EVA: A Manageable Problem

By Andrea Caudill
Courtesy of America’s Horse Daily

Equine viral arteritis is a cause of concern for horse owners and breeders, not the least because many horsemen are not very familiar with the virus. According to the U.S. Department of Agriculture’s Animal & Plant Health Inspection Service Web site, it is most commonly spread via nasal discharge. EVA can also be spread via breeding, both live cover and artificial insemination, as the virus can be carried in a stallion’s semen. “It has significant implications from the standpoint of the breeding business,” says Dr. Joe Manning, who specializes in reproduction at Equine Sports Medicine Services at Weatherford, Texas. “EVA is a virus and is one that causes abortion in mares pretty much throughout gestation. Mares that become infected while pregnant can abort their fetuses pretty much throughout, but they don’t always.”

What is EVA? 

EVA is infectious only to horses, said Peter Timoney, a professor and researcher at the Gluck Equine Research Center in Lexington, Kentucky. It is generally not a life-threatening illness, except to equine fetuses and very young foals. 

EVA can be passed via respiratory infection, similar to influenza and rhinopneumonitis (both caused by viruses). It is transmitted via direct horse-to-horse contact, but can also be transmitted via indirect methods, such as shared equipment or handlers. 

Unlike the other viruses, however, EVA can also persist in a stallion’s semen and infect mares when the stallion is bred via live cover or AI. 

Mares, geldings and sexually immature stallions will not become persistent carriers of the disease. They will acquire the virus and, within weeks, overcome and shed it. Foals born to vaccinated mares might test positive for EVA. 

Stallions, however, can become carriers for an extended period of time, because EVA is a testosterone-dependent virus. Once a stallion recovers from the original infection, he sheds the virus only through his semen. The virus will remain in the semen even if cooled or frozen. 

“It’s the only known testosterone- or androgen-dependent carrier state that I’m aware of among the mammalian viruses,” Timoney said. “That’s why it only occurs in the intact, sexually mature male.” 

The virus can remain in the infected stallion’s reproductive tract for weeks, months, years or for the rest of his life. Some stallions have spontaneously eliminated EVA from their systems and ceased to be carriers. 

Symptoms of EVA 

Symptoms include fever, edema or swelling (especially in the legs, genitals and around the eyes), abortion, nasal discharge, skin rash (localized or generalized) and loss of appetite. 

It is common for horses not to show symptoms while still being carriers. 

The greatest danger to horses infected with EVA is abortion in pregnant mares. It is also possible, although rare, for a foal to be carried to term while infected with the disease. Typically weak, the foals can die within a few days of birth. 

Abortion, if it occurs, typically happens late in the acute phase or early in the convalescent phase of the infection, according to Timoney. This means within one to three weeks following exposure to the virus. Unlike equine herpesvirus-1, the stage of pregnancy does not seem to be critical – abortion can happen from two months to term. 

“Abortion due to this virus does not occur as a sequel to infecting a mare with infected semen,” Timoney says. “Abortion occurs in mares that are already pregnant at the time of exposure to the virus, so (if breeding via AI), the exposure is invariably by the respiratory route through that mare coming in contact with a mare that’s acutely infected and shedding large amounts of the virus.” 

In other words, an EVA-negative mare that was artificially inseminated with semen from an EVA-positive stallion is not at risk to abort. She is, however, at risk for being infected with EVA and, if allowed to intermingle with pregnant mares, can cause them to abort. The mare should be isolated for three weeks after breeding. Horses that have been recently exposed to the virus should be isolated as well, to prevent the spread of the virus. 

Diagnosis is most commonly made via a blood sample that checks for EVA antibodies, indicating exposure to the virus has occurred. 

Treatment and Prevention 

Because EVA is a virus, once contracted, there is no direct treatment for it. Treatment is focused on alleviating symptoms to ease the horse’s recovery. 

To prevent spread via respiratory methods, all hygiene rules commonly used to prevent the spread of viruses should be followed. 

To prevent the disease, a live-virus vaccine is available. The vaccine, called Arvac and marketed by Pfizer Animal Health, has been around for nearly 20 years. It requires a yearly booster and is recommended for stallions, horses over the age of 6 weeks and non pregnant mares. 

After a serious outbreak of EVA in the early 1980s, regulations required all Thoroughbred breeding stallions in Kentucky and New York to receive the EVA vaccine. Timoney says there have been no instances of adverse reactions to the stallions from receiving the vaccine. 

“As part of an annual breeding soundness evaluation of stallions, people should be screening them for EVA before they start shipping cooled semen or freezing semen,” Dr. Manning says. 

There is, however, a catch to vaccinating. Many countries around the world will not allow the importing of horses, semen or embryos that test positive for EVA – even if the horse tests positive solely because it was vaccinated against the disease. Some countries, however, will recognize a horse that has been vaccinated under proper procedures by a licensed veterinarian. 

“The client has to weigh the pros of protecting their horse against a potential EVA infection and the cons, which are definitely export of the animal and/or semen and frozen embryos,” 

Dr. Manning says. Dr. Manning recommends discussing the advantages and disadvantages with your veterinarian. Specific countries’ guidelines for importation of horses, semen and embryos can be obtained from your veterinarian, but there is also a list at the USDA’s animal and plant health inspection service web site.

What Should I Do? 

If there is a chance your horse or farm has been exposed to EVA, the first step is to separate all possibly infected horses from broodmares. Quarantine procedures should be followed to prevent any possible spread of the disease. 

Second, test all horses. Have your veterinarian draw blood and send it for testing. The serum will be checked for antibodies against the virus. Unvaccinated and unexposed horses should have a zero reading, says Dr. Joe Manning of Equine Sports Medicine Services of Weatherford, Texas. When exposed, the horse will produce antibodies to the virus, elevating the number. 

Once exposed to the disease, whether by infection or vaccination, a horse will always have an elevated reading. 

Stallions testing positive need to have their semen checked for the virus to see if they are shedding the virus in the semen. 

“Not all stallions do shed (the virus),”Dr. Manning says. “So they can be a negative status and still have a serum that’s positive.” 

In the face of an outbreak, Dr. Manning says, it might be beneficial to administer an immunostimulant drug.  “Because it’s a virus, we have no antibiotics that make a difference,” he says. “But (an immunostimulant), in the face of exposure, has a potential to be beneficial. There is at least some information that says in stallions even that have been exposed, it has converted some horses who are shedders of the virus in their semen to non shedders by use of immunostimulants. So during the outbreak, finding horses that don’t necessarily have a fever or a serum titer and boosting their immune system would probably be another good tool to use.”  

If horses test negative, consider administering the EVA vaccine to prevent infection.  Peter Timoney, a leading researcher in EVA, recommended that if you are worried about your horses having been exposed to the virus, contact your veterinarian to discuss options.