The USDA has reported the detection of vesicular stomatitis (VS) in nine horses on five premises in eastern Wyoming. Vesicular stomatitis, which normally moves up from the Southwest along waterways, has not appeared elsewhere in the country this year. This has lead researchers to believe VS might have overwintered in Wyoming, and they’re trying to figure out how.

Vesicular stomatitis is a viral disease that primarily affects horses, cattle, and swine, but it can also affect sheep and goats. The disease causes blister-like lesions in the mouth and on the dental pad, tongue, lips, nostrils, hooves, prepuce, and teats of livestock. When the blisters break, they can leave raw, painful areas that can precipitate lameness and reluctance to eat. Animals with VS should be isolated from other livestock to ensure that troughs and feed buckets are not shared. Affected farms are encouraged to increase their insect control measures because biting flies such as Culicoides midges might be responsible for carrying the disease.

The two affected counties–Converse and Natrona–border one another, and the affected premises (one in Converse and four in Natrona) are situated near waterways. Donal O’Toole, MVB, MRCVS, PhD, Dipl. ECVP, FRCPath, director of the Wyoming State Veterinary Laboratory and head of the Department of Veterinary Sciences at the University of Wyoming in Laramie, said VS typically moves up out of the southwestern United States and appears in other states before springing up in Wyoming. This year there was no warning.

“The virus isolate from the first horse to come up positive in Natrona this year has been compared by the USDA to isolates from from Wyoming and Montana last year,” said O’Toole. “Although I don’t know the specific details, I understand it is a close match. The inference is that vesicular stomatitis virus somehow managed to overwinter in Wyoming in