West Nile Virus

What is West Nile virus?

West Nile virus is a virus carried by mosquitos that can cause serious, even fatal, illness in humans and animals. In 1999, West Nile virus first appeared in the United States in New York City. The virus spread rapidly throughout the country and was first detected in Washington in 2002. Cases of West Nile virus in Washington residents are reported most years. Current data and statistics on human cases and environmental detections in Washington are available on the  West Nile Virus surveillance maps and statistics.

How is West Nile virus spread?

West Nile virus is almost always spread to people and animals by the bite of an infected mosquito. Mosquitoes become infected after feeding on birds that carry the virus. West Nile virus is not spread by direct contact with infected people or animals.

Who is at risk?

Through monitoring for West Nile virus in mosquitos, animals, and people, we know that the virus is most often found in south central Washington, and less commonly in other parts of the state. Disease due to WNV occurs during mosquito season. The earliest detections in the state have occurred in May, and the latest in October. If a mosquito infected with WNV bites a person, they are at risk of becoming infected. People over the age of 60 years and people with certain medical conditions, such as cancer, diabetes, hypertension, and kidney disease are at greater risk of serious illness if they become infected with WNV. Information from mosquito monitoring, climate data, and disease in animals and people gives some indication of where West Nile virus is currently spreading. Check the DOH West Nile virus data website for up-to-date information.

How can I protect myself and my family?

The best protection against diseases carried by mosquitos, like WNV, is to prevent mosquito bites and reduce mosquito habitat.

Avoid mosquito bites:

  • Use an EPA-registered insect repellent when spending time in areas with mosquitos. Read the label and carefully follow instructions.
  • Use screens on windows and doors, and repair holes in screens to keep mosquitos outside.
  • Stay indoors at dawn and dusk, if possible, when mosquitos are the most active.
  • Wear a long sleeve shirt, long pants, and a hat when going into areas with mosquitos.
  • Treat clothing and gear with permethrin to kill or repel mosquitos.

Take steps to remove and reduce the places that mosquitos live and breed around your home:

  • Mosquitos require water for their larvae. Identify sources of standing water such as tires, buckets, planters, toys, pools, birdbaths, animal troughs, flowerpots, trash containers, or other places where standing water may collect.
  • Empty anything that holds standing water.
  • If emptying is not possible, cover water sources or change water at least twice per week.
  • Make sure roof gutters drain properly and clean clogged gutters in the spring and fall.
  • Fix leaky outdoor faucets and sprinklers.

What are the symptoms of West Nile virus?

Most people who are infected with West Nile virus (8 out of 10) will not get sick. About 1 in 5 people infected develop fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people recover completely. Very few people, about 1 in 150, will have more severe symptoms. Severe symptoms may include headache, high fever, neck stiffness, stupor, disorientation, tremors, convulsions, muscle weakness, paralysis, and coma. For a small number of people, the disease can be very serious and lead to permanent neurologic effects or death. If you experience symptoms of West Nile virus, contact your healthcare provider.

What is an effective mosquito repellent?

The most effective mosquito repellents contain the active ingredient of DEET (N, N-diethyl-m-toluamide), picaridin, IR3535, oil of lemon eucalyptus, para-methane-diol (PMD, the synthesized version of oil of lemon eucalyptus), or 2-undecanone. These active ingredients typically offer long-lasting protection against mosquito bites. Repellents come in lotions, creams, gels, sprays, and towelettes. Use the EPA insect repellent tool to find the repellent that is right for you.

Mosquito repellents must be used properly. Read and follow instructions on the label.  If you are using both sunscreen and repellent, use separate products- not a product with both sunscreen and repellent combined- and apply sunscreen first, then repellent.

How does West Nile virus affect animals?

West Nile virus naturally cycles between mosquitos and various birds. Infected mosquitos can pass the virus to birds when they bite. Some infected birds develop high levels of the virus in their bloodstream, and mosquitoes become infected by biting these infected birds. Mosquitos with West Nile virus also bite and infect people, horses and other mammals; however, people and mammals do not develop high enough levels of virus in their blood to infect other biting mosquitos.

West Nile virus can cause illness and death in some bird species, especially corvids such as crows, ravens, magpies, and jays). You can report dead birds to Department of Fish & Wildlife. They investigate the cause of death involving large wild bird die-offs, including suspected diseases such as West Nile virus.

West Nile virus can also cause serious illness in horses, mules, and donkeys. Though most horses that become infected will not get sick, about 1 in 3 horses infected will die. To protect your horse, ask your veterinarian about the West Nile virus vaccine. West Nile virus can infect dogs and cats, but it is rare.

What is DOH doing?

Each mosquito season, DOH traps and tests mosquitos for West Nile virus and supports trapping and testing by local health jurisdictions and mosquito control districts around the state. Information on mosquito trapping and testing is collected from partners. Additionally, DOH works with the Washington Department of Fish & Wildlife, the Washington State Department of Agriculture, and local public health agencies to monitor infections in birds, horses and other animals, and people to understand the spread of West Nile virus in our state.

When cases of West Nile virus are reported, local health and DOH investigate to ensure we understand where the virus is spreading in our state, and to provide prevention recommendations to people at risk.

Find more surveillance data

The Washington Tracking Network presents 2006-2019 vector mosquito species, Culex pipiens and Cx. tarsalis, test data by county. It also shows bird and mammal test data. View data in a table, chart, and map format and export data for your own use.

See Washington Tracking Network - mosquito samples, birds, and mammals tested for West Nile virus.

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