horse is a big responsibility. You work hard to ensure your equine best friend is well cared-for. This includes doing everything possible to protect against disease. What can be done to prevent a disease that can’t always be controlled with a vaccine but can cause devastating results? This is the dilemma now facing the horse community with equine herpes myeloencephlopathy (EHM). Read on to learn more about this disease in our horses.

Horse against concrete wall
Photo: Nastia Gomanova/Shutterstock

EHM is a devastating neurological disease in horses that is caused by equine herpes virus 1 (EHV-1). It has gained more notoriety over the last 10 years, affecting horses at boarding facilities, large stables, racetracks and horse shows, resulting in widespread quarantines. One of the largest EHM outbreaks in U.S. history was in 2011; it started at a horse show in Utah and ended up exposing 242 horse premises in 19 states.

The first sign of the disease is often fever, which can go unnoticed. About seven to 10 days later, the most common symptoms include hind limb weakness and/or incoordination, incontinence (dribbling urine) and stumbling. These signs can worsen into more severe neurological symptoms such as sitting like a dog, severe ataxia, paresis (“drunk walking” and weakness), and difficulty getting up. The worse the clinical signs, the worse the prognosis. Those that can’t get up are often humanely euthanized.

By the Numbers

Equine herpesvirus is a widespread, common virus in horses. It has nine different forms, but the most common are types 1 and 4 (also known as rhinopneumonitis)—and each type can have subtypes. Type 4 typically localizes in the upper airway (nose and throat), causing a snotty nose and mild to moderate fever. In other words, type 4 is usually just a horse cold, and thankfully it’s the more common form.

Type 1 is much more invasive and causes not only respiratory disease but also abortions and myeloencephalopathy (disease of the nervous system). Respiratory disease and abortions are the most common effect of this virus.

Vaccines for EHV and EHM

Both viruses can be vaccinated against; this is the “rhino” part of the flu/rhino vaccine. EHM, the neurological disease caused by subtypes of EHV-1, is very uncommon. It’s most often associated with a subtype of the herpes virus referred to as the “neuro form.” This is a very important factor when managing outbreaks where horses are tested to determine quarantine status.

The vaccines currently available are not labeled to protect against the neurological form of herpes (EHM), but researchers are currently studying if there is some protection provided.

By vaccinating the general horse population, the overall amount of herpes virus shed is decreased, and thus decreases the chances of the neuro form from occurring.

What we do know is that by vaccinating the general horse population, the overall amount of herpes virus shed is decreased, and thus decreases the chances of the neuro form from occurring.

Horse owners and facility managers can also help protect horses by becoming aware of the disease and learning good biosecurity. Follow vet-recommended vaccination programs to manage shedding of the virus among the general horse population.

The Challenge of EHV-1

EHV-1 is a difficult virus in many ways. It is highly contagious, gaining access to the body through the nose from nasal secretions of infected horses. At the nose level, it replicates and causes disease there. But type 1 goes deeper into the tissue, getting into white blood cells that transport it throughout the body.


Here is where it creates the most harm. These infected white blood cells attach to the lining of blood vessels, most notably in the uterus of pregnant mares or the blood vessels of the spinal cord and brain stem.

It invades those cells, lining the blood vessels, causing damage and essentially destruction of the blood vessel. Once this happens, the blood supply to the tissue is compromised and the tissue dies. In the case of abortions, the placenta dies. In EHM, the spinal cord tissue dies, causing the varying advancing neurological symptoms, depending on location and severity of damage.

Immune to Immunity

In addition to causing such a damaging disease, herpes virus in general has also adapted itself to evade the immune system. It moves from cell to cell without entering the blood (where the antibodies are). This is one reason why the vaccine, which stimulates the body to produce antibodies, has difficulty protecting against this disease.

