Key Insights
  • EHM is a serious, often fatal neurologic disease caused by equine herpesvirus-1 infection
  • The virus spreads easily through nasal secretions, shared buckets, tack, or contaminated surfaces
  • Symptoms appear 6–10 days post-exposure and include weakness, incoordination, urinary issues, and paralysis
  • Older horses and mares face higher risk, though all horses are susceptible during outbreaks
  • No cure exists; treatment focuses on supportive care and managing complications
  • Strict biosecurity and three-week quarantine protocols are essential for outbreak prevention

Equine Herpesvirus-1 Myeloencephalopathy (EHM) is a potentially fatal complication of equine herpesvirus-1 (EHV-1) infection in horses. EHV-1 spreads through contact with an infected horse or surfaces contaminated with the virus.

Horses affected by EHM develop incoordination, weakness, and paralysis within 6 – 10 days after exposure. Older horses and female horses have a higher risk of developing EHM following infection with EHV-1. [1]

There appears to be a rise in cases of EHV-1 and EHM, with several outbreaks affecting horse shows and other densely populated equine gatherings. [1] These occurrences are highly concerning due to the risk of mortality and the potential for long-term side effects from the disease.

When an EHV-1 infection is confirmed, proper biosecurity and quarantine measures are critical for preventing further transmission within the herd. Best practice is for horse owners to follow these same principles whenever horses from different farms interact or are housed in a shared environment, even if there is no confirmed case in the population.

Equine Herpesvirus-1 Myeloencephalopathy

Equine Herpesvirus-1 Myeloencephalopathy (EHM) results from infection with equine herpesvirus-1 (EHV-1), a highly contagious virus in horses. [2] Horses acquire EHV-1 through direct contact with infected horses, or after contact with surfaces contaminated with nasal secretions from infected horses. [2]

Equine Herpesvirus Infections

EHV-1 is extremely common in horse populations, and almost all horses will become infected at some point in their lives. [2] Estimates suggest that 80% of horses carry the virus. [3]

Horses acquire EHV-1 infection after inhaling the virus. [2] In addition to nose-to-nose contact with other horses, common sources of the virus include water troughs, feed buckets, and other items shared between horses. [2]

Most horses acquire EHV-1 during their first year of life, likely from their dam or pasture mates. [1][2] These infections are typically mild, causing self-limiting respiratory symptoms such as nasal discharge or coughing. [2] Symptoms of the initial infection usually resolve within 1-2 weeks. [2]

Although most horses recover uneventfully from an EHV-1 respiratory infection, the virus can remain latent within certain nerves and tissues in the body, establishing a carrier state. [2]

When carrier horses are healthy, their immune systems prevent the virus from multiplying and causing recurrence of disease. However, when the horse experiences a stressful event, their immune system may wane, which can activate the latent virus and trigger its proliferation. [2]

Depending on the degree of viral proliferation, horses may show no symptoms of viral activation, or may develop mild respiratory signs. [2]

Horses can also be contagious and shed the virus in their nasal secretions for three weeks or more, even if they are not showing symptoms. [2]

EHM Infection in Horses

EHM is most commonly associated with infection by equine herpesvirus-1 (EHV-1), although equine herpesvirus-4 (EHV-4) has rarely been implicated in neurologic disease. [4]

Most EHV-1 infections result only in respiratory disease. However, in some cases, the infection progresses to EHM. Certain strains of EHV-1 are considered more neuropathogenic and are associated with a higher risk of neurologic disease, although both neuropathogenic and non-neuropathogenic strains can cause EHM. [4]

Equine Herpesvirus-1 Myeloencephalopathy develops when the virus infects the horse’s spinal cord and brain. [2]

Once EHV-1 enters the body, the virus can infect white blood cells in the horse’s bloodstream. [2] From there, it transfers to endothelial cells, the cells that line blood vessels. [2]

EHV-1 is highly damaging to endothelial cells, and can cause inflammation and inappropriate blood clotting in affected vessels. [2] These outcomes can reduce blood supply to the brain or spinal cord, resulting in death of nervous tissue and symptoms of EHM. [2]

Risk Factors

Studies of outbreak situations involving EHM suggest several potential risk factors that may make some horses more likely to develop EHM compared to others following infection with EHV-1. Horse owners should consider these risk factors when planning to attend equine events.

