Part of Animal diseases

Equine herpesvirus-1

Affects horses worldwide causing serious illness including a neurological disease known as equine herpesvirus myeloencephalopathy (EHM).


Equine herpesvirus-1 (EHV-1) is associated with several disease syndromes including abortion, upper respiratory tract disease, and a neuropathogenic form known as equine herpesvirus myeloencephalopathy (EHM). Young foals are also at risk and can die from the infection. EHM is a disease that only affects horses.

EHM is not a new disease and is reported sporadically throughout the world including North America. Recent emergence of a more virulent mutant strain of the virus nEHV-1 has resulted in EHM outbreaks in several countries. The increased neuropathogenic ability of nEHV-1 has resulted in higher than previously reported morbidity and mortality. In Alberta, nEHV-1 is a provincially notifiable livestock disease.

How to report

If you suspect nEHV‑1 in your herd, call your veterinarian within 24 hours.

Neurotropic equine herpesvirus Type‑1 is a provincially notifiable disease under Alberta's Animal Health Act and must be monitored.

All suspected or confirmed cases must be reported to the Office of the Chief Provincial Veterinarian (OCPV) within 24 hours:

Hours: 8:15 am to 4:30 pm (open Monday to Friday, closed statutory holidays)
Phone: 780-427-3448
Toll free: 310-0000 before the phone number (in Alberta)
Fax: 780-415-0810

Clinical signs

The clinical signs of EHV-1 depend on the strain of the virus affecting the horse.

An alphaherpesvirus, EHV-1 is associated with:

  • fever
  • respiratory disease
  • abortion
  • death in young foals
  • neurological problems

According to the CFIA, 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.

Horses that are severely affected by the neurological form of the disease may have to be euthanized.

Where it’s found

Equine Herpesvirus is found worldwide. Outbreaks of the neurologic form, EHM, seem to be on the rise in North America. There are 9 equine herpesviruses that affect horses. EHV-1 and EHV-4 are the most common in Canada and the US. See confirmed cases of EHV-1 in Alberta.


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. For more information, see Equine herpesvirus myeloencephalopathy – fact sheet (CFIA)

Brain or spinal cord of horses that died with nervous disease can be submitted to a laboratory for virology or histopathological examination for confirmation.

How it spreads

EHV-1 is spread in the air when an infected horse coughs. It can also spread by direct (nose to nose) and indirect (such as water buckets, grooming equipment) 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-1 through repeated exposure to the virus. However, horses are not protected from the forms of the disease that cause abortions or neurologic signs, even with repeated exposure.

Risk to humans

Equine herpesvirus does not affect humans.

Prevention and control

Vaccination offers no protection against EHM. A stress-free environment, effective segregation and optimal biosecurity protocols help reduce the chances of introducing and spreading the disease within a barn.

An effective biosecurity plan is essential to prevent the introduction of infectious diseases like EHM into a herd. Continuous evaluation of biosecurity procedures is important. Commitment, dedication and persistence by all farm staff including owners and visitors is required for successful implementation of biosecurity.


Equine herpesvirus myeloencephalopathy – fact sheet (CFIA)