Some equine diseases come and go with little impact on the horse industry as a whole. Others affect only local or state industries when they rear their ugly heads. But when a disease has the potential to shutter the global horse breeding industry, controlling it becomes crucial. One of those diseases is equine viral arteritis (EVA). Fortunately, veterinarians have vaccines against this disease at their disposal to help control its spread.
During a recent presentation in Lexington, Kentucky, Peter J. Timoney, FRCVS, PhD, reviewed EVA vaccines and the current vaccination recommendations. Timoney is a professor and former department chair and director of the University of Kentucky Gluck Equine Research Center, also in Lexington.
Why Horses Need Protection
Before deciding whether to vaccinate against a disease, it’s important to understand how that disease impacts affected animals and how it could impact breeding industries around the globe. So Timoney began with a review.
Equine viral arteritis is a contagious viral disease (caused by the equine arteritis virus, or EAV) that can cause fever, respiratory illness, ocular inflammation, edema (swelling, especially of the limbs), birth of weak or sick foals, and abortion. Not all affected horses show clinical signs, and when they do, signs can be subtle. It is mainly transmitted via semen through natural cover or artificial insemination and can also be spread via the respiratory route.
Adult horses usually make full clinical recoveries with or without supportive treatment such as non-steroidal anti-inflammatories and antibiotics.
However, if the virus isn’t cleared from a stallion’s body within a few weeks he runs the risk of becoming a carrier, meaning he won’t show clinical signs of disease, but he’s still capable of shedding—and spreading—the virus. Persistently infected carrier stallions function as EAV’s natural reservoir, and they disseminate the virus to susceptible mares at breeding at a rate of 85-100%. Such an outbreak could devastate breeding industries—local, national, and global. Once a stallion is a carrier, rendering him EVA-negative can be a serious challenge. Because EVA relies on testosterone to persist, some carriers are gelded to eliminate the virus.
Scientists first isolated the virus and characterized it as a disease agent in 1957, but it wasn’t until 1984 that the first case was recorded in a Thoroughbred horse in Kentucky, Timoney explained. The disease spread rapidly across Kentucky breeding farms, and authorities worldwide took notice.
From this outbreak, Timoney said, several industry concerns arose, including:
- The risk of disease spread through the international movement of horses and/or semen;
- The potential for major outbreaks in populations of naive mares;
- The risk of illness and death in young foals; and – Stallions becoming carriers.
International movement regulations and testing requirements quickly took effect following the 1984 outbreak.
“To this day, EVA remains one of the most internationally regulated equine diseases,” Timoney said.
Current prevention and control programs are primarily focused on restricting EAV spread to both minimize the risk of virus-related abortions and to prevent stallions from becoming carriers, he said. And one of the main ways to control EAV spread is via vaccination.
Currently Available Vaccines
Timoney said the two vaccines available to today’s breeders (Arvac and Artervac, both produced by Zoetis) are based on research conducted in the 1960s, but are both safe and effective for reducing the risk of EVA.
Arvac is a modified-live virus vaccine (one that includes a weakened virus that no longer produces clinical disease, but retains the ability to induce a protective immune response) that’s been used in the United States and Canada since 1985; it’s also been used on a limited scale in New Zealand, he said. It’s been confirmed safe for use in all horses except for mares in the final two months of gestation and in foals less than six weeks of age, Timoney said. Additionally, he said, adverse vaccine reactions are uncommon.
Regarding efficacy, Timoney said, Arvac “stimulates effective protection against clinical disease and establishment of the carrier state in stallions.” Further, he said, vaccinated stallions and their semen can be exported to other countries including Japan, which will only accept stallions that have been vaccinated against EVA.
Meanwhile, Artervac is an inactivated vaccine (one that does not contain viable or infectious virus, but still stimulates an immune response) licensed for use in a number of European countries; it is not commercially available in North America. This vaccine has also been proven safe and “reputedly effective” for use in stallions and mares, Timoney said.
Current Vaccination and Management Recommendations
“Vaccination is primarily targeted at protecting breeding stallions against establishment of the carrier state,” Timoney said. He recommended annual vaccination for noncarrier stallions, three to four weeks before the breeding season, and many breeders and veterinarians are taking notice: According to statistics from Kentucky Department of Agriculture Equine Programs manager E.S. “Rusty” Ford, nearly 100% of stallions in the commonwealth are vaccinated against EVA.
Timoney noted that mandatory vaccination only applies to Thoroughbred stallions and only in the states of Kentucky and New York.
Timoney also recommended vaccinating immature colts (6 to 12 months of age) to minimize the risk that they’ll become carriers later in life.
Timoney said that while vaccination protocols are primarily targeted at protecting stallions from becoming carriers rather than preventing abortion in mares prophylactically, mares should still be vaccinated in certain situations.
“Vaccination of pregnant mares is strongly indicated in the face of high risk of exposure to natural EAV infection,” even if they’re in late gestation, he said.
Timoney also provided several other recommendations for controlling EVA:
- Identify carriers, separate them from other breeding stallions, and restrict their matings to mares that have been vaccinated against EVA or are naturally seropositive;
- If a naive mare is bred to a stallion shedding EAV, isolate her from other seronegative horses for three weeks to prevent virus spread;
- If using artificial insemination, screen semen prior to breeding; if the semen tests positive for EAV, only use it in mares that have been vaccinated against EVA or are naturally seropositive;
- Employ sound management and biosecurity practices for broodmares; and
- If breeding stock resides on a facility that often sees horses coming and going with limited isolation space, it’s highly recommended to vaccinate all at-risk equids.
He also directed attendees to the USDA Animal and Plant Health Inspection Service’s resources on managing and controlling EVA and rules and regulations associated with the disease.
When it comes to EVA, many horse owners and veterinarians around the world have a similar goal: minimize disease spread. One of the most important steps in achieving that goal is to implement a proper vaccination regimen for breeding stock. Consult your veterinarian to determine when it’s most appropriate to vaccinate your stallions and broodmares to help prevent this disease from spreading.
About the Author
Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.