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West Nile virus is a mosquito-borne virus first identified in the West Nile region of Africa in 1937. It can cause a serious, even fatal, illness in people, horses, certain types of birds, and other animals. The virus has caused outbreaks of disease in Africa, Asia, Eastern Europe and the Middle East since 1937, but it did not appear in the United States until 1999. After first being discovered in birds and people in the metropolitan New York City area, it spread westward across the US and into Canada and Mexico. It is not known how West Nile virus arrived in the United States.

Wild birds become infected with West Nile virus and carry the virus in nature. Mosquitoes become infected after feeding on infected birds. People bitten by a mosquito carrying West Nile virus may have no symptoms at all or they may become ill with symptoms ranging from mild to severe. Horses, crows, hawks, owls and eagles are especially susceptible to West Nile virus disease, whereas the virus rarely causes illness in dogs, cats or chickens.

Fortunately, most people who become infected with West Nile virus do not get sick–their body fights off the infection and protective antibodies develop. About 20% (1 person out of 5) develop West Nile fever with symptoms that may include fever, muscle aches, fatigue, headache, rash, and joint pain. Some people with West Nile fever are quite ill for up to several weeks and may see their doctor, but hospital care is not usually needed.

Less than 1 percent (about 1 in every 150) of persons who become infected with West Nile virus develops the more serious neuroinvasive form of the disease. Types of neuroinvasive disease include: West Nile encephalitis, West Nile meningitis, and West Nile meningoencephalitis. Encephalitis refers to inflammation of the brain. Meningitis is inflammation of the membrane covering the brain and spinal cord. Meningoencephalitis is a combination of the two syndromes. Symptoms may include fever, neck stiffness, confusion, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.

Persons who survive West Nile neuroinvasive disease may have long-term symptoms or disabilities, but recovery from the milder forms of infection is usually complete. It is believed that once someone has had an infection caused by West Nile virus they develop long-term protection against being infected again.

In October of 2006 West Nile virus was detected in King County for the first time when 6 birds and a horse tested positive for the virus. Also In 2006, Washington State had its first human cases when West Nile virus fever was diagnosed in residents of Pierce County (2 cases) and Clark County (1 case).

In 2008, West Nile virus was reported in 3 people, 41 horses, 22 birds and 57 mosquito pools in Washington State. Three birds from King County tested positive in the fall of 2008 and one of the state's 3 human cases was from King County (the other 2 cases were from Yakima County).

The year 2009 saw the most West Nile virus activity ever recorded in Washington State. That year there were 73 horses with confirmed disease and 38 human cases. One of the human cases was a King County resident but upon interview of the patient it was determined he was likely to have acquired West Nile virus while visiting Yakima County. In 2009, human and equine cases were concentrated in Yakima and Benton counties in eastern Washington. No human cases occurred in King County in 2010 or 2011. Statewide, there was only one human case reported in 2010 (in Grant County) and none in 2011.

Some of the variation in case numbers can be attributed to climate and weather conditions that affect mosquito and bird populations. A wet, warm spring followed by a long, hot summer generally favors increased mosquito numbers and a higher risk of transmission of West Nile virus to people and horses.

For case counts and maps of area affected by West Nile virus see the following websites:

Since the virus first appeared in the US in 1999, the federal Centers for Disease Control has received reports on over 31,000 cases and 1200 deaths. Many cases of the milder forms of West Nile virus illness are never confirmed by specific laboratory testing and therefore are not reported and counted by public health agencies.

The most West Nile cases in a single year was in 2003 when there was West Nile virus activity in 46 states and nearly 10,000 human cases and 264 deaths. The states hardest hit that year were Colorado and Nebraska.

The risk for more serious illness starts to increase after about age 50 and especially after age 70.

Based on reports to the CDC, the median age of persons who developed the more serious neuroinvasive form of the disease was 57 years and the median age of those who died was 75 years (median age means that half the individuals were older and half were younger).