Currently known risk factors include: [1]

  • Age: EHM primarily affects horses over 3 years of age, with some studies suggesting horses over 20 years of age may have an even higher risk
  • Fever: Horses that maintain a high fever following initial onset due to EHV-1 infection
  • Introduction of new horses: Introducing new horses to the herd or exposure to other horses is common before EHM outbreaks
  • Breed: Ponies and small horse breeds appear less likely to develop EHM
  • Sex: Mares and fillies may be more likely to develop EHM compared to males

Some studies report an increased risk of EHM in vaccinated horses compared to unvaccinated horses. [2][7] However, these studies had a vaccinated horse population that was significantly older than the unvaccinated group, which may have affected this finding. [2][7] The effect of vaccination on EHM development is currently unknown. [1]

Symptoms

Symptoms of EHM develop around 6 – 10 days after exposure to EHV-1 or EHV-4. [2] Symptoms typically worsen rapidly, but then stabilize after 1 – 2 days. [2]

The primary symptoms of EHM are: [2]

  • Uncoordinated movement
  • Weakness, including stumbling, toe dragging, or knuckling over onto the front of the fetlock
  • Paralysis of the tail and anus
  • Urinary incontinence
  • Inability to rise after laying down
  • Circling
  • Difficulty swallowing
  • Head tilt

Typically, weakness and incoordination primarily affect the hind limbs, but can also affect the front limbs in severe cases. [2] Some horses may develop seizures, coma, or death within the first 24 hours of showing symptoms. [2]

Respiratory Symptoms

Equine herpesvirus most commonly causes respiratory disease, and some horses may show respiratory symptoms prior to developing neurologic symptoms. [2]

Symptoms of respiratory disease include: [2]

In many cases, multiple horses on the premises develop symptoms, as EHV-1 is highly contagious through direct contact. [2] Studies from previous outbreaks suggest between 10-40% of horses will develop symptoms of EHM during an outbreak. [3]

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Diagnosis

Veterinarians perform a complete neurologic examination on horses displaying symptoms of nervous system infection or injury. [2] This examination evaluates the horse’s mentation (awareness of their environment), gait, posture, and the function of their limbs and tail. [2]

Based on this examination, the veterinarian can identify what portion of the horse’s nervous system is most severely affected. [2]

Differential Diagnosis

Several diseases affecting the brain, spinal cord, and nerves can cause similar symptoms and neurologic examination findings to equine herpesvirus-1 myeloencephalopathy. [2]

Additional tests that may help the veterinarian identify the cause of nervous system disease include: [2]

  • Bloodwork to identify inflammation
  • Evaluation of cerebrospinal fluid, the fluid surrounding the brain and spinal cord (a.k.a. “spinal tap“)
  • X-rays to identify traumatic injuries or Wobbler’s syndrome
  • Viral, bacterial, or parasitic testing for specific diseases
  • Evaluation of the horse’s feed and environment to identify potential toxins

Diagnostic Tests

Diagnostics that specifically confirm a diagnosis of EHM include: [2]

  • Testing cerebrospinal fluid for viral DNA
  • Nasal swabs or blood collection to identify viral DNA
  • Blood testing to identify antibodies against the virus

In some cases, these tests return inconclusive results, even in horses with EHM. [2] Veterinarians often move forward with a presumptive diagnosis of EHM and continue with treatment and ongoing monitoring. [2]

Dr. Fran Rowe DVM, PAS

“Neurologic signs of EHM can progress rapidly. Diagnostic testing can be inconclusive, so we often have to initiate supportive care and biosecurity protocols based on clinical suspicion alone. Early detection of fever and strict isolation protocols are critical to limiting spread, especially in multi-horse facilities.”

— Dr. Fran Rowe DVM, PAS
Mad Barn Veterinary Nutritionist

Treatment

Currently, there is no specific treatment available for EHM. [2] Treatment typically involves supportive care until the horse’s immune system can resolve the viral infection. [2]

Depending on the severity of disease, supportive care may involve: [2]

  • Intravenous nutrition and fluids
  • Manual evacuation of the bladder and rectum
  • Regular washing of the hind limbs to prevent urine scalding
  • Steroids to reduce inflammation
  • Antiviral medications

Horses that become recumbent (cannot rise from lying down) require caretakers to roll them frequently, every 2 – 4 hours, to prevent pressure sores from developing on their skin. [2] Some recumbent horses may benefit from slings to hold them in a standing position to encourage use of their limbs. [2]

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Prognosis

Horses that stay standing have a good prognosis and typically improve within a few days. [2] It can take up to a year for coordination to return completely, and some horses may have permanent deficits. [2]

Horses that continue to show signs of incoordination have a poor prognosis for future performance and riding, due to the risk of human injury from falling or stumbling. [2]

In some cases, the remaining deficits may be so severe that handling the horse is dangerous. Owners often elect to euthanize these horses due to safety concerns. [2]

Horses that become recumbent during the course of disease have a poor prognosis for recovery, particularly if they stay recumbent for more than 24 hours. [2] Recumbent horses also have a much higher risk of complications including pressure sores, colic, dehydration, and urinary tract infections. [2]

Although recumbent horses may recover, it typically requires extended hospitalization at a significant expense, making it unfeasible for many horse owners. [2]

Prevention

Prevention and control of EHM relies on two major strategies: biosecurity for uninfected horses and quarantine of potentially infected horses. [3][5] Horse owners should employ both strategies to minimize risk for their horses.

