An outbreak of equine herpesvirus-1 in a boarding stable in northern France has killed four horses so far, with a fifth horse becoming recumbent, according to the treating veterinarian. Quarantine has been established and will continue for at least three more weeks.

Jeroen Lissens, DVM, said seven of the 63 horses at the stable have tested positive for the disease since July 14. All but one had been vaccinated against EHV-1 within the past year. Boosters were administered immediately to all horses at the site.

“What’s particularly concerning about this outbreak is that every horse affected so far has shown neurological signs, including ataxia and incontinence,” Lissens said. “This leads us to believe we’re dealing with a mutated strain that we have not yet seen [in France].”

Laboratory analyses and a necropsy are underway in an attempt to reveal more details about the virus and its possible origin.

In Bridgeville, Pa., the attending veterinarian at Rolling Hills Ranch noticed symptoms consistent with EHV-1 in several horses. At least three horses have tested positive for the EHV-1 virus, and the Pennsylvania Department of Agriculture has placed a quarantine on the property, which houses more than 100 horses in five barns.