In addition to age, diabetes may also increase the risk for severe illness. Children are not at greater risk than young- or middle-age adults. Pregnancy is not known to increase the risk of developing the severe forms of West Nile virus infection; however there have been a small number of cases where a pregnant mother contracted West Nile virus and passed it to her unborn baby.

West Nile virus is transmitted by the bite of a mosquito. Mosquitoes become infected when they feed on birds carrying the virus. When the mosquito takes a blood meal from the infected bird, the virus becomes located in the salivary gland of the mosquito. Then when the mosquito bites a human or animal, the mosquito injects the virus into its victim. Persons who become ill develop symptoms 3 to 14 days after infection. The West Nile virus remains in infected individuals for a relatively short time and does not cause chronic infections.

West Nile virus is not transmitted directly from person-to-person except for rare cases attributed to blood transfusion or organ transplantation (see the next question). It is also not transmitted from animal-to-person except in very rare instances related to occupational exposure. There is no evidence that persons can become infected from eating meat from a West Nile virus infected bird or other animal.

Although rare, West Nile virus has been transmitted through transfusions of whole blood or blood components such as plasma or platelets. Blood component suppliers take preventative measures to screen out blood donors who may have been infected with West Nile virus at the time of donation. A laboratory test for West Nile virus has been in use for screening blood donors since mid-2003. There have been documented instances of West Nile virus transmission by organ transplantation, but the risk of this is extremely low.

Fortunately, clinical illness due to West Nile virus is rare in dogs and cats, and chickens are resistant as well. Persons with pet birds should be careful to protect them from mosquito bites. Horses and other equines like mules and donkeys are susceptible to West Nile virus infection and severe illness and death can result. An equine West Nile virus vaccine is available and horse owners are strongly encouraged to consult with their veterinarian about immunization. Horses will also benefit from mosquito control efforts.

At one time, Public Health–Seattle & King County conducted active surveillance for West Nile virus in both birds and mosquitoes. Detection in these species was intended to provide an early warning of the risk of human cases. As of 2009, surveillance for the virus in mosquitoes was ended in King County and surveillance in birds was reduced due to budget cuts.

Health care providers and hospitals are required by law to report to Public Health suspected cases of viral encephalitis including cases suspected to be caused by West Nile virus. For more information, consult the West Nile virus webpage for health care providers. Veterinarians report cases in horses to the state Department of Agriculture, which shares this information with Public Health.

Public Health provides information on personal protective measures and environmental measures that can be taken to reduce the risk of mosquito-borne diseases. Care is taken to provide West Nile prevention education to non-English speaking groups within the county. In past years, Public Health worked with municipal governments and other agencies throughout King County to promote mosquito surveillance, reduction of mosquito habitat and other control measures but these efforts were greatly reduced by budget constraints in recent years.

Crows and other corvid birds (like jays, ravens and magpies) are particularly susceptible to West Nile virus, and often die from it. Therefore, in partnership with the Washington State Department of Health, Public Health tests a sample of dead crows and other corvids each summer to see if they have died from West Nile virus.

Mosquitoes like still or standing water to lay their eggs. Stagnant water containing organic materials like dirt or plant matter is especially attractive to many types of mosquitoes. Once laid, mosquito eggs hatch into larvae and develop into adults in as few as seven days in very hot weather. Some species need only a few ounces of water to lay eggs.

Larvae ("wigglers") are 1/4 to 1/2 inch long, or smaller. They move by vigorously wiggling or flexing their bodies. They are usually dark in color and look like tiny aquatic worms. Though there are variations dependent on weather and temperature, mosquito larvae are most likely to be present in King County from March through October.

