Equine Viral Arteritis


Equine viral arteritis (EVA) is an infectious viral disease of horses that causes a variety of clinical symptoms, most significantly abortions. The disease is transmitted through both the respiratory and reproductive systems. Many horses are either asymptomatic or exhibit flu–like symptoms for a short period of time. An abortion in pregnant mares is often the first, and in some cases, the only sign of the disease. EVA has been confirmed in a variety of horse breeds, with the highest infection rate found in adult Standardbreds. Breeders, racehorse owners, and show horse owners all have strong economic reasons to prevent and control this disease. While it does not kill mature horses, EVA can eliminate an entire breeding season by causing numerous mares to abort. In addition, U.S. horses that test positive for EVA antibodies and horse semen from EVA-infected horses can be barred from entering foreign countries. As the horse industry becomes increasingly internationalized, nearly all major horse-breeding countries are ncluding in their import policies measures to reduce the risk of EVA. The U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service’s (APHIS) Veterinary Services (VS) program provides the equine industry with EVA diagnostic and surveillance support.


More than a century ago, a disease fitting the clinical description of what we now call EVA was reported in European veterinary literature. However, the virus was not isolated from horses in this country until 1953 during an epidemic of abortions and respiratory disease. The most recent EVA epidemic occurred in 1984 when this disease affected 41 thoroughbred breeding farms in Kentucky. This outbreak brought to light two very important findings about EVA: the efficiency with which an acutely infected stallion could venereally transmit the virus and the high carrier rate that immediately occurred in stallions following natural infection with the virus.


EVA is primarily a respiratory disease. Particles from acutely infected horses’ nasal discharges are inhaled, often during the movement of horses at sales, shows, and racetracks. Horses are herd animals that tend to commingle, and this close contact facilitates the spread of the virus. However, unlike other respiratory diseases, EVA can also be transmitted venereally during breeding, either naturally or by artificial insemination. When a mare, gelding, or sexually immature colt contracts the disease, the animal will naturally eliminate the virus and develop a strong immunity to reinfection. On the contrary, infected stallions are very likely to become virus carriers for a long time. Once stallions are in the carrier state, they transmit the virus to mares during breeding. While the mare will shed the virus easily, a pregnant mare infected with EVA may pass the virus to her unborn fetus. Depending on the stage of pregnancy, the fetus can become infected, die, and be aborted. If the infected foal is born, it will only live for a few days.