Equine herpesvirus

Equine herpesvirus is category 1 restricted matter.

Under Queensland legislation, if you suspect the presence of this disease in any species of animal, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Disease Watch Hotline on 1800 675 888.

There are 5 strains of herpes viruses that cause disease in horses and donkeys in Australia.

  • EHV1 is the most significant because it is widespread and causes abortion, stillbirths, septicaemia in newborn foals, respiratory and nervous signs.
  • EHV2 causes mild respiratory disease.
  • EHV3 is spread during mating and causes ulcerative sores on the vulva and vagina of mares, and penis of stallions. Pain may prevent mating, but recovery is usually complete within 2 weeks.
  • EHV4 usually causes upper respiratory tract disease. On rare occasions it may be associated with abortions in individual mares.
  • EHV5 causes mild respiratory disease.

Of the equine herpesviruses (EHV) types 1 to 5, EHV1 is nearly always the cause of abortion outbreaks, while EHV4 has been the cause of the odd sporadic case. EHV1 is also a common cause of respiratory disease in young horses, and has occasionally caused neurological disease with incoordination, paralysis and subsequent death. EHV3 causes equine coital exanthema, which is a mild vaginal infection spread venereally. EHV4 is a major cause of respiratory infection (usually called rhinopneumonitis). EHV2 and EHV5 have been associated with mild respiratory disease. There is no evidence that any of these equine viruses affect humans.

EHV associated with abortion or neurological signs in horses is a notifiable disease under legislation, and all suspect and confirmed cases in horses should be reported to Biosecurity Queensland.


EHV1 virus is one of the common causes of 'colds' in young horses and most have antibodies to it by the time they are 2-3 years old. Older horses seldom show clinical signs of infection. Once infected, horses carry the virus for life and, when they are stressed, the virus is reactivated and excreted.

While EHV4 is restricted to the respiratory tract and nearby lymph nodes, EHV1 has a predilection for the cells lining blood vessels, and so can manifest as abortion or as nervous symptoms in some cases.

Infection leading to both respiratory disease and abortion occurs mainly by inhalation, but also by ingestion. The aborted foetus, foetal membranes and fluids, and uterine discharges from the mare contain large amounts of infective virus. Both discharges and aerosols from an aborted mare, clinically-affected young horse or carrier horses are also sources of virus infection. Virus contamination of pasture, feed, feed bins, water troughs, halters, rugs, bedding, transport vehicles, and staff clothing can lead to infection. Weak EHV1-infected foals can be a source of infection to other foals that were born healthy.

After aborting, the virus in the mare's genital tract disappears in a few days, but she may shed virus through the respiratory route for up to 2 weeks. Virus may remain infective in the environment and on horse hair for up to 6 weeks in cool moist conditions, if cleaning and disinfection are not adequate.

Other names


Distribution in Queensland

The disease occurs wherever horses are kept. Abortion and neurological signs are reported infrequently compared to respiratory signs. Neurological cases have been reported, particularly in the United Kingdom and the United States.

Only the respiratory form of the disease was recognised in Australia before 1977. Since that time, single abortions and abortion storms due to EHV1 have occurred sporadically. Neurological disease associated with EHV1 infection is extremely rare in Australia.

Life cycle

The virus can survive 14-45 days outside the body, depending on environmental conditions.

Affected animals

horses; donkeys


Symptoms of both EHV1 and EHV4 include:

  • fever
  • depression
  • nasal discharge and a cough
  • lack of appetite
  • possible swelling of the lymph glands around the throat.

Milder respiratory disease may involve only conjunctivitis and a cough with little impairment of respiratory function. The nasal discharge and cough can last up to 3 weeks. Secondary bacterial infection can result in pneumonia.

Abortion in horses due to EHV1 usually occurs between 7-11 months' gestation and, occasionally, as early as 4 months' gestation. Abortions occur anywhere from 10 days to 12 weeks after virus infection. The mare may not show signs of respiratory infection prior to aborting and sometimes the foetus is expelled still covered with the placenta.

Mares infected late in pregnancy may have a live foal. Infected foals may be normal at birth, but become weak, very depressed and die in a few days with signs of respiratory disease.

Adult horses can be affected by the nervous form of the disease. Signs can vary from slight temporary ataxia to severe incoordination and recumbency, requiring euthanasia.