At this time, veterinarians recommend vaccination for EHV-1 to reduce symptoms of respiratory disease and viral shedding upon re-exposure or re-activation of the virus. [1][3][5] There are currently no vaccines available that protect against or reduce the symptoms of EHM. [2][8]

Biosecurity

Biosecurity is a set of management practices designed to minimize exposure to infection. Biosecurity protocols are necessary whenever there is a risk of contact with new horses, or in an environment where other horses have been housed previously.

Horse owners should seek the advice of their veterinarian when developing biosecurity protocols for their farm and horses.

General recommendations for biosecurity at equine events include: [3][5][6]

  • Avoiding direct nose-to-nose contact with other horses
  • Avoiding human contact between horses or groups of horses
  • Avoiding sharing brushes, halters, lead ropes, tack, buckets, etc. between horses
  • Regularly disinfecting the horses’ stabling environment or any commonly contacted surfaces
  • Avoiding placing the end of a shared hose into the water bucket during filling
  • Keeping hay, feed, and equipment out of barn aisles where horses or personnel may contact them

Quarantine

All horses returning from an event with other horses and new horses arriving at a farm should be treated as if they are potentially infected. [3] Quarantining these horses reduces the risk of spread to other horses on the farm, minimizing the severity of any outbreaks that may occur. [3]

Horses require at least three weeks of quarantine upon arrival at the property or return from an event. [3]

During this time, recording the horse’s body temperature twice daily can help identify any fever that develops. [5] Symptoms such as nasal discharge, cough, lethargy, and loss of appetite should also be noted. [5]

Proper quarantine requires strict isolation protocols to prevent the virus from spreading to other animals. [5] All equine properties should have a quarantine plan developed with the assistance of a veterinarian. [5]

Quarantine Protocol

Common components of a typical quarantine protocol include: [5]

  • A designated area for horse stabling at least 30 feet (9 meters) from all other horses
  • Designated personnel to care for quarantined horses
  • Provide care to quarantined horses last so as to reduce exposure to healthy horses
  • Dedicated care and cleaning equipment only used for quarantined horses

Quarantine for Horses with EHV

Any horses that develop symptoms of EHV require immediate veterinary care for diagnosis and management. [5] These situations require stricter quarantine protocols, as the risk of disease spread is much higher. [6]

Additional protocols may include: [5]

  • Quarantine of the entire premises, meaning no new horses allowed on property and no horses allowed off property
  • Moving the quarantine area further than 30 feet (9 meters) from other horses
  • Personal protective equipment requirements for handlers, including gloves and disposable coveralls
  • Disinfectant stations, including foot baths and hand sanitizer
  • Strict restrictions for animal, human, and vehicle movement into the quarantine area
  • Fever monitoring for all other horses on the property
  • Thorough disinfection of the premises, including any potentially shared surfaces

Frequently Asked Questions

Here are some frequently asked questions about equine herpesvirus and EHM in horses:

Summary

Equine herpesvirus-1 myeloencephalopathy (EHM) is a potentially fatal disease in horses resulting from equine herpesvirus-1 infection.

  • EHV-1 spreads through contact with an infected horse or contaminated surfaces
  • Symptoms of EHM include incoordination, weakness, and paralysis
  • There is no specific treatment available for EHM
  • The prognosis of EHM varies, with some horses requiring euthanasia due to symptom severity
  • Strict adherence to biosecurity and quarantine protocols is necessary to minimize risk of exposure
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References

  1. Lunn. D. P. et al. Equine Herpesvirus-1 Consensus Statement. Journal of Veterinary Internal Medicine. 2009.View Summary
  2. Reed. S. M. et al. Equine internal medicine, 3rd ed. St. Louis, Mo: Saunders Elsevier, 2010.
  3. Pusterla. N. and Hussey. G. S. Equine Herpesvirus 1 Myeloencephalopathy. Veterinary Clinics of North America: Equine Practice. 2014.View Summary
  4. AAEP: Infectious Disease Guidelines: Equine Herpesvirus (EHV-1 & EHV-4). AAEP. 2025.
  5. Equine Herpesvirus Myeloencephalopathy Incident Guidelines for State Animal Health Officials. United States Animal Health Association, 2018.
  6. Equine Herpesvirus Myeloencephalopathy (EHM) & EHV-1 Frequently Asked Questions. American Association of Equine Practitioners.
  7. Henninger. R. W. et al. Outbreak of Neurologic Disease Caused by Equine Herpesvirus‐1 at a University Equestrian Center. Veterinary Internal Medicine. 2007.View Summary
  8. Osterrieder. K. et al. Vaccination for the Prevention of Equine Herpesvirus‐1 Disease in Domesticated Horses: A Systematic Review and Meta‐analysis. Veterinary Internal Medicine. 2023.View Summary