Equine Herpes Virus

Equine herpes virus (EHV) is found in horse populations worldwide.1 There are different strains of the virus, the most common being EHV-1 and EHV-4.1

EHV-1 can cause respiratory diseases, especially in young horses, neurological diseases, and abortion.1 EHV-4 causes respiratory disease and may cause abortion in some mares.EHV can persist in the horse with reactivation and shedding of virus occurring during periods of stress.

If your horse is infected, you may notice the following signs:1

  • fever, which may be the only sign in some cases, and may be missed if the horse’s temperature is not measured
  • coughing
  • nasal discharge
  • abortion, which usually occurs without warning, late in the pregnancy.

The respiratory disease caused by EHV most commonly affects young horses.1 Shedding via respiratory secretions typically lasts 7-10 days but may be longer, with aerosol being the primary means of transmission, either directly or indirectly through poor hygiene.

Mares that have aborted also shed virus in respiratory secretions with virus also being present in the foetus, placenta, foetal membranes and foetal fluids. Affected foals that are born alive typically die within days. A single abortion can be a precursor to abortion outbreaks.

The neurologic disease caused by EHV-1 infection is known as equine herpes virus myeloencephalopathy (EHM), and is due to damaged blood vessels, including damage to the blood-brain barrier.

EHM may occur without any signs of respiratory disease and commonly affects the hind limbs and the urinary system.

Signs include:1

  • lack of co-ordination
  • urine retention incontinence
  • inability to stand up, if severely affected.

If your horse is only mildly affected, there’s a good chance of recovery, but there’s a poor outlook for those that have been severely affected. It may take several weeks or months to recover from neurologic problems, and some horses are affected for the rest of their lives.1

The equine herpes virus is transmitted in various ways.

  1. Horses breathe in the virus from infected horses – shedding via the respiratory system can last for 7 to 10 days1
  2. The virus can be spread indirectly when secretions from an infected horse (either from their nose or due to an abortion, which results in infected tissue and fluids) are transmitted by people or by sharing equipment1

You can vaccinate your horse against equine herpes virus with Duvaxyn® EHV-1,4, available from your vet.

Vaccination reduces the clinical signs of respiratory diseases caused by both strains of the virus, EHV-1 and EHV-4.

It also helps to control EHV-1 abortion when used in conjunction with appropriate management practices, as advised by your vet.

Your horse can get this vaccination from 5 months of age, or from 3 months if at high risk of infection. Your horse will need a second dose 4–6 weeks after the first dose, followed by a booster every 6 months.

Breeding mares should receive doses at 5, 7 and 9 months of pregnancy.

For more details about Duvaxyn® EHV-1,4 and correct dosing information, visit the Products section of this site.

Myth: Only horses showing clinical signs can spread equine herpes virus infection

Horses can transmit the virus without showing any clinical signs. Usually, the respiratory disease caused by EHV causes symptoms in young horses – weanlings and yearlings.1

Myth: Once recovered, a horse is no longer infectious

This is not true – a horse can transmit the equine herpes virus even after they have recovered.1

Myth: Once infected, a horse cannot have a recurrence of symptoms

Once infected, a horse can have a relapse of symptoms, caused by a reactivation of the virus. This may occur on and off throughout the horse’s lifetime.1


  1. Queensland Horse Council Inc. Equine Herpes Virus Fact Sheet, March 2009

Don't Wait to Vaccinate