Abortions due to EHV1 are common in the United Kingdom and the United States, and the virus is found worldwide. Cases of EHV abortion have been confirmed in Queensland in mares allegedly coming from southern states where other cases of abortion due to EHV1 have been diagnosed.

How it is spread

This disease is highly contagious and spreads easily in horse populations. The virus is found in nasal secretions or in the products of abortion. Horses contract the disease from inhaling infected droplets or eating contaminated material.

Monitoring and action

Paired blood samples should be collected to demonstrate rising antibody levels. When aborted foetuses are autopsied, sections of a range of organs should be collected, both preserved and either fresh chilled or in virus transport medium (include thymus and gut contents).

Actions if an aborted foal is found

  • Isolate the mare from contact with all other horses.
  • Leave her halter and lead rope with her in isolation.
  • Call your veterinary surgeon.
  • Wear gloves to pick up the foetus and placenta and put them in a strong plastic bag.
  • Keep the foetus and placenta cool (not frozen) for the veterinarian to examine and to preserve it until it, or samples from it, can be sent to the laboratory.
  • Fence off and treat the contaminated pasture with lime.

Anyone who has had contact with the aborted mare or the foetus or placenta should:

  • disinfect hands and boots with a good surface disinfectant, for example iodophors (Iovone Scrub), hexachlorophenes (Hibitane or other chlorhexidine disinfectant) or phenols (Polyphen Polyphenolic Microbiocide)
  • shower and change clothes. Washing and sun drying clothes will effectively kill the virus
  • disinfect the vehicle used to transport the aborted mare to quarantine (inside and wheels) and anything that has come in contact with the foetus, foetal fluids or discharge from the mare.

Until a diagnosis has been made:

  • keep the mare completely isolated
  • avoid entering the quarantine yard at all if possible or use a 5% formalin footbath outside the yard to disinfect footwear
  • provide the isolated mare with her own feed bin and other equipment, and do not use them for any other horse
  • make sure the isolated mare is cared for by someone who does not work with other horses or who disinfects, showers and changes before they do.



Disease introduction can be prevented and disease spread restricted by good management practices:

  • Keep weanlings, yearlings and non-breeding stock in a separate part of the farm with their own facilities and staff.
  • Separate pregnant mares from non-pregnant mares and from mares with foals.
  • Keep resident mares separated from mares visiting only for the stud season.
  • Isolate visiting mares on arrival at stud for 3-4 weeks. Keep them in small groups of 2 or 3, which can be combined if they remain disease-free.
  • Minimise stress factors for pregnant mares such as overcrowding and transport in the last 2 months of gestation, and provide access to good quality feed and ensure adequate parasite control.
  • Establish isolation facilities equipped with gloves, plastic bags, disinfectant etc.
  • Clean and disinfect horse floats after each journey.
  • Train staff so they understand about isolation and the principles of preventing disease spread. Implement hygiene procedures that will help prevent spread of disease.
  • Keep accurate records of mares (e.g. date of arrival, where from, previous stud details) and of movements of horses on the farm. These are essential for identifying the source of infection.
  • Consider vaccinating pregnant mares in the 5th, 7th and 9th months of pregnancy, which will help reduce the risk of abortion.

Treatment if EHV1 is confirmed

There is no specific treatment for EHV1 infection in mares. Most foals infected with EHV1 before birth succumb within a few days despite nursing and antibiotic administration.

Management if EHV1 is confirmed

  • The aborted mare must be isolated for at least 30 days. After that (provided there are no further abortions on the property), she can be treated as a dry mare and bred.
  • Mating should stop as should horse movements on and off the property.
  • Consult your veterinarian about a vaccination program. While vaccination does not induce total immunity to these viruses, it may prevent some abortions and it will reduce the risk of spread of infection.
  • Communication with other stud farms is essential. Simple precautions will reduce the risk of disease spread to other farms and minimise disruption to horse movements usual during the stud season.
  • Where there is only one case of abortion and the mare has been rigorously isolated and thorough disinfection has occurred, there is minimal risk of spread of infection and the management of the unexposed mares can continue as normal.
  • The long-term prospects for the mare that aborted are good. Her conception rate is not adversely affected and it is rare for her to abort again due to EHV1.


In-contact pregnant mares should be isolated until they foal and for 30 days after foaling or, if another mare aborts, 30 days after the last abortion. In-contact mares include those in the same paddock as the aborted foetus and those in direct contact with the mares in the same paddock (e.g. sharing a water trough or in direct contact over a single fence line).