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JAMA Patient Page
July 23/30, 2003

West Nile Virus

JAMA. 2003;290(4):558. doi:10.1001/jama.290.4.558

West Nile virus (WNV) affects humans, birds, horses, and other animals. The virus is carried from infected birds (the host animal) to humans and other mammals through mosquito bites. When a mosquito bites, a small amount of blood is injected into the skin of the bitten person or animal. This infected blood can transmit WNV or other similar diseases.

West Nile virus is more commonly found in the eastern United States. West Nile virus disease activity has spread rapidly since 1999, when it was first reported in the United States. The main period of risk is from July through September, but cases have occurred from May through December. The outbreak of WNV infection in the United States during the summer of 2002 was the largest reported. There were 284 deaths and 4156 human cases of WNV infection reported in 2002. The July 23/30, 2003, issue of JAMA includes articles about WNV.


  • Fatigue

  • Headache

  • Loss of appetite

  • Nausea or vomiting

  • Muscle aches

  • Rash

  • Fever

  • Confusion, delirium, or coma

Most people who become infected with WNV do not develop severe illness. Mild WNV infections usually last 3 to 6 days after an incubation (delay) period of 3 to 14 days after the initial exposure to the virus. Approximately 1 in 150 persons infected with WNV develops a more severe form of WNV infection, including encephalitis (inflammation of the brain). Some individuals with a severe WNV infection develop rashes, paralysis, inflammation of nerves, and seizures. Older persons are more susceptible to severe infection with WNV.


Special blood tests may be done by local or state health departments if your doctor suspects you might have WNV infection. Because WNV is a virus, antibiotics will not work to treat WNV infection. Persons who are ill with WNV infection usually require hospitalization for supportive care. There is currently no vaccine for WNV. Research studies are ongoing to find a way to immunize persons against WNV infection.


Preventing WNV transmission by mosquitoes is the most important way to decrease WNV infection in humans:

  • Drain all standing water where mosquitoes breed (birdbaths, old tires, flower pots, buckets, stagnant puddles or ponds).

  • Wear protective clothing when mosquito exposure is possible ((long sleeves, long pants, socks).

  • Use an insect repellant containing DEET on exposed skin.

  • Stay indoors during peak mosquito time (dawn, early evening).

  • Keep windows and screens in good repair.

Also, do not handle dead birds. More than 162 species of birds have been reported to have WNV infection. Crows have a particularly high death rate from WNV. Local and state health departments may collect dead birds to test them for the presence of WNV.



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The JAMA Patient Page is a public service of JAMA . The information and recommendations appearing on this page are appropriate in most instances, but they are not a substitute for medical diagnosis. For specific information concerning your personal medical condition, JAMA suggests that you consult your physician. This page may be photocopied noncommercially by physicians and other health care professionals to share with patients. Any other print or online reproduction is subject to AMA approval. To purchase bulk reprints, call 718/946-7424.

Sources: Centers for Disease Control and Prevention, National Institute of Allergy and Infectious Diseases, Environmental Protection Agency, National Pesticide Information Center