Six horses in Washington have been infected by West Nile virus in the past 10 days, already surpassing the number of equine cases in all of 2014, according to the Washington State Department of Agriculture.

The virus is appearing in horses earlier than last year, when the first of five cases was confirmed Aug. 19, WSDA spokesman Mike Louisell said Friday. The first case in 2013 was confirmed in late September.

Horses contract the disease from mosquitoes that have fed on infected birds. “Look at the hot weather we’ve been having. That’s no doubt spurred mosquitoes,” Louisell said.

The virus is most common in south-central Washington, though it has infected horses and humans in other regions of the state.

So far this year, the state Department of Health reports five human cases, including four in Benton County, where two horses have been infected.

The fifth person sickened by the virus lives in King County in Western Washington, but was likely infected in Adams County in Eastern Washington, according to the health department.

WSDA previously reported that a gelding in Kennewick in Benton County was confirmed July 21 to have the virus. The horse survived, and it continues to improve, according to WSDA.

Since then, five more cases have been confirmed. The six cases are the most in Washington since 2009, when the virus infected 78 horses, by far the worst outbreak ever in the state. Equine cases have been relatively rare in the past several years, with two cases confirmed in 2013, one in 2012 and none in 2011 and 2012.

The cases so far this year include:

• A 2-year-old Andalusian stud colt in Harrah in Yakima County. The horse’s condition is improving.

• A 4-year-old Andalusian stud in Kennewick was euthanized.

• A 3-year-old Quarter horse gelding in Mesa in Franklin County died.

• A 9-year-old Appaloosa mare in Othello in Adams County. The horse appears to be recovering.

• A 3-year-old Azteca male in Mabton in Yakima County. The horse appears to be recovering.

Idaho confirmed July 23 that a horse in Washington County was infected this year, the state’s only case this year.

The California West Nile Virus website, jointly run by several government agencies, does not report equine cases. California’s surveillance system includes chickens set out to detect mosquito-borne illnesses. So far, 45 chickens have been sickened in nine counties, including the Northern California counties of Shasta, Butte, Glenn, Sutter, Yuba and Placer.

Oregon has not confirmed any equine cases this year, state Department of Agriculture spokesman Bruce Pokarney said Friday.

The West Nile virus appeared in the United States in 1999. The disease sickens people, horses, birds and other animals, but it does not spread from horses to people or other animals. Most horses that contract the virus show no ill symptoms. About one-third of the horses with symptoms die.

WSDA recommends horses be vaccinated against the virus and that owners take steps to protect their horses from mosquitoes.

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