Equine Herpes Virus (EHV) 

The Disease:

EHV is a common virus, which occurs in horse populations world wide and is endemic in the U.K.

There are many different types of herpes virus in the horse, but the two most commonly seen types of EHV are EHV-1 and EHV-4.
Typically EHV-1 causes respiratory disease in young horses, abortion in pregnant mares and paralysis in horses of all ages.
EHV-4 usually only causes low-grade respiratory disease, but it can, in rare circumstances, cause abortion.

Following infection with an EHV horses carry the virus as a latent (silent) infection, which can reactivate at intervals throughout life – often brought on following a stressful situation. Once reactivated the virus causes clinical symptoms in the affected horse, and the affected horse then sheds the virus, thus spreading the virus to other horses. A similar virus causes “cold sores” in humans, with the development of cold sores happening throughout the affected person’s life, and the spread of infection generally happening when the “cold sore” is present on the affected person.
Figure 1: Life cycle of EHV-1 infection

Central Equine

Clinical signs of EHV -1:

Respiratory disease (by far the most common symptoms of EHV1 infection):
•    Elevated temperature
•    Serous (clear) nasal discharge (may progress to purulent (pus like))
•    Coughing

•    Abortion 
•    Birth of weak foals which are abnormal from birth and show signs of jaundice, neurological signs and may have difficulty breathing

Neurological signs:

•    Incoordination of hind limbs (occasionally incoordination of forelimbs)
•    Urine and/or faecal retention
•    Recumbency (lying down and unable to stand)

Clinical signs of EHV-4

Respiratory disease only:
•    Elevated temperature
•    Serous (clear) nasal discharge (may progress to purulent (pus like))
•    Coughing


•    An inactivated vaccine is available which reduces the clinical signs of respiratory disease caused by EHV-1 and EHV-4 and also reduces the risk of abortion caused by EHV-1.

Commonly Asked Questions about EHV-1 and EHV-4

How long does the virus survive for in the environment?

The virus can survive for several days in grass or bedding which virus rich bodily secretions have contaminated. As such, if you suspect a horse may have EHV infection you are best to isolate, remove any contaminated bedding and thoroughly disinfect the area.

I have pregnant mare – should I vaccinate?

This depends on the perceived risk of EHV-1 infection on the premises (consult your vet with regard to this). Most stud farms will request that mares are vaccinated. Pregnant mares should be vaccinated at the 5th, 7th and 9th month of pregnancy. In addition mares and young stock should be kept separate to reduce the risk of spread of the disease to pregnant mares.

If there is an outbreak of EHV infection should I vaccinate my horse?

Generally speaking vaccination is not performed in the face of an outbreak. The vaccination does not protect against the neurological form of the disease. Obviously it is always best to pre-empt a problem, so incorporating an EHV-1-4 vaccination into your horse’s preventative medicine programme may be a good idea. This coupled with good biosecurity should help minimize the risk of your horse contracting an EHV-1 or EHV-4 infection.

What do I do if my horse comes into contact with a horse, which has been suffering from EHV?

In this situation you should monitor your horse’s rectal temperature twice daily, and should it exceed 38.5°C and / or show any of the afore mentioned clinical signs you should immediately isolate the horse, contact your vet and monitor the other in contact horses. Fortunately most cases do not develop neurological symptoms.

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