Montana Horse Owners Urged to Vaccinate for West Nile Virus

Montana Horse Owners Urged to Vaccinate for West Nile Virus

Based on Montana Department of Livestock data, no equids that were current on vaccinations have ever contracted the disease in Montana.

Photo: Anne M. Eberhardt/The Horse

Montana animal health officials are encouraging horse owners to consult with their veterinarians about vaccinating their animals for West Nile virus (WNV) after a spike in the number of cases last year.

“We had 32 cases last year, the most since 2007, and the third highest total in the nation,” said assistant state veterinarian Tahnee Szymanski, DVM. “That’s concerning because the disease is highly preventable.”

Based on Montana Department of Livestock data, no equids that were current on vaccinations have ever contracted the disease in Montana. In contrast, one-third of unvaccinated equids that contracted the disease either died or were euthanized (161 of 492 since 2002).

Greg Johnson, MS, PhD, professor of veterinary entomology in the Montana State University Department of Animal and Range Sciences, says vaccination—which is recommended as a core vaccine by the American Association of Equine Practitioners—is prudent given WNV’s unpredictability.

“It’s kind of like the flu season,” Johnson said. “We can look at the existing data and forecasts and make some guesses, but we can’t really predict what West Nile is going to do or how bad it’s going to be.”

However, with snowpack running at 150% of normal throughout much of the state, environmental conditions could be right for WNV activity later this year.

“With all of that snowpack, it looks like we’ll have lots of water, and that can mean more mosquitos,” he said.

A mosquito-borne disease, WNV was first found on the east coast of the U.S. in 1999. Since then, the disease has spread westward, arriving in Montana in 2002. The disease knows no climactic or geographic boundaries in Montana, and has been found statewide.

West Nile is a viral disease transmitted to horses by infected mosquitoes. Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

Stressing the importance of vaccination, Szymanski said, is that there is no specific treatment for horses that contract the disease.

West Nile is a reportable disease in Montana. Any confirmed or suspected case should be immediately reported to the Montana state veterinarian’s office at 406/444-2043.

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