At least four horses have died from West Nile virus since early August.
Since August 9, there have been 10 confirmed cases of the virus in unvaccinated horses living in Washington's Spokane, Lincoln, and Pend Oreille counties, and Boundary County in Idaho.
Washington State University has tested around 40 horses for the virus since the beginning of 2016. Kevin Snekvik, the lab's director of operations, says warmer weather might be why the virus is striking farther north than it normally does.
Infected mosquitoes transmit the virus after feeding on infected birds. The American Association of Equine Practitioners reports that horses account for nearly 97% of reported non-human mammalian cases of the virus.
The equine symptoms of West Nile virus include: lack of coordination and stumbling, weakness in the hind limbs, depression, droopy lip, muscle spasms, teeth grinding, lack of appetite, excessive sweating, disorientation, convulsions, head pressing, colicky behavior, aimless wandering, and, in some cases, total paralysis.
West Nile virus peaks in intensity near the end of summer and into the fall. The virus can be fatal. Health officials are urging all horse owners to vaccinate their horses if they have not already done so.
You can learn more about West Nile virus in horses here.