Equestrian Network Magazine

Equine Viral Arteritis:
A Concern For California Breeders

by Alexandra Eastman

As the 2007 breeding season gets under way, many stallion and mare owners are surprised to be asked about their horse's EVA status. In the wake of an outbreak of Equine Viral Arteritis (EVA), which occurred in Quarter Horses in 2006, many horse breeders and trainers are rethinking their approach to this

disease. Testing and vaccination protocols are becoming stricter in all breeds of horses. The 2006 outbreak originating in New Mexico had a devastating impact on the Quarter Horse breeding industry. The virus was confirmed on premises within five other states (Kansas, Montana, Oklahoma, Utah, and Alabama).

Historically the disease has impacted Standardbred horses most significantly and the last documented outbreak was in Thoroughbred horses in 1984. For these reasons, the disease has not recently been a central issue concerning the breeding program of other breeds. Because the disease can be spread by infected semen, there are widespread implications for people breeding with transported semen. With semen transport becoming easier and easier, these concerns extend beyond state and international lines.

In most instances, horses exposed to EVA exhibit either mild clinical signs of an upper respiratory illness or no apparent clinical signs. Affected horses may have a fever, decreased appetite, depression, nasal discharge, and edema (swelling) of the ventral trunk, scrotum, mammary gland and/or lower limbs. The incubation period from exposure to onset of signs is typically 3 to 14 days. Clinical signs may be present for a week or more. The mortality rates in adult horses are extremely low and most horses recover without incident. The primary concern is for stallions and pregnant mares.

The concern in stallions is that after infection the virus localizes within the accessory sex glands where it can continue to be shed with every ejaculate, potentially for the lifetime of the stallion. 30%-60% of stallions infected become chronic carriers. Because the virus is present in the ejaculate, stallions can infect mares

during live cover or artificial insemination. There is no known cure for the carrier state in stallions. After the initial infection and illness, the disease itself does not cause decreased conception rates or abortion in mares bred with semen from infected stallions. The problem lies with the infection of other horses by mares bred with infected semen.

Mares bred to infected stallions can infect other horses two possible ways. Post breeding, infected semen eliminated from the mare's reproductive tract is potentially infective to other horses. The more common mode of transmission is for the mare to become transiently ill from the virus and transmit the virus to stable mates via nasal secretions. Infected nasal secretions can also be carried on people or equipment in contact with multiple horses. The infection rate for naive mares bred to carrier stallions is 85%-95%. These mares

can infect other pregnant mares (abortion is caused in mares more than three months pregnant) and if those mares are infected, they may abort within 1-3 weeks. Exposure to the virus causes abortion in up to 50-75% of non-vaccinated pregnant mares. Mares exposed very late in pregnancy may deliver term foals that may be born alive and die within days of severe pneumonia.

Mares and geldings do not serve as reservoirs for the disease. Once the disease has run its course, the animal will stop shedding the virus within three weeks. Clinical disease will usually cause future immunity. Because the shedding is testosterone dependent, if a stallion that is a chronic shedder is castrated, he will no longer be a source of the disease.

There is a blood test available to determine if a horse has been exposed to or previously vaccinated for EVA. This test looks for antibodies in the blood to the EVA virus. Presently, it is not possible to differentiate between natural exposure and previous vaccination. Unless they are sick, in mares, geldings, and immature stallions it does not matter why they have a positive antibody titer. The positive test would simply indicate that they might not need to be vaccinated. To determine if a sick horse has been infected with EVA, paired titers may be helpful to determine if the antibody titer is indicative of recent exposure. For mature stallions with a positive antibody titer, the semen from 2 or more ejaculates should be tested to determine if the stallion is a chronic shedder.

There is only one vaccine currently approved for vaccination of horses against equine arteritis virus (the virus which causes EVA). The vaccine is manufactured by Fort Dodge. Because of the increased demand for the vaccine in the wake of the recent outbreak, there is currently a waiting list to get the vaccine in some areas. The vaccine effectively prevents the establishment of the carrier state in stallions and abortion in mares. Proper documentation of the vaccination procedure, including accurate horse identification, serologic status prior to vaccination and vaccination date(s) is critical. The American Association of Equine Practitioners has posted vaccination recommendations for EVA on their website (www.aaep.org). Since each situation is unique, it will be important to discuss specific vaccination protocols for your horse(s) with your veterinarian.

The American Quarter Horse Association began documenting horses that have been tested for and vaccinated against the disease. The program, already available to AQHA owners, is purely voluntary. When a horse is vaccinated for EVA, the owner can complete the official AQHA EVA report form (available from AQHA www.aqha.com use the link on the left side of the site for forms ) and return it to AQHA, along with the horse's certificate of registration. The name of the federally-accredited lab that performed the test along with the accession number and test results must be provided as well as the manufacturer, serial number, and expiration date of the vaccine and date of vaccination. Once received by AQHA, the form will become part of the horse's permanent records. Other breed registries may follow the lead set by the AQHA.

Equine arteritis virus is not considered to be very stable or resistant outside a horse. The virus is sensitive to sunlight, high temperatures and low humidity. It may persist in the environment for up to 24 hours in cold or damp conditions and in the presence of organic material. EAV may be transmitted on hands, halters, lead ropes, and other equipment that may be in contact with infective secretions or semen. A broad-spectrum disinfectant may be used in contaminated examination areas. Indoor areas such as laboratories or clinics may be disinfected with dilute bleach and water. It may be important to disinfect breeding phantoms periodically or even between uses.

More information on EVA is available at the USDA website www.aphis.usda.gov/vs/nahps/equine/eva.