Equine Herpesvirus (EHV)

What is equine herpesvirus (EHV)?

Equine herpesviruses are DNA viruses that are found in most horses all over the world. Almost all horses have been infected with the viruses and have most of the times no serious side effects. It is currently unknown what causes some infected horses to develop the serious neurological forms associated with EHV1 that may be fatal. EHV stands for equine herpes virus.  This is a family of viruses which are named by numbers such as EHV 1, 2, 3, 4 and 5.  There are more viruses in this family, but EHV 1, 3, and 4 pose the most serious health risks for domestic horses. Equine herpesvirus myeloencephalopathy (EHM) is another name for the neurologic disease associated with equine herpesvirus (EHV) infections.

EHV-1: Can cause four manifestations of disease in horses, including neurological form, respiratory disease, abortion and neonatal death.

EHV-3: Causes a venereal disease called coital exanthema that affects the external genitalia but has not been shown to affect fertility.

EHV-4: Causes a nonfatal upper respiratory tract disease in foals and is uncommonly associated with abortion and rarely with neurological disease.

EHV is a common DNA virus that occurs in horse populations worldwide. The two most common species are EHV-1, which causes abortion, respiratory disease and neurologic disease; and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion and rarely neurological disease. Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. Adult horses are more likely than younger ones to transmit the virus without showing signs of infection.

EHM results from widespread vascular or blood vessel injury after damage to the lining of the blood vessels of the blood-brain barrier. Neurologic signs result from inflammation of the blood vessels, blood clots, and death of neurologic tissue. Cases of EHM occur singly or can affect multiple infected horses. They may or may not be associated with a previous or ongoing EHV-1 respiratory or abortion disease outbreak.

How does EHV spread?

EHV-1 is contagious and spread by direct horse-to-horse contact via the respiratory tract through nasal secretions. It is important to know that this virus can also be spread indirectly through contact with physical objects that are contaminated with the virus:

Human contaminated hands or clothing

Contaminated equipment and tack

Contaminated trailers used for transporting horses

Contaminated wipe rags or other grooming equipment

Contaminated feed and water buckets

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

How long can the virus live outside of the horse’s body?  This includes on clothing, footwear, walls, buckets, tack, etc.

The virus can survive up to 7 days in the environment under normal circumstances but can remain alive for a maximum of one month under perfect environmental conditions. The most important measure is to first clean equipment and horse housing areas. (Please note: It is important to wash and rinse surfaces when possible prior to applying disinfectants.) Cleaning first allows for removal or organic material which makes the disinfectants more effective. After cleaning the surface, follow with a disinfection process. The virus is easily killed in the environment by most disinfectants. Conventional disinfectants and detergents are the best. It is important to perform hand hygiene (wash hands with soap and dry thoroughly or use alcohol based hand sanitizer) when moving between horses that are grouped separately to avoid spreading pathogens that may contaminate your hands.

What are some of the signs of EHV?

After infection, the incubation period may be as short as 24 hours, is typically 4-6 days, but can be longer. EHV-1 typically causes a biphasic (two-phase) fever peaking on day 1 or 2 and again on day 6 or 7. With respiratory infections there is often serous or mucoid nasal and ocular discharge, but not a lot of coughing. There may be some persistent enlargement of submandibular lymph nodes (lymph nodes under the jaw). With the neurologic form, there are typically minimal respiratory signs, fever (rectal temperature greater than 101.5 degrees F) being the only warning sign. Neurologic disease appears suddenly and is usually rapidly progressing, reaching its peak intensity within 24 to 48 hours from onset of initial neurologic signs. It is also pertinent to mention that horses may develop EHM even without any preceding fever and/or respiratory signs. Clinical signs of the neurologic disease may include:


Hind limb weakness

Loss of tail tone


Urine dribbling

Head tilt

Leaning against a fence or wall to maintain balance

Inability to rise

Source FEI


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