Equine herpes virus (EHV) is a highly contagious viral infection which can cause respiratory disease, abortions and neurological disease. There are two different types of EHV which cause disease in horses: EHV-1, is less common, but can be severe in its effects, causing abortion and neurological disease. EHV-4 is very common and causes respiratory disease.
In breeding mares, an infection of EHV-1 can seriously endanger the life of the foal, and is the most commonly diagnosed cause of infectious abortion. As a previous infection can leave a horse as a carrier, the nature of the virus is such that there may be no signs of respiratory disease present in the mare prior to abortion. Most abortions occur in the last trimester (or third) of pregnancy. The foal is usually born dead. Those infected in the womb and born alive usually die shortly after birth.
EHV is a contagious viral disease which passes from horse to horse through coughing and inhalation of infective viral particles. Horses that become carriers can intermittently shed virus and so infect other in-contact horses. Typically, carriers show no signs of the disease, but will shed virus at times of stress or excitement, such as when mixing with new horses at stud. The tissues and membranes of an aborted foetus are highly infectious and every effort should be made to keep potential infection away from susceptible mares.
If a mare aborts, or a foal dies or becomes ill within 14 days of birth, then seek veterinary advice immediately, and place the mare (and, if appropriate, the sick foal) in strict isolation.
A vaccine is available to reduce the risk of abortion due to EHV. It is administered to the mare during pregnancy. It is also recommended that other animals be vaccinated, including stallions, as the herd is far better protected as a whole against this potentially serious disease.