Equine herpesvirus type 1 (EHV-1)

EHV-1 virus is one of a large group of DNA viruses and is an important cause of disease in horses. EHV-1 causes both sporadic and epidemic abortion (abortion storms), as well as respiratory disease and neurological disease. The neurological strain of EHV-1 is very rare in Australia.

EHV-1 is a common cause of respiratory disease of horses. Once infected, it is assumed horses carry the virus for life. When horses are stressed, the virus can be reactivated and excreted.

EHV-1 is endemic in Australia and disease can occur wherever horses are kept.

Symptoms of both EHV-1 include:

• fever (39-40.5^oC)
• conjunctivitis
• depression
• nasal discharge and a cough
• lack of appetite
• possible swelling of the lymph glands around the throat.

Mild 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 EHV-1 usually occurs between 8-10 months' gestation and, occasionally, as early as 4-5 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.

Horses with neurological disease caused by EHV-1 infection show signs of incoordination, weakness and have trouble standing. Rear limbs tend to be more severely affected than forelimbs. Other signs can include difficulty urinating and defaecating, extreme lethargy and a coma-like state.

EHV-1 infection is likely to result in loss of training time and reduced performance of working or racing horses. Losses due to abortion and peri-natal deaths may be significant, particularly during abortion storms.

This disease is highly contagious and spreads easily in horse populations by direct horse-to-horse contact or by contaminated fomites such as equipment (e.g. feed and water buckets) and tack (e.g. halters, bridles).

Infection of EHV-1 occurs mainly by inhalation, but also by ingestion of material contaminated by nasal discharges or aborted foetuses.

Actions if an aborted foal is found or abortions are suspected

• Isolate the mare from contact with all other horses.
• Leave her halter and lead rope with her in isolation.
• Call your veterinarian.  Your veterinarian will provide you with advice on determining the cause and biosecurity measures to take to protect other horses.  Your veterinarian will also provide advice on notifying Biosecurity Queensland.
• Use a disposable respirator and 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 ) for veterinary examination:
  • When aborted foetuses are necropsied, gross foetal lesions may include subcutaneous oedema, jaundice, increased thoracic fluid volume and an enlarged liver with yellow-white lesions.
  • Foetal membranes and a range of foetal tissues, both preserved and either fresh chilled or in virus transport medium should be submitted to the laboratory for diagnostic testing.  Lung, liver, adrenal glands and lymphatic tissues should be provided.
  • In cases where EHV-1 abortion is suspected but the foetus and foetal membranes cannot be found, paired serum samples from the mare may help determine if EHV-1 was the cause of abortion.
• Keep other horses away from the abortion site for at least two weeks after the area has been decontaminated.

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 chlorhexidine or phenols
• shower and change personal clothing. Washing and sundrying clothes will effectively inactivate the virus
• disinfect vehicles and floats used to transport the aborted mare and anything that has come in contact with the foetus, foetal fluids or discharge from the mare.

Until a diagnosis is reached:

• keep the mare completely isolated
• avoid entering the contaminated area at all if possible or use a disinfectant footbath outside the area 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 clothes before they do.

Prevention

Disease introduction can be prevented and disease spread restricted by good biosecurity and 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.
• Talk to your veterinarian about vaccinating pregnant mares to help reduce the risk of abortion.

Treatment

There is no specific treatment for EHV1 infection in horses.

Control

• Isolate mares that have aborted for at least 30 days.
• Consult your veterinarian about a vaccination program . While vaccination does not provide 100% protectio, 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 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.
• 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).