Two unvaccinated horses residing in western Idaho have tested positive for West Nile virus (WNV), according to a news release from the Idaho Department of Health and Welfare.

According to Idaho WNV surveillance information, a horse residing in Canyon County was diagnosed with the virus on July 19. Less than a week later, on July 25, a horse residing in Ada County tested positive for the disease.

These two cases mark the first equine WNV cases diagnosed in Idaho in 2013. In 2012 veterinarians confirmed seven equine WNV cases.

Clinical signs for WNV—a mosquito-borne disease—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 (incoordination). Equine mortality rate can be as high as 30-40%.

Owners can take steps to help lessen their horses’ risk of contracting WNV, including:

  • Vaccinating horses against WNV annually, and sometimes more frequently if the animals reside in a mosquito-dense area;
  • Minimizing the mosquito population around horses by eliminating standing water sources and removing muck from areas near the animals;
  • Using equine-approved mosquito repellents and/or protective fly gear;
  • Keeping horses stalle