Equine Herpesvirus


Equine herpesviruses (EHV) are DNA viruses that occur in horse populations worldwide. This is a family of viruses which are named by numbers such as EHV 1, 2, 3, 4 and 5. There are more viruses in this family, but two of the most common species, EHV-1 and EHV-4, pose serious health risks for domestic horses.

  • EHV-1, causes abortion, respiratory disease and neurologic disease; and
  • EHV-4, causes respiratory disease and can occasionally cause abortion and rarely neurological disease.

EHV-1 is contagious and spread by direct horse-to-horse contact via the respiratory tract through nasal secretions. The virus can also be spread indirectly through contact with physical objects that are contaminated with the virus such as: hands and clothing; equipment and tack; trailers used for transporting horses; wipe rags and other grooming equipment; and feed and water buckets.

The air around the horse that is shedding the virus can also be contaminated with infectious virus, but it is difficult to establish the distance the virus can spread in this manner under typical horse management and environmental conditions. Also, the virus can survive up to 7 days in the environment under normal circumstances but can remain alive for upwards of one month under perfect environmental conditions.

Safety and precaution are always recommended, even when an outbreak has not occurred. Biosecurity is important at all times and means doing everything you can to reduce the chances of an infectious disease being carried onto your farm by people, animals, equipment or vehicles, either accidentally or on purpose. Anything that touches an infected horse or sheds secretions from sick horses has the potential to transfer pathogens to other horses.

NDA encourages all horse producers to follow biosecurity measures on their operations, including: requiring individuals to wash their hands before and after contact with each horse; disinfect boots and change clothes that come into contact with horses other than your own; isolate horses returning from shows or events for a minimum of 3 weeks.

USDA Biosecurity Tips for Horse Owners

American Association of Equine Practitioners (AAEP) Biosecurity Guidelines

Symptoms of EHM
Equine herpesvirus myeloencephalopathy (EHM) is another name for the neurologic disease associated with EHV infections.

Neurologic disease appears suddenly and is usually rapidly progressing, reaching its peak intensity within 24 to 48 hours from onset of initial neurologic signs. Horses may develop EHM even without any preceding fever and/or respiratory signs. Clinical signs of the neurologic disease may include:

  • Loss of coordination
  • Hind limb weakness
  • Loss of tail tone
  • Lethargy
  • Urine dribbling
  • Head tilt
  • Leaning against a fence or wall to maintain balance
  • Inability to rise

Diagnosis of EHM is based on clinical signs and detection of the virus by diagnostics performed by your veterinarian.

If your horse develops fever, respiratory signs or neurological signs, immediately notify your veterinarian and do not move the horse or horses in the immediate area. Alert those who have horses in the adjacent area to cease all movement of horses in and out of the facility until a diagnosis is confirmed by testing and a targeted plan for the control of spread of disease is developed in consultation with your veterinarian.

The AAEP EHV Control Guidelines can be used by your veterinarian to assist you in developing a more detailed response plan.

Reporting a Disease
The Equine Disease Communication Center (EDCC) reports cases and diseases that have been diagnostically confirmed by a State Animal Health Official or an attending veterinarian.

Veterinarians are encouraged to report any and all cases with a confirmed diagnosis of an infectious or vector borne disease, including outbreaks and cases of non-reportable diseases.


Additional Resources