SPOKANE, Wash. (AP) — Ten cases of the West Nile virus have been confirmed in horses in eastern Washington state and northern Idaho, and at least four of the animals have died.

The Spokesman-Review reports all 10 cases were identified in unvaccinated horses in Washington's Spokane, Lincoln and Pend Oreille counties and Idaho's Boundary County.

A Washington State University laboratory has tested about 40 horses for the virus since the beginning of the year. The first case of West Nile, which is typically transmitted through the bite of an infected mosquito, was confirmed Aug. 9.

The lab's director of operations, Kevin Snekvik, says the virus has been detected farther north than usual and mosquitoes may be taking advantage of the region's warmer weather.

Health officials are urging owners to get their horses vaccinated.