EVA Prevalence in Spanish Horses
In areas with large equine populations, knowing animals’ infectious disease status is an important aspect of biosecurity. That’s even more so in places where horses cross country lines frequently for competitions. That’s why Spanish researchers recently decided to verify the exposure rate of equine viral arteritis (EVA) in their country’s horse heartland.

“The central area of Spain is an area with a high density of horse populations (both breeding and sport horses) and also the area where the majority of competitions are held,” said Fátima Cruz-Lopez, PhD, DVM, of the Complutense University Veterinary School, in Madrid. “Furthermore, it’s a frequent stop for horses traveling to northern Europe. So it was a good area for comparing the seroprevalence of EVA in breeding and sport horses.”

Blood samples from 155 breeding horses at 16 stud farms and 105 sport horses at 12 riding clubs revealed a 21% positive rate in breeding horses compared to 6.7% in sport horses, she said. That doesn’t mean these horses have active infection, but rather that they have been exposed to the virus at some point in their lives and have developed antibodies against it.

“We weren’t surprised by our results,” Cruz-Lopez said. “Our study showed that it’s quite common to find antibodies against EVA in our breeding horses, but the experience in our laboratory tells us that it is not so common to find carrier stallions or even outbreaks of the disease.”

The EVA virus has been known to stay active in stallions’ sex glands for months or years. In fact, because EVA persistence is testosterone-dependent, a carrier stallion could continue to “shed” virus in his semen as long as he remains intact. That’s why some researchers have looked into temporary castration techniques to allow horses the time to lose their “carrier” status before reproducing again.

“We hypothesize that infected stallions may cover several mares in a short space of time, and those mares are in contact with other mares, therefore increasing the seroprevalence of the stud farm (via the respiratory transmission route),” she said. “But very few of those stallions in fact remain carriers afterwards. We haven´t carried out a study to find out the percentage of stallions that remain carriers, but the experience in our lab tells us that this percentage is not high.”

Meanwhile, Cruz-Lopez said, the seroprevalence rate in sport horses is “low,” suggesting that the primary route of infection is probably venereal rather than respiratory in the Spanish horse population.

These findings shouldn’t affect the export of live animals or semen, she said. Most countries will continue to accept seropositive mares as long as the antibody rate remains stable (between two samples taken two weeks apart). Stallions, however, would need to undergo further laboratory analyses until they’re considered negative, at which time they can be accepted into most countries.

“Breeders wanting to obtain semen from Spain should have no concern since every stallion that exports its semen needs regular analysis (at least once a month) during the breeding season, and they are analyzed for equine contagious metritis, EVA, and equine infectious anemia,” Cruz-Lopez said.

As for international sporting events, the low seroprevalence in Spanish sport horses should have “virtually no effect on international transport” for sporting events, she said.

The study, “Equine viral arteritis in breeding and sport horses in central Spain,” was published in Research in Veterinary Science.