Horses in western bridles EHM
EHV-1 is spread primarily through nasal secretions of infected horses via direct contact or contaminated surfaces. There may be some risk of airborne transmission in horses shedding the virus. Photo: Nicole Ciscato/Shutterstock

The virus can also stop the infected white blood cell it has invaded from producing markers on the surface of that cell that alert the other white blood cells to the infection and signal destruction of the infected cell.

Hidden Threat

Finally, it literally “hides out” in lymph tissue and the nervous system, becoming dormant and undetectable by the body. It’s in this latent state that most horses are infected. Many are infected as foals by their dam, and the virus remains dormant until some physical stressors activate the virus.


It is not completely known what exactly reactivates the virus and causes EHM. The disease itself appears to be occurring more frequently, and is considered an emerging disease.

As such, much of what we do know has been from studying the outbreaks that have occurred. From these studies it appears that older horses and mares are more at risk. Introduction of new horses to a herd and showing are common factors in outbreaks. It’s theorized that these events cause stressors in the horses, creating the right conditions for activation of the virus.

Events that cause stress in horses create the right conditions for activation of the virus.

This disease is reportable, meaning when it occurs it must be reported to the United States Department of Agriculture (USDA) state regulatory veterinarian. All horses on the affected premises must be quarantined, and horses that have been in contact with the affected horses are monitored for disease. Alerts about these outbreaks are issued nationwide; you can stay up to date via the Equine Disease Communication Center ( and sign up for email notifications.

How to Diagnose EHM

Currently the best available diagnostic tests are nasal swabs and blood testing to look for a rise in antibody titers over a seven-day interval. Antibody levels are compared, and a four-fold increase indicates active infection. Later-stage tests include virus isolation, cerebral spinal fluid analysis and tissue sampling.

This disease can look like West Nile virus, Eastern and Western encephalitis, wobbler’s, severe arthritis of the neck, EPM or rabies, so when clinical symptoms first show it can be very challenging to make a proper diagnosis.

Treatment of EHM

There is no direct treatment for EHM. Efforts are directed at providing supportive care with fluids and IV total nutrition, as well as minimizing the damage with anti-inflammatory drugs like Banamine, steroids and DMSO.

Antiviral Drugs:

There are two antiviral drugs that have been used with some success: acyclovir and valacyclovir, with the latter currently the more promising, although neither is proven. There is a small amount of data in the literature that suggests zinc supplements might help prevent and cure the disease.


Biosecurity is key in minimizing the spread of this disease. Infected horses should be removed from the herd and isolated on the same property. All items that were in contact with the infected horse must be disinfected, since the virus can survive in the environment for a long time. Some of the techniques for managing infected horses include foot baths and separate clothes for handlers, separate stall cleaning tools, and ideally, separate handlers when possible.

Prevention of EHM

Horses that have been exposed to infected horses need to be closely monitored. This includes taking their temperature twice daily, serial nasal swabs and blood antibody titers. No vaccine has been shown to prevent EHM, but herd vaccination has been shown to decrease the virus shed and could potentially help prevent the disease.

The recommended vaccination frequency is once every six months, or three months for horses at higher risk. Horses must remain quarantined for 28 days after the last horse has shown signs of the disease and nasal swabs show no more infection. Horses that have recovered from the disease typically don’t have relapses and aren’t contagious. However, they may have lasting neurological changes.

What’s Next?

EHM’s status as an emerging disease has focused a lot of research on it and as a result, a lot more is known now than 15 years ago. Unfortunately, there are still so many unanswered questions that horse owners and veterinarians continue to struggle to conquer this disease.

EHM will most likely never be eradicated, but much work is being done to find better treatments and a vaccine. Until then, the horse community must work hard to combat this disease by implementing good vaccine protocols, quarantining horses new to a herd, and monitoring temperature for at least two weeks before introducing to the group.

Other steps include good nutrition, not overworking horses (especially while at shows), and trying to minimize physical stress as much as possible.

This article about Equine Herpes Myeloencephalopathy originally appeared in the August 2018 issue of Horse Illustrated magazine. Click here to subscribe!


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