The Washington State Department of Agriculture (WSDA) has announced that a 10-year-old Quarter Horse residing in the town of Four Lakes, in Spokane County, is the first horse in the state to be confirmed positive for West Nile virus (WNV) this year.

The horse, which was unvaccinated against the disease, reportedly had coordination problems in the hind legs and muscle tremors, but is improving.

Washington State University’s Washington Animal Disease Diagnostic Laboratory, in Pullman, reported the positive test results to WSDA’s State Veterinarian’s Office this week.

In 2016 the WSDA confirmed 27 equine WNV cases in Washington horses, seven of which died or were euthanized. Ten counties reported equine WNV cases involving horses last year, including Benton, Franklin, Grant, Kittitas, Lincoln, Okanogan, Pend Oreille, Spokane, Stevens, and Yakima counties. Spokane County lead with eight cases.

“Horses can still benefit from first-time vaccinations or an annual booster vaccination,” said State Veterinarian Brian Joseph, DVM said. “It’s never too late to vaccinate your horse, but it’s more effective to do it earlier than now.”

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West Nile is transmitted to horses via bites from infected mosquitoes. Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; 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%.

Horse owners should also consult their private practicing veterinarian to determine an appropriate disease prevention plan for their horses. Vaccines have proven to be a very effective prevention tool. Horses that have been vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters annually—one in the spring and another in the fall. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three- to six-week period.

In addition to vaccinations, horse owners also need to reduce the mosquito populations and their possible breeding areas. Recommendations include removing stagnant water sources, keeping animals inside during the bugs’ feeding times, which are typically early in the morning and evening, and applying mosquito repellents approved for equine use.

Veterinarians in Washington who learn of potential West Nile virus cases in horses or other animals should contact the State Veterinarian’s Office at 360/902-1878.