The equine herpes virus can strike mares without warning towards the end of their pregnancy, with devastating consequences. Emma Houghton MRCVS explains how precautionary measures can help protect both mare and foal

Spring is the time of year when breeding mares are at highest risk of an equine herpes virus (EHV) abortion.

Broodmares are in the last third of their pregnancy, with up to 95% of diagnosed EHV-1 (see below) abortions occurring at this stage. Breeding mares may not show any clinical signs before they abort. The infection can spread easily, so it is vital to be vigilant and, once one mare aborts, quickly alert those with others at risk, to avoid spread.

The virus remains present within our horse population mainly due to “carriers” — horses that harbour the infection in an inactive form. Under stressful conditions the virus is reactivated within the carrier and can then be transmitted to other horses by direct and indirect contact.

Transmission can be through nose-to-nose contact between horses, or via nasal secretions or items such as buckets, rugs or clothing. The virus can also be spread in the air by up to 50 metres. The aborted foetus, placenta and placenta fluids are also infectious, and must be carefully removed.

Events that might cause stress include travelling, moving locations, changes in equine social groups, competitions or another illness. In late pregnant mares, stressors can cause shedding or transmission of the virus within the mare or to other mares, with the virus crossing the placenta to cause abortion.

Respiratory signs of both EHV-1 and EHV-4 can vary, but usually involve a clear or purulent (consisting of or containing pus) nasal discharge, loss of appetite, depression, fever and occasional coughing. Neurological signs are identified by hindleg paralysis and recumbency. It’s often the case, however, that pregnant mares in the last eight to 11 months of pregnancy display no clinical signs.

In addition, abortion “storms” can occur whereby the virus is unknowingly transmitted through groups of breeding mares and multiple abortions then result. Weak foals are occasionally born alive following infection of the mare. Unfortunately, in most cases, the foal will then develop fatal pneumonia in the first few days of life.

Do vaccines work?

Vaccines are available in the UK against EHV-1 and EHV-4. In breeding mares, vaccination is recommended at five, seven and nine months of pregnancy.

Vaccination increases the level of immunity within the population against EHV, but, due to the nature of the virus with its inactive form among carriers, total protection is not possible.

Although vaccination will not prevent individual animals aborting due to EHV infection, studies show that it may be beneficial in preventing abortion storms, in addition to reducing the severity and duration of the disease if it does occur.

Additional prevention measures should be undertaken on stud farms, alongside vaccination.

Broodmares should be managed in small groups with similar due dates, from early pregnancy. They should be kept at pasture as much as possible to avoid sharing the same airspace.

Ideally, a mare should foal at home. If this is not an option, she should arrive at stud at least 28 days before her foal is due.

A pregnant mare should not travel with other horses. Mares arriving from other studs, the sales or abroad should be isolated for at least two weeks and regarded as high risk to the residential mare population.

What is EHV-1?

Equine herpes virus is present within the horse population worldwide. There are two common types, EHV-1 and EHV-4. Of these, EHV-1 is more serious as it can cause respiratory disease in young horses, abortion in pregnant mares and neurological paralysis in horses of all ages. EHV-4 usually results in a lower grade of respiratory disease, but can still cause abortion and neurological paralysis.

Several abortions were recorded in the UK last spring, and some this year too. Fortunately all were isolated events. Appropriate control measures were implemented, preventing the spread of the disease.

Ref: H&H 30 April, 2014