Equine Herpesvirus Myeloencephalopathy




Many of our equine clients are already aware of the outbreaks of Equine Herpesvirus-1 that have been occurring in many counties across Ontario since February of this year. They have occurred at many types of facilities, including:

  • boarding stables,

  • breeding farms, and

  • racetracks stables.

As late as March 25, 2021 OMAFRA has reported a total of 11 cases of horses with clinical signs including:

  • nasal discharge,

  • fever,

  • ataxia,

  • swollen limbs, and

  • difficulty urinating.

While 3 horses showing neurologic signs have recovered, the other 8 were unfortunately euthanized.


With this client communication, we hope to inform you about this complex disease and what you can do to prevent it.


What is Equine Herpesvirus Myeloencephalopathy (EHM)?


EHM is caused by the EHV-1 virus (Equine Herpesvirus 1) which is common in the horse population. In extremely rare cases, EHV-4 can also develop into EHM. The virus is spread through direct contact with nasal droplets, or spread as aerosol droplets. Furthermore, contamination of tack, stalls, feed, water, and trailers is also a transmission risk. Lastly, don’t forget that you can also spread the virus with your hands and clothing.


Although EHV-1 and EHV-4 are a relatively common cause of a mild respiratory disease (equine rhinopneumonitis), EHM, the neurologic form, is not common. It is unknown why this virus develops into neurologic disease in some horses.


The time from infection to clinical signs (incubation period) is 5-6 days. Horses can shed the virus during the incubation period even when showing no clinical signs.


Where does the EHV-1 virus originate?


It is estimated that 40-60% of infected horses can become lifelong carriers of EHV-1. Most horses have been infected at some point in their life. Horses that have had EHV-1 may be carriers and the virus may be latent and reoccur under periods of stress such as transport or a change in the horse’s environment.

How serious is an Equine Herpes Myeloencephalopathy outbreak?


EHM can be life threatening. The virus typically causes a biphasic (two phase) fever. The horse will have fever on day 1 or 2 and again on day 6 or 7. Neurological signs may not present until the second fever. Some horses may not develop a fever. Other signs include:

  • nasal discharge,

  • depression,

  • incoordination (ataxia),

  • hind limb weakness,

  • swollen limbs,

  • loss of tail tone,

  • inability to urinate properly (dribbling urine),

  • dog sitting position,

  • leaning against a fence/wall to maintain balance, and finally

  • recumbency.


How is EHM diagnosed?


The diagnosis is made by having a veterinarian collect nasal swabs and whole blood from the horse, which are then submitted for laboratory examination. Horses with neurologic signs which test positive for EHV-1 are considered positive for EHM.


Is there a cure for EHM?


Unfortunately there is no definitive cure for EHM. Supportive care is administered including the use of nonsteroidal anti-inflammatory drugs (NSAIDS) such as phenylbutazone (Bute) or flunixin meglumine (Banamine) to reduce fever, inflammation, and pain. Corticosteroids have been used but there is no evidence of benefit. Antiviral drugs such as Acyclovir and Valacyclovir have been used but their value in horses with EHV infection is the subject of continued investigation.

Prognosis for horses that test positive for EHV and then develop neurologic signs of EHM is often poor, with fatality as high as 30%. In a percentage of cases, horses with neurologic signs can recover from the infection but may retain neurologic deficits.

How can you prevent EHV-1 infection?


There are vaccines that protect against the respiratory forms of EHV-1 and EHV-4. The vaccine our clinic recommends is Calvenza, which protects against respiratory disease caused by EHV-1, EHV-4, and Equine Influenza. However, there is currently no licensed EHV-1 vaccine in Canada or the U.S.A which is proven to protect against the neurological disease associated with EHV-1. The best method of protection is always to maintain current EHV vaccinations on all horses on your property. This will reduce the amount of EHV being shed by latently infected horses.


In addition, follow correct biosecurity protocol when bringing new horses onto your premises, or when travelling, or during any activity where horses may come together. Take extra precaution at horse shows or events, in order to minimize the amount of direct and indirect contact between horses.


Proper biosecurity measures include extensive cleaning and disinfection of surfaces and equipment that come in contact with affected horses. Individuals treating or coming into contact with infected horses need to follow appropriate disinfection protocols.




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