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Animal Diseases and Biosecurity

​​​​​Species affecte​d

​Horses and other equine species.​


Equine herpesvirus myeloencephalopathy (EHM) is another name for the neurologic disease associated with equine herpesvirus (EHV) infections. Neurological signs appear as a result of damage to blood vessels in the brain and spinal cord associated with EHV infection. Interference with the blood supply leads to tissue damage and a subsequent loss in normal function of areas in the brain and spinal cord.

To date, nine EHVs have been identified worldwide. Three of these—EHV-1, EHV-3, and EHV-4—pose the most serious health risks for domesticated horses and can have significant economic impacts on the US equine industry. EHV-1 and EHV-4 can cause upper respiratory disease, neurological disease, abortions, and/or neonatal death. EHV-3 causes a venereal disease called coital exanthema.

By 2 years of age, almost all horses have been infected with EHV-1. The initial exposure generally occurs in foals from contact with their dams. The virus can then become latent, or inactive, in the horse’s body, setting up a carrier state that is life-long. Horses of any age that are carriers of EHV-1 do not show any external signs of disease when the virus is in a latent form. This is what makes this disease so difficult to diagnose and the reason it is important to make sure horses are monitor closely and any horses that may seem ill stay home. The virus can be reactivated during times of stress, such as strenuous exercise, long-distance transport, exhibition/showing of the animal, or at weaning.

The average incubation period is 4 to 7 days, but with some taking up to 14 days.


The most common way for EHV-1 to spread is by direct horse-to-horse contact. This virus is shed from infected horses via the respiratory tract or through direct or indirect contact with an infected aborted fetus and fetal membranes. Horses may appear to be perfectly healthy yet spread the virus via the secretions from their nostrils.

It is important to realize that EHV-1 can also be spread indirectly through contact with physical objects contaminated with infectious virus. 

Examples of such objects include:

  • Tack
  • Wipe rags or other grooming equipment
  • Feed and water buckets
  • People’s hands, clothing or boots
  • Equipment

The air around a horse that is shedding the virus can also be contaminated with infectious virus. Although we know that the virus can become airborne, it is difficult to establish the distance the virus can spread in this manner under typical horse management and environmental conditions.


There are many steps you can take to help prevent the spread of EHV-1.

  • Stop horse movement if your animals may be infected with EHV-1. This is the most important first step horse owners can take. Horses should neither enter nor leave a premise where EHM has been diagnosed until cleared by the veterinarian.
  • Do not allow horses exposed to EHM to have contact with unexposed horses on the premises.
  • Isolate sick horses. Horses that have aborted or shown signs of fever, respiratory disease, or neurologic disease should be separated from healthy horses. Ideally, sick horses should be moved into a separate building or paddock on the premises, or be transported to a veterinary hospital with an isolation facility.
  • Do not share equipment among horses on the facility. The virus can be spread from horse to horse via contaminated objects such as water and feed buckets or tack.
  • Practice proper biosecurity measures. People can transfer the virus from horse to horse via their hands and clothing. Personnel should wash their hands after handling one horse and before handling another. They should also change their clothes and footwear after working with a sick horse. Optimally, a person who takes care of a sick horse should not work with healthy horses. When this is not practical, healthy horses should be handled first and sick horses last. Wearing gloves and using disinfectant to sanitize footwear can also help minimize the risk of people spreading the virus between animals.​


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

From University of California, Davis. EHV-1 and EHM information from UC Davis

Resources on Equine Herpesvirus

From the American Association of Equine Practitioners. Resources on Equine Herpesvirus

USDA information on Equine Herpesvirus

Equine Herpesvirus transmission, signs, treatment, and prevention information from the United States Department of Agriculture. USDA Equine Herpesvirus webpage



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Salem, OR 97301
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