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Equine herpes myeloencephalopathy (EHM) fact sheet

What is EHM?

Equine Herpes Myeloencephalopathy (EHM) is another name for neurological signs in horses caused by equine herpesvirus-1. Equine herpesvirus (EHV) affects horses worldwide. Almost all horses have been exposed to an equine herpesvirus without becoming seriously ill.

There are nine equine herpesviruses that affect horses. EHV-1 and EHV-4 are the most common in Canada and the US.

Is EHM a risk to human health?

Equine herpesvirus does not affect humans. However, EHV can be transmitted to other horses on skin, clothing, boots, etc.

What are the clinical signs of EHM?

EHV-1 can cause respiratory disease, abortion, neonatal death or neurological problems.

Fever is often the first indication of EHV infection. Respiratory signs include cough and nasal discharge. Neurological problems include incoordination, urine dribbling, weakness, reduced tail tone and inability to stand.

Young horses are most likely to develop respiratory issues associated with EHV (equine rhinopneumonitis or rhino). Older horses are more likely to spread the virus without getting sick.

Neurological Strain

The neuropathogenic strain of EHV-1 is more likely to cause neurological signs. There are two strains of EHV-1:

  1. Non-neuropathogenic or wildtype strain, which occasionally causes neurological signs, but is considered less likely to do so.
  2. The neuropathogenic strain has a genetic mutation that makes neurologic signs more likely. Although it is not a new disease, it is currently regarded as an emerging disease by some organizations because of the increasing number of EHM outbreaks in North America.

Where is EHM found?

Equine Herpesvirus is found worldwide. Outbreaks of the neurologic form, EHM, seem to be on the rise in North America.

How is EHM transmitted and spread?

EHM is a disease that only affects horses.

The virus is spread in the air when an infected horse coughs or by direct (nose to nose) and indirect (water buckets, grooming equipment, etc.) contact with nasal secretions from an infected horse. Horses may or may not appear ill when they are shedding the virus. Contact with aborted fetuses, fetal fluids and the afterbirth associated with EHV-1 abortions will also spread the virus.

Most mature horses have developed some immunity to EHV through repeated exposure to the virus. Horses are not protected from the forms of the disease that cause abortions or neurologic signs, even with repeated exposure.

How is EHM diagnosed?

Horses suspected of having EHM may have a nasal swab and/or blood samples analyzed by a polymerase chain reaction (PCR) test performed at a laboratory. This test identifies horses that are shedding the virus by detecting the genetic material of EHV-1.

How is EHM treated?

There is no specific treatment for EHM. Your veterinarian may suggest fluid therapy, anti-inflammatory therapy and supportive care if required. Anti-viral medication may be used in some cases.

How would the CFIA respond to an outbreak of EHM in Canada?

EHM is not federally reportable in Canada. However, horse owners should always follow recommended biosecurity measures to reduce the risk of introducing infection to their animals, and immediately notify a veterinarian if illness is suspected. Infected animals may be managed using disease management and biosecurity practices employed for other non-federally reportable diseases.

CFIA's Animal Biosecurity webpage has information on biosecurity practices.

Additional information

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