Equine Herpesvirus (EHV)

There are three main types of herpes virus in horses:

Equine herpesvirus 1 (EHV-1);

which causes respiratory disease and also occasionally abortions and neurological disease.

Equine herpesvirus 4 (EHV-4);

causes respiratory disease (generally mild).

Equine herpesvirus 3 (EHV-3);

which is less commonly seen but may cause venereal disease in breeding mares and stallions.


Equine herpesvirus (EHV) 1 and 4 most commonly affects young stock and results in signs of respiratory disease including snotty nose, cough, fever and a reduced appetite.

EHV-1 can cause abortion in pregnant mares, stillbirth or the birth of weak and sick foals. The abortions can occur in “storms” where large numbers (up to 50 per cent) of unvaccinated mares lose their unborn foal. Where pregnant mares are vaccinated the abortion rate is significantly reduced.

EHV-4 can cause abortion but has not been associated with such ‘storms’.

Finally, neurological disease can occur with a variety of signs ranging from mild muscle weakness (usually the hind limbs) to paralysis of the bladder (incontinence) and collapse and paralysis necessitating euthanasia.


  • a common cause of respiratory disease in young stock:
  • may present with respiratory disease signs and/or poor performance;
  • abortion may occur, usually in the later stages of pregnancy:
  • neurological disease may be fatal but is rare;
  • vaccination is key to prevention, but it is not licensed to prevent the neurological form of the disease.


EHV is contracted via the respiratory tract but not all animals will show signs of respiratory infection prior to abortion or neurological disease.

Vaccination is the most effective method of management and will protect against both EHV-1 and 4. Whilst vaccination does not prevent infection it significantly reduces the chance of infection, the severity and length of illness, as well as reducing further spread in the equine population. However, despite good protection, the cover is temporary and boosters need to be done regularly. A standard vaccination programme is:

  • two primary vaccines four to six weeks apart;
  • booster vaccines at six month
  • intervals;
  • foals vaccinated from three months;
  • pregnant mares vaccinated at five, seven and nine months of gestation.

Vaccinating in the face of an outbreak is currently a controversial subject, because occasionally vaccination in affected herds has elicited more severe symptoms. It is suggested infection control procedures (isolation, disinfection, treatment of individuals) would be more appropriate.

As a general principle, pregnant mares should not be in contact with young horses or other show or performance horses. New arrivals should be isolated for two to three weeks and monitored before being allowed to enter the main herd.


In any equine abortion:

  • isolate mare and collect aborted foetus and associated membranes to be examined by the vet;
  • thoroughly disinfect the foaling area;
  • isolate the affected mare;
  • isolate any ‘in-contact’ mares;
  • institute vaccination only after veterinary consultation.

Treatment of respiratory disease:

  • there are no specific antiviral medicines licensed for this disease in horses, but some ‘immunomodulator therapies’ used in humans, such as Echinacea, have been tried.
  • antiinflammatories, rest and good nursing care are important.