Equine VS Cases Confirmed in Wyoming

On Aug. 17, the Wyoming State Veterinary Laboratory announced the first confirmed U.S. case of vesicular stomatitis (VS) in 2006. The primary case, a 10-year-old horse, is located near Casper in Natrona County. A second case was confirmed a few days later in Converse County.

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, reported federal authorities are examining at least 11 more suspect cases in the state. Test results are pending at the USDA's National Veterinary Service Laboratories (NVSL). The last VS outbreak in the United States involved nine western states in 2005, and was formally declared over on April 11, 2006.

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 painful raw areas that can precipitate lameness and reluctance to eat.

The Wyoming laboratory doesn't run samples on horses exhibiting clinical signs of vesicular stomatitis--that is under the purview of the NVSL, since VS is a foreign animal disease. Animals with VS can appear as if they're suffering from foot and mouth disease (which horses cannot contract), so signs of vesicular disease in livestock are generally taken very seriously. If there are horses and cattle affected with vesicular lesions on a premises, generally foot and mouth disease is not the cause.

"Vesicular stomatitis is an incredibly enigmatic disease, how it crops up out of the blue" said O'Toole. "It typically comes up out of the Southwest and moves north along riparian areas (along waterways), and it starts jumping up through the states, and it's around this time of year that we typically see it...but we normally hear it's coming." That wasn't this case this year.

The VS-affected horses are on quarantined premises and are isolated from other animals to ensure troughs and feed buckets are not shared. Insect control measures were increased since VS is thought to be spread by insects. The Natrona horse had not been moved from the premises recently, and the owners reported a large burden of Culicoides midges and large numbers of biting flies in the area.

"The virus isolate from the first (Natrona) horse...has been compared by the USDA to isolates from Wyoming and Montana last year," said O'Toole. "The inference is that VS virus somehow managed to overwinter in Wyoming in 2005-2006. This is an interesting situation since in the past we've always assumed this disease comes out of Central America or Mexico, and moves north in big jumps we don't understand."

O'Toole said USDA scientists are trying to determine what the overwintering host might be. "They don't have much time since, if it's an insect vector, it needs to be pinned down before our first hard frost in September," he said.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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