Veterinarians this month confirmed 10 cases of West Nile fever in horses in Eastern Washington and North Idaho.

All 10 cases were detected in unvaccinated horses in Washington’s Spokane, Lincoln and Pend Oreille counties and Idaho’s Boundary County, according to the Washington Animal Disease Diagnostic Laboratory. At least four of those horses have died.

The lab, based at Washington State University’s College of Veterinary Medicine, has tested about 40 horses for West Nile virus since Jan. 1. The first case was confirmed Aug. 9.

Charlie Powell, a spokesman for the college, said veterinarians are concerned but expect winter frost to eradicate mosquitoes that carry the virus.

“With these numbers we’re doing increased testing,” said Kevin Snekvik, the lab’s director of operations.

Snekvik said the virus was detected farther north than in previous years, and experts aren’t sure why. He said the reason could be that mosquitoes are taking advantage of rising temperatures in the region.

“Very rarely do we see them this far east, and certainly not this far north,” he said.

Idaho’s Panhandle Health District has said about two out of three horses infected with West Nile virus survive, although health officials urge owners to get their horses vaccinated.

“It should be the norm, but obviously it’s up to the owners’ preference,” Snekvik said.

The 10 horses reported this month showed neurological symptoms of West Nile fever. Powell said the death rate is much higher when the virus progresses to that stage.

Health officials also urge residents to wear long sleeves and rid their properties of stagnant water, a common breeding ground for mosquitoes.

Earlier this month, a Spokane County man in his 40s was hospitalized and released after contracting the virus from a mosquito in the area. And in Benton County, a woman in her 80s died of complications from the virus.

Benton County reported half of Washington’s 24 cases of West Nile virus last year. The virus has been detected in nine counties in Washington this year, all on the eastern side of the state.