September 2006
From the State Vet's Office

Equine Viral Arteritis

Not Very Common But May Cause Problems

by Dr. Tony Frazier

Equine Viral Arteritis (EVA) is a contagious disease of horses, mules, zebras, and other equids. As the name indicates, it is caused by a virus.

It is often a mild disease characterized by fever, depression, and swelling (edema) in the lower limbs and lower chest and abdomen. In healthy horses, EVA is usually not too much of a problem, although it can vary from an infection with no signs of illness to severe disease that includes a great deal of edema, respiratory problems, and even some central nervous system signs such as ataxia (incoordination). The primary concern, however, is the reproductive problems that are associated with the disease.

These reproductive problems include abortions and a possible period of subfertility in the stallion. Consistent with many diseases, clinical signs of the disease are more severe in the very young and the very old. Although death is rare, it had been reported in young foals as a result of severe pneumonia or a combination of pneumonia and intestinal inflammation.

The equine industry in the United States has traditionally attached little significance to the disease. However, since an epidemic of the disease in Kentucky in 1984, EVA has had a significant impact on international trade of horses and equine semen. The United States currently does not have an import control policy for EVA, but all other major horse-breeding countries have policies that deny entry of carrier stallions and EVA-infective semen. According to the USDA, efforts to gain a greater level of control over the dissemination of EVA within the country’s equine population have been hampered by a lack of awareness of the disease and its potential consequences in the horse industry.

In 1998, Alabama participated in a study conducted by the UDSA that, among other things, questioned randomly selected horse owners about their knowledge of EVA. Like most owners in other states that participated, those in our state indicated that most had only heard of EVA with little knowledge of the disease or had no knowledge about it at all. Another finding in this National Animal Health Monitoring System study indicated that on random blood sampling for EVA, there was a low seroprevalence, meaning there was very little exposure to the virus in our country. Because they have never been exposed to the virus, the vast majority of our nation’s horse population could be considered completely susceptible to natural infection.

The virus can be found in many countries of the world with Japan and Ireland being exceptions. Outbreaks of the disease are not very common, but are usually associated with the movement of horses or shipping of semen. Outbreaks that have occurred may be widespread, but the majority of the horses infected may never show signs of illness.

Aerosol is the primary mode of transmission, although it can be transmitted venerally, from dam to offspring, or indirectly through the environment, on feed or other materials. Aerosols from acutely infected horses are responsible for the widespread dissemination of the virus at racetracks and other places horses are congregated. Mares, geldings, and young colts that become infected usually clear the virus from their body after a period of time. After the incubation period of 3-14 days, the actual time that signs of disease may be present is 2-9 days. Stallions, however, often develop into carrier states in which the virus may exist for years in the otherwise healthy animal. Carrier stallions may constantly shed the virus in their semen for years. Therefore, when susceptible mares are bred by these stallions either naturally or by artificial insemination, they often become infected with EVA.

In late June of this year, the New Mexico Department of Agriculture and their State Veterinarian announced an outbreak of EVA in their state. According to the New Mexico State Veterinarian, one ranch was quarantined and others voluntarily restricted movement in order to stop the dissemination of the disease. They are still in the process of investigating where the virus may have come from and how it may have infected the horses involved. There are no specific numbers of horses infected or other information available. The important point to remember is that New Mexico animal health officials have taken actions to stop the spread of the virus.

What happened in New Mexico is a reminder that we as horse owners should remain vigilant against the spread of disease, whatever it may be. It is important to always practice good biosecurity measures, such as separating horses that are introduced or reintroduced into your herd after being commingled with other horses. While this may not be practical, it is ideal. Beyond that, it is important to notify your veterinarian when signs of illness do occur. Your veterinarian should be able to determine whether there is a need to submit samples or test for diseases that could be move of a problem than the original appearance might indicate.