Equine herpesvirus: EHV-1 and EHM (2024)

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.

Treatment

• 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.

Prevention

• 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

Shows or facilities with CONFIRMED cases of EHV-1

• Isolate your horse from the rest of the stable after returning. This includes using separate equipment for isolated horses and washing your hands with soap and water routinely.
• Consult with your veterinarian to determine how long to isolate your horse. You should keep your horse isolated for at least 21 days.
• Closely monitor your horse for fever, respiratory disease or neurologic signs for three weeks.
• Take your horse’s temperature twice a day. If your horse’s temperature is greater than 101.5 F, contact your veterinarian.
• Clean your equipment. You can readily kill EHV-1 with many common disinfectants including diluted bleach, quaternary ammonium compounds, accelerated peroxide and peroxygen compounds, and iodophors.

Shows with NO KNOWN EHV‐1 cases but during an EHV‐1 outbreak elsewhere in the state

• Isolate your horse from the rest of the stable for 14 days after returning. This includes using separate equipment for isolated horses and washing your hands with soap and water routinely.
• Closely monitor your horse for fever, respiratory disease or neurologic signs for 14 days. Take your horse’s temperature twice a day. If your horse’s temperature is greater than 101.5 F, contact your veterinarian.
• Clean your equipment using a common disinfectant such as diluted bleach, quaternary ammonium compounds, accelerated peroxide and peroxygen compounds, and iodophors.
• Regardless of an outbreak, you should always practice good biosecurity when traveling to and from different horse-populated areas. See Horse biosecurity: tips during peak riding season for more information.

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