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 with the disease are asymptomatic, and others exhibit flulike symptoms for short periods. In mares, abortions are among the first, and in some cases, only signs of the disease. EVA has been confirmed in a variety of horse breeds, and the highest infection rate is found in adult standardbreds.
Breeders, racehorse owners, and owners of show horses all have strong economic reasons to prevent and control this disease. Although it does not kill mature horses, EVA can virtually eliminate an entire breeding season by causing a high percentage of 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 enforcing import policy measures to reduce the risk of EVA.
EVA is primarily a respiratory disease. Healthy horses can inhale particles in the nasal discharges from acutely infected horses during movements at sales, shows, and racetracks. Because horses are herd animals that tend to commingle, this close contact facilitates the spread of the virus.
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. Infected stallions, on the other hand, 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.
Although the mare will eliminate the virus easily, a pregnant mare infected with EVA may pass the virus to her unborn fetus. As determined by the stage of the pregnancy, the fetus can become infected, die, and be aborted. If the infected foal is born, it will live for only a few days.
Many horses infected with EVA are asymptomatic. When clinical signs do appear in the acute stage of the disease, they can include any or all of the following: fever, nasal discharge, loss of appetite, respiratory distress, skin rash, muscle soreness, conjunctivitis, and depression. Other clinical signs are swelling around the eyes and ocular discharge, swollen limbs, swollen genitals in stallions, and swollen mammary glands in mares. Abortion in pregnant mares is also a symptom of EVA. Abortion rates in EVA-infected mares range from 10 percent to 70 percent.
Horse owners should suspect EVA when respiratory symptoms accompany an abortion in a mare. Because the clinical signs of EVA are similar to those of other respiratory disease and no characteristic lesions are found in EVA-aborted fetuses, only diagnostic tests can confirm the disease. Virus isolation can be attempted from swabs of the nose, throat, or eyes; semen, placentas, or fetal tissue; and blood samples. The most common method of diagnosis is testing blood for the neutralizing antibodies of the virus. Although presence of these antibodies alone does not indicate active infection, it does signify that EVA exposure has occurred. The signs of active infection are very high levels of antibodies on a single sample or a rising antibody titer from paired blood samples collected 14 to 28 days apart.
Although there is no specific treatment for EVA, care should include rest and, in selected cases, antibiotics, which may decrease the risk of secondary bacterial infection. Adult horses recover completely from the clinical disease. However, the virus commonly persists in the accessory glands of recovered stallions, and thus these carrier stallions continue to shed the virus for years and remain a significant source of infection.
A safe, effective, and low-cost avirulent live-virus vaccine is now available. Combining this vaccine with isolation of the vaccinated animal from noninfected horses can prevent the spread of EVA. Because properly vaccinated EVA-negative stallions do not become carriers, all EVA-negative colts less than 270 days old should be vaccinated. The vaccine is not approved for use in pregnant mares. All vaccinated horses should receive yearly boosters to protect against infection and, for the stallions, to prevent the development of a carrier state.
The EVA Uniform Methods and Rules contain minimum standards for detecting, controlling, and preventing EVA. These may be obtained by contacting your local APHIS-VS District Office or found at: http://www.aphis.usda.gov/animal_health/animal_diseases/eva/downloads/eva-umr.pdf