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Equine herpesvirus: EHV-1 and EHM

Quick facts

  • Equine herpesvirus 1 is a contagious virus that can cause neurological disease, respiratory disease, newborn death and abortion in horses.
  • Equine herpesvirus myeloencephalopathy refers to the neurological form of equine herpesvirus 1.
  • Practicing biosecurity and vaccination can help limit the spread of these diseases.

What is equine herpesvirus 1 (EHV-1)? 

Equine herpesvirus 1 (EHV-1) is a contagious equine virus that can cause neurological disease, respiratory disease, newborn death and abortion. EHV-1 can be either neuropathic or nonneuropathic. Despite the names, both forms can cause neurologic disease.

EHV-1 is also known as rhinopneumonitis.

What is equine herpesvirus myeloencephalopathy (EHM)?

Equine herpesvirus myeloencephalopathy (EHM) refers to the neurological form of EHV-1. Horses positive for EHV-1 and show neurological signs such as incoordination and hind-end weakness are considered positive for EHM. Once a horse is positive for EHM, other horses at the facility are at an increased risk of getting EHM. EHM is often fatal and those that make a rare recovery will likely face long-term neurologic problems.

Be safe. Handling a horse with a neurological disease such as EHM can be dangerous. Your horse may lose their balance or partial control of their body, which can put you and others at risk of injury. While waiting for a veterinarian, stay a safe distance away from your horse to prevent injury.

Signs of illness

If you suspect your horse has EHV-1, contact your veterinarian right away. Your veterinarian can take a nasal swab or blood sample to test your horse for EHV-1. Signs of EHV-1 include:

  • Fever, which commonly precedes other clinical signs
  • Respiratory disease: most common in young horses
  • Fever, coughing, and clear to thick yellow nasal discharge
  • Abortion: usually occurs in late pregnancy (over 8 months), but can occur as early as 4 months with no warning signs
  • Neurologic disease (EHM)  
    • Hind-end weakness and incoordination
    • Leaning against walls or fences for balance
    • Urine dribbling or not able to urinate
    • Down and not able to stand    

Horses usually express signs of illness 4 to 6 days after exposure to the virus. This can greatly vary and may be as short as 24 hours. When EHM occurs, it typically begins 8 to 12 days after fever begins.

How does the virus spread? 

  • The most common way to spread EHV-1 is by direct horse-to-horse contact.
  • Horses can get the virus through infected air droplets or nasal discharge of infected horses.
  • EHV-1-aborted foals can also act as an infection source.
  • Horses can carry EHV-1 but not show signs of illness. Stress such as transportation or weaning can reactivate the virus in carriers and cause disease.
  • EHV-1 can also spread indirectly through contact with physical objects contaminated with the virus. This can include grooming equipment, feed and water buckets as well as people’s hands or clothing. The virus can live for a few weeks on uncleaned objects and surfaces.


  • Supportive care and anti-inflammatory drugs are often used.
  • Antiviral medications may be used for horses with EHM.
  • Sling support can help horses with severe weakness and incoordination.
  • Isolating affected horses will help prevent the infection from spreading.
  • Your veterinarian may prescribe antibiotics if they have concerns about secondary bacterial infection.


  • Vaccinations are available to control the respiratory and abortion manifestations of EHV-1. Current vaccines don’t reliably prevent development of EHM. Your veterinarian may recommend vaccinating to help reduce disease spread. Some breed associations require vaccination prior to participating in shows or events.
  • Practice biosecurity to avoid spreading disease and bringing it home or to other facilities.  
  • Don’t enter or leave a facility where a horse has tested positive for EHV-1 or EHM until a veterinarian has cleared it.
  • Keep your horse home if it shows signs of illness.

For horses returning from shows


Does EHV-1 affect other animals? 

  • EHV-1 doesn’t affect humans, dogs, cats, sheep, goats, cattle, pigs or birds.
  • Alpacas and llamas are prone to EHV-1.

Additional resources 

American Association of Equine Practitioners

United States Department of Agriculture, Animal Plant Health Inspection Service

Acknowledgments: This article was adapted from the Equine Herpesvirus 1 factsheet prepared by the University of Minnesota Veterinary Medical Center.

Reviewed in 2019

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