Herpes – Equine Herpes Virus

Equine herpes virus (EHV) is a contagious viral infection which can cause respiratory disease, abortions and neurological disease.

There are two different types of EHV which mos commonly cause disease in horses:

EHV-1: In breeding mares, an infection of EHV can seriously endanger the life of her foal, causing abortions in the last third of pregnancy. EHV-1 is the most commonly diagnosed cause of infectious abortion and we strongly advise protecting breeding mares. See our information sheet – Guidelines for reducing risk of EHV-1 in pregnant mares

In rare cases an EHV-1 infection can cause a neurological disease. Infected animals can become paralysed, collapse and have a very poor prognosis for survival.   This is because the virus damages the blood vessels supplying the spinal cord, causing the sinal cord to die.  We don’t know why some horses go wobbly and collapse 1-6 days after having a high temperature and others just have a cough.

EHV-4: This is very common and causes respiratory disease. Although overtly less serious in nature, it can cause a low grade infection – the strain on the immune system can drag your horse down for a number of weeks and cause great disruption to competition and racing yards.

EHV2+5 can cause uveitis (an often painful and sometimes blinding eye condition) – never ignore changes in the appearance of your horse’s eyes.

EHV does not stimulate significant natural immunity after initial infection and the main concern for the equine population as a whole is the way the disease persists in our horses. Once a horse is infected, that individual can harbour the virus throughout its life and potentially shed the disease to other animals without showing any outward signs. Re-activation and shedding from infected horses tends to happen when they are stressed – typically when the horse moves yards or is in hard work.

Any carrier or infected horse can start shedding the virus through nasal droplets (ie. like an aerosol spray from the nose), which can then infect others in close contact.  Nose to nose contact is the most common method of spread, but people and tack can spread the virus too.

In most cases of EHV infection the signs are limited to mild respiratory disease (runny nose, cough, swollen glands or legs, off food), but there may be no signs at all.  A high temperature is usually one of the first signs, (be suspicious of anything above 38.5oC (101oF)

Prevention and Management

Stable management and vaccination, are key to the control of the disease. Minimising the potential stresses such as travelling, other diseases/ill health, that set off disease spread is important as well as reducing issues such as overcrowding. Vaccinating your horse at an appropriate time will help prevent disease and perhaps more importantly will also help reduce the amount of infective virus that is shed to other in-contact horses, thus reducing the amount of disease present on your yard.  In our experience, yards which vaccinate against herpes virus have less problems with coughs and breathing problems.

In the event of an outbreak of EHV, the advice is much more specific.  Horses that are sick must be isolated along with any in-contact horses. The virus can survive in bedding or on grass for a few days.

  1. To be considered isolated, the isolated horses must be kept in a building that is physically separated from other horse buildings by a public highway or equivalent.
  2. Isolated horses must be attended to by separate staff and riders who are not in contact with horses on other (non-isolated) premises.
  3. To minimise risk of disease spread within an isolation premises, biosecurity measures on the property must include
    1. hand-sanitising facilities so that staff can clean their hands between handling each individual animal
    2. foot dips outside every stable
    3. separate coveralls outside each stable that are changed between caring for different horses
    4. separate tack, equipment, feed bowls for each horse
    5. facilities for disinfection of mucking out equipment must be in place
  4. All horses on the premises, including those which have not been in-contact, must have close clinical monitoring with twice daily temperature recording. Temperature records should be kept, any raised temperatures should be reported to your vet (normal rectal temperature 37 -38  degrees centigrade.
  5. All horses on the premises including those which have not been in-contact should not leave their premises.
  6. Laboratory diagnosis of EHV can be via blood samples (usually 2 samples 10-14 days apart are required) and/or nasopharyngeal

EHV vaccination

  • Vaccination decreases nasal shedding of virus if a vaccinated horse does become infected. Therefore, vaccination decreases the total amount of virus in the environment which in turn reduces the likelihood that other horses become infected.
  • However, no EHV-1 vaccine is registered for use to prevent neurological disease and vaccination has not been shown to reduce the risk of neurological signs.
  • Travelling, housing in large groups, and mixing of horses are all recognized risk factors for precipitating clinical signs of neurological EHV. Therefore, our strong advice is that travel should be avoided in the face of an outbreak regardless of whether horses are vaccinated or not.
  • It is essential that vaccination is not considered a means to overcome sub-optimal biosecurity. Avoiding mixing in at risk populations, close monitoring, early diagnosis and isolation of suspect horses remain the cornerstones of prevention regardless of vaccination status.
  • Vaccination of animals known or suspected to have recently been in contact with EHV-1 is not recommended.
  • Although vaccination will reduce the risk for the horse population as a whole, there is some evidence from previous outbreaks that recent vaccination is a risk factor for development of neurological signs in individuals thus advice for this group of horses is less definitive.
  • EHV is an endemic disease in Europe therefore it will continue to represent an ongoing threat after the current outbreak is over. Therefore, we advise EHV vaccination for horses. For horses to remain protected it is important that once the primary course has been given, vaccination should be continued in future with boosters required every six months.

Contact the practice for more information about EHV vaccination and the risks to your horses.

Last reviewed March 2021