Three horses have been diagnosed with West Nile virus in Okanogan County since the beginning of September.

The prognosis for one horse was very good, but the other two have been euthanized, according to Okanogan County Public Health. None of the horses had been vaccinated against the virus. All three horses were reported to have acquired the infection in Washington state.

West Nile is a virus carried by mosquitoes that feed on an infected bird. Once infected, the mosquito can infect people, horses and other birds. Scientists do not believe the virus is spread from person to person or from animal to person, according to Public Health.

No human cases of West Nile virus have been reported in Okanogan County this year, but the infection of local horses means that the virus may be present in the county.

West Nile virus affects horses, mules and donkeys more often than other animals, according to the Washington State Department of Agriculture. Most horses infected with the virus do not become ill or show any symptoms. Those that do become ill may have a loss of appetite, loss of coordination, confusion, fever and muscle tremors. Approximately one-third of horses that become ill will die.

The risk for a person to become infected with West Nile virus is very low, according to the state Department of Health. Most people infected with the virus do not get sick, but one in five have mild symptoms such as fever, headache and body aches.

One in 150 people have more severe symptoms, which can include high fever, neck stiffness, disorientation, convulsions, muscle weakness and paralysis. Anyone experiencing these symptoms should contact a health care provider. There is no human vaccine for the virus.

Public Health recommends people take precautions to prevent mosquito bites, including staying indoors at dawn and dusk, using mosquito repellent, wearing long sleeves and pants, and using window screens.

People should reduce mosquito breeding areas by emptying anything that holds standing water, such as old tires, buckets and toys. People should also change water in birdbaths, wading pools and animal troughs at least twice a week; make sure roof gutters are not clogged; and fix leaky outdoor faucets and sprinklers.

West Nile virus first appeared in the U.S. in New York City in 1999 and has spread rapidly throughout the country. The first human cases in Washington were reported in 2006, according to the Department of Health.

This year in Washington, there have been six confirmed human cases, 16 cases in horses, and one in birds. All cases have been in eastern Washington, with the human cases in southeastern Washington.

The Department of Agriculture recommends that horse owners talk to their veterinarians about the vaccine for the virus. Report suspected cases of West Nile virus in horses to the state veterinarian at (360) 902-1881 or ahealth@agr.wa.gov.

For more information on West Nile virus, visit the Department of Health at www.doh.wa.gov/YouandYourFamily/IllnessandDisease/WestNileVirus, or call 1-866-788-4787. Information for horse owners is at the Department of Agriculture at agr.wa.gov/FoodAnimal/AnimalHealth/Diseases/WestNileVirus.