Animal Health - Equine Herpesvirus

Factsheet - ISSN 1198-712X   -   Copyright King's Printer for Ontario
Agdex#: 400/660
Publication Date: January 2013
Order#: 13-003
Last Reviewed:
Written by: Tim Pasma

Table of Contents

  1. Cause
  2. Clinicial Signs
  3. Treatment
  4. Protection and Management
  5. Transmission to Humans
  6. Reporting

Equine herpesvirus is a highly contagious disease that usually causes respiratory illness in horses. It can also result in abortion, neurological symptoms and even death, so it is important to prevent the spread of the disease.


Equine herpesvirus is caused by a common virus that is carried and spread by horses. Most horses have been exposed to the virus by the age of 2 years.

There are several types of the virus, including EHV-1 and EHV-4. The EHV-4 type is the most common and only causes respiratory disease. The EHV-1 type also causes respiratory disease but can create even more severe symptoms, including abortion and death of newborn foals. In rare cases, the EHV-1 type leads to severe neurological symptoms and death, a condition called equine herpesvirus myeloencephalopathy (EHM).

Every year, the Animal Health Laboratory in Guelph diagnoses several cases of respiratory disease or abortion due to EHV-1 in Ontario.

Clinical Signs

A horse infected with the disease may have a fever, little to no appetite and show signs of respiratory illness, including runny nose and cough. In pregnant horses, if the disease is more severe, the horse may abort several weeks after becoming sick. A foal that is born with the disease will usually die shortly after birth.

In rare cases with EHV-1, a horse will develop neurological signs ranging from slight incoordination to severe paralysis and death.

Horses can also carry the virus without becoming sick (Figure 1).

Herd of horses on pasture

Figure 1. Horses may carry the virus but not show signs of being sick. Source:


Call your veterinarian if you believe your horse is sick. Keep your sick horse separate from other horses and use different equipment to feed, water and care for it. Work with healthy animals first, then with sick animals. When working with sick horses, always wear separate clothes and boots and wash your hands when finished.

If your horse is diagnosed with EHM, your veterinarian will provide instructions on giving rest and care for your horse. Your veterinarian can make recommendations on medications to treat secondary infections. Steroids and antiviral medications can be effective in some situations. Horses may have to be euthanized due to the severity of symptoms. When a horse with neurological symptoms dies or is euthanized, it is extremely important that your veterinarian test the horse for EHM and other neurological diseases, such as rabies or Eastern equine encephalitis.

Horse wearing bridle standing in stable

Figure 2. Separate sick horses from healthy ones. Change clothes and wash hands and tools after handling a sick horse. Source:

Prevention and Management

The virus spreads easily by direct contact with an infected horse or contaminated equipment. It can also be spread on the clothing, hands or tools of people who have visited a farm affected by the disease. Biosecurity is important to prevent the spread of equine herpesvirus and EHM.

Practise disease prevention every day. Maintain a herd health program and have your veterinarian visit regularly. Keep new horses separate from the main herd for at least 2 to 3 weeks after arrival, and separate sick horses from healthy ones (Figure 2). Use signs to notify visitors of your biosecurity protocols, and keep a log of persons visiting your farm. Clean and disinfect equipment after use, and keep your farm clean and free of pests.

Your veterinarian can also make recommendations about quarantining and monitoring horses that have been to a horse show or other place where equine herpesvirus infection or EHM has been diagnosed.

There is a vaccine for equine herpesvirus that reduces shedding of the virus. However, the vaccine does not prevent the disease or minimize clinical signs and may not protect against EHM.

Transmission to Humans

This disease does not affect humans.


Veterinary laboratories in Ontario and veterinarians who use a laboratory outside of Ontario must notify the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) if EHM is diagnosed in an animal. OMAFRA will work with the equine industry to help prevent the spread of equine herpesvirus and EHM.

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