Equine Herpesvirus-1 (EHV-1) and Equine Herpesvirus Myeloencephalopathy (EHM)

What is EHV-1, and why is it a concern for horses?

Equine herpesvirus-1 (EHV-1) is a highly contagious virus that can cause respiratory disease, abortion in pregnant mares, neonatal death, and neurologic disease. The neurologic form, known as equine herpesvirus myeloencephalopathy (EHM), can lead to severe brain and spinal cord damage, causing symptoms like ataxia (incoordination), difficulty standing, and even death. EHM has a high mortality rate of 30–50%.

How is EHV-1 transmitted?

EHV-1 spreads through:

• Direct horse-to-horse contact, especially nose-to-nose.
• Contaminated surfaces, including tack, water troughs, and human hands or clothing.
• Aerosolized droplets from coughing or sneezing.
• Horses can also carry the virus in a latent (inactive) state, reactivating and shedding it during periods of stress.

What are the symptoms of EHM?

Horses with EHM may show:

• Hind leg weakness or incoordination.
• Difficulty standing or walking.
• Difficulty urinating.
• Severe lethargy.
• Nasal discharge or fever.
• In advanced cases, horses may become recumbent (unable to stand) or require euthanasia.

How is EHV-1/EHM diagnosed?

Diagnosis involves:

• PCR testing of nasal swabs or blood samples to detect the virus.
• Clinical signs: Neurologic symptoms paired with a positive test for EHV-1 confirm EHM.

How can I protect my horse from EHV-1?

Preventative measures include:

1. Biosecurity Practices:
   • Isolate new or sick horses with the help of veterinarian instructions.
   • Disinfect equipment, clothing, and hands after handling horses.
   • Avoid shared water or feeding areas during outbreaks.
2. Vaccination:
   • Vaccines exist to reduce respiratory symptoms and viral shedding but are not proven to prevent EHM. Modified-live virus (MLV) vaccines, like Rhinomune®, show promise in reducing viral shedding and fever.

What should I do during an outbreak?

1. Contact your veterinarian immediately. This disease is reportable, and your veterinarian can make recommendations regarding quarantine and best biosecurity practices.
2. Quarantine affected horses and farms to prevent the spread.
3. Isolate exposed horses for at least 21 days but confirm with your veterinarian regarding isolation instructions.
4. Enforce strict biosecurity protocols such as footbaths, dedicated equipment, and limiting personnel contact with multiple horses.

What treatments are available for EHM?

Treatment focuses on supportive care and experimental therapies:

1. Supportive Care:
   • Providing accessible food and water.
   • Assisting recumbent horses with slings to prevent ulcers and improve lung function.
   • Using anti-inflammatory drugs like Banamine.
   • Monitoring for secondary complications, like bladder dysfunction or cystitis.
2. Antiviral Medications:
   • Antiviral medications may be prescribed by a veterinarian. However, this treatment does not completely reverse or treat clinical signs of EHM.
     1. Valacyclovir hydrochloride: Reduces viral load, fever, and ataxia severity but requires further study.
     2. Acyclovir: Less effective due to poor absorption and limited activity against EHV-1.

Is there a cure for EHM?

Currently, no cure exists for EHM. Treatment focuses on reducing symptoms and supporting the horse’s recovery. Experimental treatments like valacyclovir and siRNA therapy show potential, but further research is needed.

What is the prognosis for horses with EHM?

EHM has a poor prognosis, with a mortality rate of 30–50%. Horses diagnosed early and receiving supportive care or experimental treatments may have better outcomes, but severely affected horses often require euthanasia.

What research is being conducted to improve treatment and prevention?

Studies are exploring:

• More effective vaccines to prevent EHM.
• Improved antiviral treatments, like valacyclovir and heparin.
• Field studies during outbreaks to collect data on viral loads, symptoms, and treatment efficacy.
• Larger clinical trials with standardized protocols are needed to confirm the effectiveness of these interventions.

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Acknowledgment:

This FAQ was developed using information from the senior paper of Krista Adams (’25 DVM), “Equine Herpesvirus-1/Equine Herpesvirus Myeloencephalopathy (EHM) Treatment and Prevention” submitted on June 6, 2024, under the guidance of Dr. Erin Pinnell.

This information is not meant to be a substitute for veterinary care. Always follow the instructions provided by your veterinarian. Washington State University assumes no liability for injury to you or your pet incurred by following these descriptions or procedures.