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West Nile Virus - Safety Information, Causes, Mosquito Control and More

What Is West Nile Virus?

West Nile virus (WNV) is a potentially serious illness. Experts believe West Nile Virus is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall. This fact sheet contains important information that can help you recognize and prevent West Nile virus. The rate of fatality is 3-15% among those who develop the disease. WNV encephalitis primarily affects persons over 50 years of age. Those with a compromised immune system may also be more susceptible to acquiring encephalitis. Currently no approved human vaccine is available for WNV.

What Can I Do to Prevent West Nile Virus

The easiest and best way to avoid West Nile Virus is to prevent mosquito bites.

• When you are outdoors, use insect repellents containing DEET (N, N-diethyl-meta-toluamide). Follow the directions on the package.  There are also some other formulations available; but deet has been used since World War II and is considered safe and effective.  Use an insect repellent containing 10 per cent or less DEET  for children and no more than 30 per cent DEET for adults.
  • For children between six months and two years of age, use one application per day of a product containing 10 per cent or less DEET in situations where a high risk of complications due to insect bites exist
  • For children between two years and 12 years of age, up to three applications per day of a product containing 10 per cent or less DEET can be used.
  • Individuals 12 years of age and older can use a DEET products of up to 30% DEET concentration.
  • Information about which repellent to use; what is most effective and safe mosquito repellent, is found on this page. And more information on the safe use of insect repellents can also be found on Health Canada's Web site.
     
• Many mosquitoes are most active at dusk and dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours. Light-colored clothing can help you see mosquitoes that land on you.
   
• Make sure you have good screens on your windows and doors to keep mosquitoes out.
   
• Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.
   

What Are the Symptoms of West Nile Virus?

West Nile Virus affects the central nervous system. Symptoms vary.

• Serious Symptoms in a Few People. About one in 150 people infected with West Nile Virus will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

• Milder Symptoms in Some People. Up to 20 percent of the people who become infected will display symptoms which can include flu-like symptoms: fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as short as a few days, though even healthy people have been sick for several weeks. Symptoms of WNV in humans generally appear within 3-14 days of infection, following a period of incubation. Some individuals who become infected with the virus (less than 1%) develop encephalitis or meningitis, which is an inflammation of the brain. Symptoms of this disease develop rapidly and may include high fever, headache, muscle weakness, stiff neck, confusion, and coma.

• No Symptoms in Most People. Approximately 80 percent of people (about 4 out of 5) who are infected with West Nile Virus will not show any symptoms at all.

How Does West Nile Virus Spread?

• Infected Mosquitoes. Most often, West Nile Virus is spread by the bite of an infected mosquito. Mosquitoes are West Nile Virus carriers that become infected when they feed on infected birds. Infected mosquitoes can then spread West Nile Virus to humans and other animals when they bite.
   
• Transfusions, Transplants, and Mother-to-Child. In a very small number of cases, West Nile Virus also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.
   
• Not through touching. West Nile Virus is not spread through casual contact such as touching or kissing a person with the virus.

How Soon Do Infected People Get Sick?

People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito.

How Is West Nile Virus Infection Treated?

There is no specific treatment for West Nile Virus infection. In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.

What Should I Do if I Think I Have West Nile Virus?

Milder West Nile Virus illness improves on its own, and people do not necessarily need to seek medical attention for this infection though they may choose to do so. If you develop symptoms of severe West Nile Virus illness, such as unusually severe headaches or confusion, seek medical attention immediately. Severe West Nile Virus illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be West Nile Virus.

What Is the Risk of Getting Sick from West Nile Virus?

People over 50 at higher risk to get sick. People over the age of 50 are more likely to develop serious symptoms of West Nile Virus if they do get sick and should take special care to avoid mosquito bites.
Being outside means you're at risk. The more time you're outdoors, the more time you could be bitten by an infected mosquito. Pay attention to avoiding mosquito bites if you spend a lot of time outside, either working or playing.
Risk through medical procedures is very low. All donated blood is checked for West Nile Virus before being used. The risk of getting West Nile Virus through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. If you have concerns, talk to your doctor.

Pregnancy and nursing do not increase risk of becoming infected with West Nile Virus. The risk that West Nile Virus may present to a fetus or an infant infected through breastmilk is still being evaluated. Talk with your care provider is you have concerns.

What Else Should I Know?

If you find a dead bird: Don't handle the body with your bare hands. Contact your local health department for instructions on reporting and disposing of the body.  Click here for information about reporting birds in your state.

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For more information call the Center for Disease Control (the CDC) public response hotline
at (888) 246-2675 (English), (888) 246-2857 (Español), or (866) 874-2646 (TTY)

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How to Control Mosquitoes

There are about 200 different species of mosquitoes in the United States, all of which live in specific habitats, exhibit unique behaviors and bite different types of animals. Click here for a page on how to control mosquitoes in your yard and neighborhood.

More Information:

• US map of West Nile virus (http://westnilemaps.usgs.gov/us_human.html)

• Bird Identification

  The following bird identification pages illustrate the most common bird species affected by West Nile virus. To date the species most affected are: 1) American Crows 2) Western Scrub-Jays 3) Yellow-Billed Magpies and 4) Steller's Jays.

  Corvids (Crows, Ravens, Magpies, and Jays)...............................Click Here

  Raptors (Birds of Prey).................................................................Click Here

  Sparrows (House Sparrow, White-Crowned Sparrow, etc.).........Click Here

  Finches (Lesser Goldfinch, House Finch, etc.)...............................Click Here

Other Information

• Frequently Asked Questions About Birds and West Nile Virus, WNV
• 2003 case count from CDC (http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount03.htm)
• http://www.cdc.gov/ncidod/dvbid/westnile/
• World map of West Nile and related viruses (http://www.cdc.gov/ncidod/dvbid/westnile/map.htm)
• Canadian case count (http://www.hc-sc.gc.ca/pphb-dgspsp/West Nile Virus-vwn/)
• Nature news article on West Nile paralysis

• American Mosquito Control Association (AMCA)  

General mosquito control information and listings of local mosquito abatement districts. See their page on mosquito traps or call (904) 215-3008.
• California West Nile Surveillance Information Center
• California Department of Food and Agriculture (veterinary information)
• U.S. Department of Agriculture (veterinary information)
• U.S. Centers for Disease Control (English)
   

This page was updated on 21-Feb-2008