What causes virus abortion in mares?

Virus abortion is caused by Equine Herpes Virus (EHV).

The two most common types of this virus are:

EHV-1, which causes respiratory disease in young horses, abortion in pregnant mares and paralysis in horses of all ages and types.

EHV-4, which usually only causes low-grade respiratory disease but can occasionally cause abortion.

Following the initial infection, most of these horses carry the virus as a silent or latent infection that can reactivate at intervals throughout their lives.

Is this a commonly occurring problem in Ireland?

Equine herpes is a common virus that occurs in horse populations worldwide.

At what stage of pregnancy does the mare abort?

Abortion caused by this virus usually occurs in late pregnancy from eight months onwards. However it can happen as early as four months and can occur from two weeks to several months following infection with the virus.

This abortion can be the result of either a recent infection or re-activation of latent infection in a carrier horse.

Some mares infected with virus abortion can carry to full term but the foal usually dies within a week.

What are the signs of EHV infection in a pregnant mare?

There are usually no specific warning signs of abortion caused by equine herpes virus.

A mare will usually “bag up” or develop increased udder size fairly suddenly, a few days before aborting, even though she may not be due to foal for three–four months.

Therefore when checking pregnant mares each day, it is important to take special note of the udder size of these mares.

A sudden and unexpected abortion should always be treated with suspicion, the mare should be isolated and veterinary help sought to confirm or rule out EHV infection without delay.

Many owners/managers tend to rug pregnant mares, which is a disadvantage in these abortion cases as a sudden ‘springing’ in her udder, may go unnoticed while concealed by the rug.

Foals born alive but infected in utero are usually abnormal from birth, showing weakness, jaundice, difficulty in breathing and occasionally neurological signs. They usually die, or have to be put down within a few days.

What are the signs in cases of the respiratory form of EHV?

The signs of EHV respiratory disease include mild fever, occasional coughing and discharge from the nose.

The most common sign in older foals, usually following weaning, is a nasal discharge.

Less commonly, secondary bacterial infection may cause pneumonia in infected animals.

How is EHV infection spread?

One of the commonest methods of spread is via the respiratory system.

Infected older horses, or indeed foals with a herpes respiratory infection that come in contact with pregnant mares, can spread it and cause abortion.

The older horses or mares in the vicinity may inhale the infection, or it can be spread by utensils or the grooms in attendance.

When the infected mare aborts, the aborted foetal fluids and the placenta are a major source of infection for other mares.

If this abortion occurs in the midst of a herd of unvaccinated pregnant mares, it could result in an “abortion storm”.

The virus can remain infectious for up to one month. Therefore the paddock or stable where the abortion occurred could remain infected for that length of time if the areas have not been thoroughly disinfected.

There is a continual cycling of the herpes respiratory virus disease in young horses and an intermittent reactivation of latent virus in older horses. This maintains the risk of virus abortion in pregnant mares and of herpes paralytic (neurological) form of the disease in horses of all types and ages.

The respiratory disease caused by herpes virus is most common in weaned foals and yearlings, usually in autumn and often winter and referred to as “autumn snots”.

However, older horses that become infected are more likely than younger ones to transmit the virus without showing clinical signs of infection.

What types of horses are ‘carriers’ of EHV?

Horses can be ‘carriers’ of EHV in a latent form (meaning that they carry a silent form but are not always infectious to others). This can, under conditions of stress, be reactivated, so they can then transmit infection without showing signs of illness.

In carriers, illness (respiratory, abortion or neurological) may become apparent from time to time, especially after stress (particularly travelling and changing of location and social groups) or after suffering another disease. The virus is potentially contagious at these times and may be transmitted to otherwise healthy but susceptible horses, who may then develop EHV disease.

Are the risks of spread very high if virus abortion occurs on a stud?

As the disease is easily spread, owners visiting the premises where the herpes virus abortion infection is present, should be notified of the risks.

In the case of thoroughbreds, the ITBA should be notified. In the case of sport horses, Horse Sport Ireland should be notified.

Can EHV be effectively treated?

There is no specific treatment.

Treatment of clinical abnormalities and complications will be determined by the treating veterinary surgeon.