Bay Area Medical Information (
West Nile Virus (WNV)
Health Depts:
State and Local
(For more information about mosquito control in your area or to report dead birds)

2006 Map of Diagnosed Cases from the US Geological Survey

It takes from 2 to 15 days after the bite of an infected mosquito for an individual to develop symptoms of the West Nile Virus.
Symptoms (1)(6)

It takes from 2 to 15 days after the bite of an infected mosquito for an individual to develop symptoms of the West Nile Virus.

About 80% of people who get West Nile virus infections get such a mild illness they may not even notice it. However, those who get serious disease can suffer from partial paralysis, permanent nerve damage, chronic fatigue, or even die.

Up to 20 percent of the people who become infected will have milder symptoms such as 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 a few days to several weeks. In general, the likely outcome of a mild West Nile virus infection is excellent.

Less than 1% of people infected with WNV will develop severe illness. Those at highest risk for serious symptoms are the elderly and those with lowered immune systems, however, people of all ages can develop life-threatening illness. Severe West Nile illness happens when the virus invades the spine, the brain, and/or the surrounding membranes (the meninges).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, but neurological effects such as paralysis may be permanent. Approximately 10% of patients with brain inflammation do not survive.

How is the virus spread?
  • Infected Mosquitoes. The easiest and best way to avoid WNV is to prevent mosquito bites.
  • In a very small number of cases, WNV also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.
  • WNV is not spread through casual contact such as touching or kissing a person with the virus.
To reduce the risk of being bitten by mosquitoes, the U.S. Center for Disease Control strongly recommends that individuals remain diligent in preventing bites from mosquitos. Precautions should include the Five "Ds" of Prevention:(12)
Remember the 5Ds--Important Preventive Measures for West Nile Virus (1)

Dress, Dusk and Dawn: Consider staying indoors during the hours between Dusk and Dawn. Dress with long Hats are protectivesleeves, pants, and a hat when outdoors between dusk and dawn as many mosquitoes are most active during this time. Be aware that a smaller number of mosquitos are also active during the day.

Infants should be kept indoors, or mosquito netting should be used over infant carriers when mosquitoes are present.


DEET: Use insect Insect repellents are the best protectionrepellents on exposed skin and spray clothes with repellent because mosquitoes can bite through thin clothing.

Don't apply repellant to skin that will be under clothing.

Don't apply repellents to the hands of children. Adults should apply repellent first to
their own hands and then transfer it to the child’s skin and clothing.


Drainage: Eliminate all mosquito breeding sites Plant by emptying containers with standing water from flower pots,wading pools, clogged gutters, buckets and barrels. Change thewater in pet dishes and replace the water in bird baths regularly. For best results, wash out the containers with soap and water to destroy any eggs that might have stuck to the sides of the container.

Keep mosquitoes out of your Screens should be in good conditionhome with screens, in good condition, on your windows and doors.

Don't rely on unproven products such as vitamin B or ultrasonic devices

Mosquito Repellents
DEET, Picaridin, and oil of lemon eucalyptus are ingredients recommended by the CDC and are registered with the EPA.
DEET is the most effectiveDEET: DEET-containing products are the most effective mosquito repellents available. DEET also is effective as a repellent against a variety of other insects, including ticks. The greater concentration of DEET a product contains, the longer it lasts. Repellents that contain more DEET don't work better, they just last longer. DEET should not be applied more than once a day, under clothing, or over open skin. It's important that the user read the label for further precautions. (3)
  • DEET has been the subject of many studies over the years. In rare cases, exposure to high levels of DEET has been reported to be toxic to the brain. In 1998, the EPA re-evaluated DEET, and found that it to be very safe when used according to label directions. If used as directed, The American Academy of Pediatrics has approved it use in children over 2 months. Young children shouldn't be allowed to apply DEET repellent themselves, but it is safe for them to use. This year, the American Academy of Pediatrics issued a statement saying that DEET is safe for children aged two months and older. Previously, the age limit was set at two years. The academy also raised the maximum DEET concentration in mosquito repellent for kids from 10% to 30%.
  • The most common complaint is when DEET gets in the eyes, as it can be irritating.(10)
Picaridin Has similar effectiveness to DEET(4) Its chemical name, which you might find in the list of "active ingredients" on a product, is KBR 3023. Years of safe use of picaridin in other parts of the world have proven that it's safe and effective.(7)
Oil of lemon eucalyptus, a plant- based repellent, provides protection slightly less than DEET and Picaridin but is approved by the CDC and EPA and provides longer lasting protection than other plant-based repellents.(5) Oil of lemon eucalyptus is also known as P-menthane diol, or PMD, for short. If you have a child older than age 3 and you feel more comfortable using plant-based products, a repellent with oil of lemon eucalyptus may be a good option. It is not recommended for kids under the age of three.(4)(7)
2% soybean oil is slightly less effective than oil of lemon eucalyptus(4) A product called Bite Blocker for Kids is a 2% soybean oil product.
Permethrin (Permanone) is a repellent that is intended for application to clothing, shoes, bednets and gear, but not directly on the skin.(5) Permethrin-treated clothing repels and kills ticks, mosquitoes and other arthropods. Aerosol and pump-spray products are available for treating clothing.
Oil of citronella is the active ingredient in many of the candles, torches, or coils that may be burned to produce a smoke that repels mosquitoes. These are useful outdoors only under windless conditions. Their effectiveness is less than repellents applied to the body or clothing.(8) Citronella candles have been used since 1882 as a means of drawing mosquitoes away from people, but one study shows that they're not much more effective than plain candles, which also give off heat, carbon dioxide, and moisture.(10)
Note: With all insect repellants, it's important to follow the directions on the label in order to note precautions as well as duration of protection, which varies widely among products.
Research on Insect Repellents

In a New England Journal of Medicine article in 2002, researchers compared several types of mosquito repellents in laboratory tests.