How to reduce the number of mosquitoes

Removing sources of standing water on your property and around your home reduces mosquito breeding habitat. Examples of things you can do include:

  • Tip water out of barrels, buckets and wheelbarrows and turn them over so water cannot collect
  • Tip out containers that could hold water such as toys, cans or plant saucers weekly
  • Empty children's wading pools weekly
  • Change water in birdbaths, pet watering dishes and animal troughs at least once a week
  • Discard, properly store or drill holes in used tires
  • Clean debris from ornamental ponds and keep fountains running during the summer
  • Stock ornamental ponds with fish
  • Maintain swimming pools and hot tubs
  • Recycle old bottles, buckets, and cans
  • Clean leaf-clogged gutters
  • Drain flat topped roofs
  • Dump water off of tarps and plastic sheeting
  • Drain water from covers on pools, boats and hot tubs
  • Repair leaky outdoor faucets
  • Cover rain barrels with mosquito screens
  • Cover garbage cans with an appropriate lid
  • Repair ripped window and door screens and make sure they fit tight; use a screen door on doors that often are left open
  • Consult a licensed pest control operator for mosquito habitat that cannot be controlled by drainage or other means

Help your elderly or frail neighbors and relatives with these activities (older people are more vulnerable to severe West Nile virus)

Be aware of the times of day when mosquitoes are most likely to be biting. The prime biting periods are often at dusk and dawn. If you do go outside when mosquitoes are biting, wear long sleeve shirts, long pants, and socks. Hats are also useful. If you are frequently outside when mosquitoes are biting, consider wearing special clothing that has been treated with permethrin to repel mosquitoes.

To help keep mosquitoes out of your home, ensure that window and door screens are in good repair and fit tightly. Reducing mosquito breeding habitat around your home will also help decrease the number of biting mosquitoes (see previous section).

Consider wearing an insect repellent. Repellents containing DEET (N,N-diethyl-meta-toluamide) or picaridin are known to be very effective. Oil of lemon eucalyptus can also be effective but may not offer protection for as long as DEET or picaridin. About mosquito repellents, CDC.

The American Academy of Pediatrics advises that DEET products not be used on infants under 2 months of age. The Academy has no age recommendations on picaridin products. Whenever using any repellent product, it is important to read the label and follow the instructions carefully. About repellent use for children, CDC.

Follow the general safety precautions listed below when using insect repellents:

  • Always follow product label instructions.
  • Use only enough repellent to cover exposed areas of skin; reapply if mosquitoes start biting.
  • Do not apply repellent to areas that are covered by clothing.
  • Never use repellents over cuts or irritated skin, and do not spray into eyes or mouth.
  • Do not apply directly to face, but spray on hands first and then apply to face.
  • Wash off treated areas of skin with soap and water after returning indoors.
  • Wash treated clothing before wearing again (this may vary by product, follow label instructions).
  • If you get a rash or another reaction, stop using the repellent and wash off the treated area with mild soap and water, and call the Washington Poison Center at 1-800-222-1222. If you see a health care provider, bring the product with you.
  • The Centers for Disease Control and Prevention (CDC) recommends against use of single combination products containing DEET and sunscreen because the instructions for insect repellent and sunscreen use are different (sunscreen usually needs to be applied more often than insect repellent). Instead use separate insect repellent and sunscreen products. Apply sunscreen first, then repellent.

Reduce breeding habitat on your own property where possible (see above). If you notice that mosquitoes are a problem, please see detailed control information. You may also choose to consult a licensed pest control company.

Public Health does not have regulatory authority to require property owners to remove mosquito habitat except if the habitat is in violation of solid waste law. However, we can send you educational materials that provide guidance on reducing habitat.

Wetlands and other natural water features may potentially be mosquito breeding habitat under certain conditions. Fortunately, wetlands also have many natural predators that feed on mosquito larva or adult mosquitoes and this often helps keep mosquito populations down.

It is undesirable to drain or fill wetlands because they play an important role in cleaning and holding storm run off - they play a critical public health role. Man-made retention and detention ponds play an important role as well. Visit the King County Wetlands website.