Environmental Health
Rodents, Bats, Insects & Other Vectors
West Nile Virus (WNV)
  WNV — Frequently Asked Questions  

What is West Nile Virus (WNV)?

West Nile virus is a mosquito-borne virus that affects the central nervous system. In the most severe cases, this potentially serious illness can result in encephalitis and meningitis (inflammation of the brain or lining of the brain and spinal cord) and a polio-like paralysis.

How is WNV spread?

WNV is usually spread by the bite of an infected mosquito. Mosquitoes carry the virus after they become infected by biting a bird that is infected with the virus. In addition to humans and birds, other animals, such as horses, that mosquitoes bite can become infected.

In a small number of cases, WNV also has been spread through blood transfusions and organ trans-plants (since 2003, blood banks screen for WNV).

WNV is not spread by casual contact, such as touching and kissing.

How many people have been infected? Has anyone died?

Between the first detection in the U.S. in 1999 and 2016, over 46,000 people have contracted some form of the disease and over 2,000 individuals have died. For U.S. statistics, see Maps and Data, Centers for Disease Control.

How many cases have been reported in Washington?

In 2017, there were eight human cases in the state, all from Eastern Washington. Previous years have had similar numbers. For current Washington statistics, see WNV Surveillance, WA Dept of Health.

Who is at greatest risk?

Although anyone can become ill from a mosquito bite, the risk for severe illness and death is greatest among people 50 and older, and individuals who spend a lot of time outdoors.

What are the symptoms?

Once an individual is infected, symptoms will start showing within 2 to 15 days. However, most people infected with WNV have no symptoms or experience mild illness, such as headache, body aches, tiredness, and possible rash that usually last a few days (but can last up to several weeks, even in healthy people). Individuals with mild symptoms should recover completely and do not require any specific medication or laboratory testing.

Only 20% of those who become infected will develop flu-like symptoms. Less than 1% of those infected develop severe illness, including a high fever, severe headache, neck stiffness, confusion, and/or your eyes become sensitive to light. If you develop severe symptoms, seek medical attention immediately.

If I get bitten by a mosquito, should I be tested for WNV?

No. Most mosquitoes are not infected with WNV. However, if you develop any of the severe symptoms mentioned above, seek medical attention as soon as possible.

If I live in an area where WNV has been found, am I likely to get sick if I am bitten by a mosquito?

No. Very few mosquitoes—less than 1%—are infected with WNV, even in areas where WNV is known to exist.

What happens to WNV in the winter?

Infected females in some mosquito species can pass WNV onto their eggs, which lie dormant over the winter and then hatch in the spring and summer. WNV can also return to an area from migrating birds returning from warmer climates where WNV can reside year round. See Mosquito 101 [PDF] for additional information.

What can I do to protect myself and my family?

The best way to protect yourself and your family is to avoid mosquito bites and reduce places where mosquitoes live and breed around your home. For a list of preventative measures you can take, see Preventing West Nile Infection.

Is it safe to use mosquito repellent on my children?

The American Association of Pediatrics (AAP) and other experts suggest using repellent with low concentrations of DEET on infants over 2 months old. Other guidelines suggest using repellents containing DEET after children are 2 years of age. For guidelines on applying repellent on children and other pertinent information, see Preventing Mosquito Bites webpage.

Are there any alternatives to DEET-containing repellents?

There are other repellents without DEET on the market, but they may not protect as well and do not last as long. For additional information, see EPA Repellent Information.

What good is emptying small containers around my house when there are natural wetlands all around?

Culex pipiens, which can transmit West Nile virus, prefer polluted waters and small containers in which to breed. Most mosquitoes found in natural wetlands are not effective transmitters of WNV.

What about the nearby wetland, stormwater pond, etc.?

Wetlands and "wet" stormwater ponds have natural mosquito predators present. Many aquatic insects eat young mosquitoes, as do salamanders, tadpoles, and fish. Birds, bats, dragonflies, salamanders, and frogs eat adult mosquitoes. Encourage these predators, and avoid using pesticides that will harm beneficial insects.

What about my or my neighbor's tarps, tire pile, or other garbage?

Small amounts of stagnant water where mosquitoes breed are a concern, because there are fewer predators to eat the young mosquitoes. Once the mosquitoes can fly, they can be eaten by birds, bats, and dragonflies. We encourage everyone to identify areas on their property where stagnant water collects, and either remove the containers or empty them at least once a week. Encourage others to do the same.

In the case of dumping or unmanaged garbage, contact Thurston County Code Enforcement at 360-867-2581. Be aware that enforcement can be a slow process. You may wish to be extra careful to use the recommended personal mosquito precautions (see Preventing WNV Infection).

Can I add fish or frogs to my pond?

In general, you should not add fish or frogs to stormwater ponds or natural ponds and wetlands. Some species are known to threaten native species, and even native animals may spread wildlife diseases from one pond or lake to another. A permit from the Washington Dept. of Fish and Wildlife (360-902-2724) is needed to release species into large ponds or water that connects to natural bodies of water.

It is permissible to release fish commonly available in pet stores into small, contained backyard garden ponds if they do not connect with, or occasionally flood into natural water bodies. Aerating small ponds and adding native plants around the edges to attract mosquito-eating birds are recommended.

I want to treat my pond, wetland, and/or yard for mosquitoes. What or who do you recommend?

It is more effective to protect yourself from WNV by following the personal protection recommendations (see Preventing WNV Infection). Treatment of any water besides lined garden ponds requires both a permit from the Dept. of Ecology and a licensed pesticide applicator to do the work. For permit information, see Aquatic Pesticide Permits, Dept. of Ecology.

For lined garden ponds, consider aerating, adding fish (see question above), and adding wildlife-attracting plants.

A low-toxic pesticide is Bti (Bacillus thuringiensis israeliensis). Bti is a bacterium that is toxic to mosquito larvae. As with any pesticide, follow label directions carefully.

Can WNV infect animals other than birds?

While a relatively small number of dogs and cats have been infected with WNV, horses are shown to be more susceptible and can result in severe illness or death. An equine WNV vaccine for horses is available; consult with your veterinarian about immunization.

WNV is transmitted by mosquito bites, the same way as humans. There is no evidence that an infected animal can transmit the disease to humans or other animals.

There is no need to destroy a pet or horse that has contracted the disease. In most cases, animals infected with WNV recover completely, and usually no unusual behavior or symptoms may be noticed.

What should I do if I find a dead bird?

We no longer track or test dead birds for West Nile virus. With several years of experience tracking West Nile Virus, Washington State has found that monitoring mosquitos has been a more reliable indication of West Nile virus activity than tracking or testing dead birds.

What is the County doing about mosquitoes?

The first rule of medicine is "Do no harm." The County is approaching WNV in this way. we want to be sure our response does not cause more problems than the virus. We are preparing a phased response that calls for careful monitoring of the virus, and responding prudently to the level of risk.

In the first and current phase we recommend taking personal protection actions, such as wearing long-sleeved shirts, long pants, and hats. We also suggest avoiding mosquito areas at dusk, using repellents and cleaning up small containers where mosquitoes breed. If we reach a stage where humans are at high risk for contracting West Nile virus, we will alert the community and give appropriate recommendations.

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