  • OFF! Deep Woods repellent, a product containing about 24% DEET, fared the best. Its protection lasted an average of five hours.(10)
  • The least effective products were wristbands treated with DEET or citronella, which offered almost no protection.(10)
  • Skin-So-Soft, Avon's original product that was never marketed as a mosquito repellent, worked as a mosquito repellent for an average of roughly 10 minutes, which hardly matched DEET products, or even soybean oil.(10) Avon has come out with several mosquito repellent formulas, some containing IR3535, a new EPA-approved mosquito repellent. IR3535 belongs to drug maker Merck, and it has been used as a mosquito repellent in Europe for 20 years. Avon also has a product with Picaridin, another EPA approved ingredient.
  • Bite Blocker for Kids, a 2% soybean oil product, protected against bites for an average of 94 minutes.(10)
What attracts mosquitos? from WebMD
Do you seem to get eaten alive when others are left alone? You're probably not just imagining it. Everyone's body chemistry is a little different, and some people are more likely to attract unwanted insect advances than others are. Mosquitoes can sense your presence from far away. When you breathe out, you emit a plume of carbon dioxide that carries on the breeze, and CO2 also seeps from your skin. "Mosquitoes are attracted to carbon dioxide as well as the warmth and humidity you're giving off", says Renee Anderson, PhD, a medical entomologist at Cornell University in Ithaca, N.Y. "They follow the trail, flying in a zigzag pattern, until they find the source. In addition, they are also attracted to certain chemicals in your sweat. And mosquitoes love a moving target -- it helps them zero in". WebMD
Pathology and Diagnosis (2)

West Nile virus is a type of organism which can cause encephalitis or meningitis, depending on where it spreads.

  • Encephalitis is an inflammation of the brain, usually caused by infections.

  • Meningitis is an infection which causes inflammation of the tissues covering the brain and spinal cord.

The most accurate way to diagnose this infection is a blood test or spinal tap to detect the presence of antibodies against West Nile virus in the CSF (cerebrospinal fluid) or serum (a blood component). A lumbar puncture, or spinal tap, will look for antibodies and signs of infection in the cerebral spinal fluid, which surrounds the brain and spinal cord. An MRI may be performed to diagnose encephalitis.

West Nile Virus Infections as of July 25, 2006 Pink areas: Positive Test Results for WNV; Green areas: Samples submitted; Yellow areas: No Data This Map of WNV infections is up-to-date as of July 25, 2006 from the USGS and CDC, U.S. Dept of Interior. To see a more updated map, click here

West Nile Virus as of July 25, 2006West Nile virus (WNV) is a potentially serious illness usually spread by the bite of an infected mosquito. The virus is spread when a mosquito bites an infected bird and then bites a person. The mosquito feeds on blood and is the carrier of many diseases, such as encephalitis, West Nile, dengue fever, yellow fever, and others.

WNV first emerged in the 1930s in the West Nile district of Uganda. Over time it spread into Europe and Asia, eventually arriving in New York in 1999. It has since rapidly spread across the North American continent. 15,000 people in the U.S. have now tested positive for WNV infection resulting in over 500 deaths. Many more people have likely been infected, but have experienced little or no symptoms. For an interesting illustration from the CDC on the trend of WNV in the United States since 1999, click here.

The incidence of WNV peaks in late summer and early fall, but in the southern climates where temperatures are milder, West Nile virus can be transmitted year round. Many of the mosquitoes that carry West Nile virus die during the winter in colder climates, but still enough survive to carry on the virus.

The extent of the outbreak varies from year to year and nobody knows how big a problem it will be in any given year, but all experts agree that: The easiest and best way to avoid WNV is to prevent mosquito bites. (1) (6)

Internet References (Academic & National Organizations)
  1. "What you need to know about West Nile Virus" from the CDC, 2005 (CDC phone: 970-221-6400)
  2. "West Nile Virus" from Merck Source
  3. West Nile Virus Information from the American Academy of Pediatrics
  4. DEET Alternatives from the American Academy of Pediatrics, June 2005
  5. Q & A about West Nile Virus from the CDC
  6. West Nile Virus from the National Biological Information Infrastructure, a program that provides increased access to data and information on the nation's biological resources.
  7. CDC adds to approved list of mosquito-fighting ingredients from the Nemours Foundation, April 2005
  8. Mosquito Control from the EPA
  9. Repellents from the American Mosquito Control Association
  10. Mosquito Repellents, What Works from WebMD
  11. Health Depts: State and Local For more information about mosquito control in your area, contact your state or local health department. Report dead birds to your health dept.
--Written by N Thompson, ARNP in collaboration with C Thompson and M Thompson, MD, Internal Medicine, Last Updated July 2006

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