Equine herpesvirus type 1 (EHV-1)

EHV-1 is a highly contagious viral disease of horses. In Australia, it:

• commonly causes mild respiratory disease
• occasionally causes sporadic or epidemic abortions (abortion storms) and neonatal deaths
• very rarely causes neurological signs.

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

Respiratory disease

EHV-1 (and EHV-4) infection in young horses usually presents as an acute febrile respiratory illness that can spread rapidly. Signs include:

• fever (39–40.5°C)
• conjunctivitis
• depression
• nasal discharge and a cough
• lack of appetite
• possible swelling of the lymph nodes around the throat.

Subclinical infections are common, even in young animals.

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 and neonatal death

Abortion in horses due to EHV-1 usually occurs between 8 and 10 months gestation but occasionally is as early as 4–5 months gestation. Abortions occur any time from 10 days to 12 weeks after virus infection. The mare may not show signs of respiratory infection before aborting and sometimes the foetus is expelled still covered with the placenta.

Mares infected late in pregnancy may have live infected foals. Sometimes these foals appear normal at birth, while other times they are born very sick. Infected foals become weak and very depressed, and often develop respiratory signs. They respond poorly to treatment and often die after a few days.

Neurological disease

Horses with neurological disease caused by EHV-1 infection can show signs such as:

• fever
• incoordination, weakness and trouble standing (with rear limbs often more severely affected than forelimbs)
• difficulty urinating and defecating
• decreased tail tone
• rarely, 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.

Foal losses due to abortion and perinatal deaths may be significant, particularly during abortion storms.

Neurological disease due to EHV-1 is very rare in Australia; however, neurological outbreaks overseas have caused horse deaths and cancellation of events.

This disease is highly contagious and spreads easily in horse populations by:

• direct horse-to-horse contact
• contaminated fomites such as equipment (e.g. feed and water buckets) and tack (e.g. halters and bridles).

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

Once a horse is infected, it will carry the virus for life. When the horse is stressed, the virus can be reactivated and excreted. During this time, the virus can be spread to other horses.

Legal requirements

If you own, treat or manage horses, you have a general biosecurity obligation to take all reasonable and practical measures to prevent or minimise the effects of a biosecurity risk. This means that if you are dealing with horses and other possible carriers, you must:

• reduce the risk of EHV-1 infection
• limit the spread of the virus.

Recommended actions following an abortion or suspected abortion

Immediate actions

• Isolate the mare from contact with all other horses.
• Leave her halter and lead rope with her in isolation.
• Call your veterinarian. They will give you advice on:
  • determining the cause of the abortion
  • biosecurity measures to take to protect other horses.

Samples to gather for diagnosis

• 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 veterinary examination and sampling.

Biosecurity measures

• Anyone who has had contact with the mare, foetus or placenta should:
  • disinfect their hands and footwear
  • shower and change their personal clothing
  • wash and sun-dry their clothes (to inactivate the virus).
• Disinfect vehicles and floats that have been used to transport the mare.
• Disinfect anything that has been in contact with the foetus, foetal fluids or discharge from the mare.
• Keep other horses away from the abortion site until at least 2 weeks after the area has been decontaminated.

Actions until a diagnosis is reached

• Keep the mare completely isolated.
• Avoid entering the contaminated area. If you must enter the area, use a disinfectant footbath outside the area to disinfect your 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. If this is not possible, before working with other horses the person should:
  • disinfect their hands and footwear
  • shower and change their clothes.

Reporting and management

The abortogenic and neurological strains of EHV-1 are category 1 restricted matter under the Biosecurity Act 2014. Respiratory disease caused by EHV-1 is common and not reportable.

If you become aware of the presence of EHV-1 causing abortion or neurological disease in horses, you must report it as soon as practicable by phoning either:

• Biosecurity Queensland on 13 25 23
• or
• the Emergency Animal Disease Hotline on 1800 675 888.

If EHV-1 infection linked to abortion or neurological signs is confirmed in a horse, Biosecurity Queensland will work with the animal owner and their veterinarian to manage the situation.

Prevention

Following good biosecurity and management practices will help to prevent introducing disease and to restrict its spread.

Guidelines

• Ensure all people working with horses understand isolation and infection control procedures, and implement hygiene practices that will help prevent the spread of disease.
• Establish isolation facilities stocked with biosecurity tools such as gloves, plastic bags and disinfectant.
• Isolate visiting and newly introduced horses for at least 14 days.
• Clean and disinfect horse floats after each journey.
• 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
  • mares with foals.
• Keep resident mares separated from mares that are visiting for the stud season only.
• Minimise stress for pregnant mares such as overcrowding and transport in late gestation. Provide them with good-quality feed and adequate parasite control.
• Keep accurate movement records.
• Consult your veterinarian about a vaccination program.

Treatment

There is no specific treatment for EHV-1 infection in horses.