Genetics Test Clarifies EVA Risk in Stallions

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Photo: Anne M. Eberhardt
Dr. Peter Timoney with the University of Kentucky Gluck Center

A new genetics test that determines whether a stallion could be a persistent carrier of the equine arteritis virus has been developed by the University of Kentucky Gluck Equine Research Center.

Outbreaks of equine viral arteritis may result in significant economic losses to the equine industry because of pregnancy loss in mares, death in young foals, and cause longer-term problems through stallions that continue to carry the virus between breeding seasons, according to Gluck researchers.

The latest research done at the Gluck Research Center by Drs. Udeni Balasuriya, Ernie Bailey, and Peter Timoney shows stallions identified with a "susceptible haplotype" (a group of four specific nucleotide changes in the CXCL16 gene) are more likely to retain the virus in their reproductive tracts than horses that possess the "resistant haplotype."

Resistant stallions initially shed the virus in their semen following infection, but in most cases clear the virus from the reproductive tracts within months. Stallions possessing even one copy of the susceptible haplotype are at greater risk for becoming long-term shedders of EVA.

"Since surgical castration can be resorted to in stallions that are confirmed carriers of EVA, this test can help identify those horses that may spontaneously clear themselves of the virus, thus avoiding the loss of a valuable breeding animal," said Kathryn Graves, director of genetic testing at Gluck laboratory.

Timoney stressed the test is a step toward identifying animals at risk for persistent infection, but it does not provide black-and-white answers. Some stallions carrying the resistant haplotype have still been found to be carriers.

The test does give stallion owners a clearer picture of their risk, which can be minimized through vaccination and appropriate management practices, according to Timoney.

"We have ample evidence that this vaccine conveys a very high level of protection," Timoney said. "Even though the vaccine is a modified live strain, we have not confirmed yet a stallion that has become infected following the recommended treatment."

The Kentucky Department of Agriculture reports no known EVA shedding Thoroughbred stallions standing at stud in the state. State law requires all Thoroughbred stallions be vaccinated annually against EVA, and stallions standing their first season in Kentucky are required to be tested prior to vaccination in order to determine that they are not affected with the arteritis virus. According to Gluck, New York is the only other state that mandates EVA vaccination. These two states represent about 17% of the U.S. stallion population in 2015 and are an even smaller percentage of the total stallion population for all breeds.

Annual EVA vaccinations, which cost about $75 in Central Kentucky if given by a vet, are recommended and particular important for stallions that are shuttling overseas, according to Timoney. 

"This issue is like water dripping on a stone, though. You cannot give up on the education about EVA to people who haven't experienced the devastating losses from this disease," he said.