Medicine for the Outdoors
Medicine for the Outdoors

Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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West Nile Virus

The County of Santa Clara (California) Public Health Department reports that 5 pools of mosquitoes were found within the county infected with West Nile Virus (WNV) the week of July 3, and that at least 24 dead birds have tested positive for WNV. So, the WNV season is in full swing. WNV has also been reported from many other counties in the state, and will likely begin its seasonal upswing across the western U.S.

WNV is an arbovirus endemic in Africa, Europe, Asia, and the Middle East that established its presence in the U.S. beginning in 1999. It has since spread to be found in most states. It is transmitted to humans by the bite of an infected mosquito. While much of the clinical WNV activity is noted in summer and autumn, it is certainly possible to acquire the disease in winter from the bite of an infected mosquito. The four top species of wild birds affected by WNV are American crows, Western scrub-jays, yellow-billed magpies, and Steller's jays. Mosquitoes bite the birds and thus acquire the virus.

Most (80%) people infected with WNV never realize that they have had the disease, because they remain without symptoms. 20% of infected people develop West Nile fever, and less than 1% of people infected develop severe medical illness, including meningitis and/or encephalitis (characterized by seizures, loss of vision, and disorientation) or paralysis. Death is uncommon.

West Nile fever incubates in a human for 3 to 14 days after the bite of an infected (with WNV) mosquito, and is usually characterized as a mild illness lasting for 3 to 6 days. However, it can be more severe, with fatigue as a residual symptom lasting for up to a month. Symptoms include fever, fatigue, headache, muscle aches, and skin rash (chest, stomach, or back). Lymph glands (nodes) may become swollen.

WNV infection does not appear to spread from human to human. There are blood tests for WNV infection. These measure antibodies to the virus, and show positive in most infected people within 8 days of the onset of symptoms. However, they may initially be "negative" and need to be repeated at a later date. Treatment for West Nile fever is based upon symptoms - there is no antidote. For severe WNV disease, there are studies looking at specific drugs to combat the virus. 

Prevention is essential. First and foremost, that means preventing mosquito bites. Here are some recommendations:

  1. Do not maintain standing water that serves as a breeding ground for mosquitoes, which lay eggs in the water. Drain or dump all standing water on a weekly basis. This includes water as shallow as one inch deep, as may be found in flower pots, planter bases, old tires, child pools, etc.
  2. Be sure that all doors and windows have tight-fitting screens. Repair any holes or rips, and if possible, treat screens and door jambs with mosquito control products.
  3. Most bites occur at dawn and dusk, so limit outdoor activities during these times.
  4. Use effective insect repellents, such as those containing DEET (N,N-diethyl-m-toluamide) or picaridin (KBR 3023). Use repellents according to the manufacturer's labeled instructions, and reapply frequently, particularly if you swim or become sweaty.
  5. Wear clothing designed to cover your arms and legs, including long sleeves and pants.
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About the Author

Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.