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West Nile Virus (WNV)


What is West Nile Virus?

West Nile Virus (WNV) is a mosquito-borne disease that is common in Africa, west Asia, the Middle East, and as of 1999 in North America. WNV has been found in mosquitoes, birds, mammals or humans in every US state except Hawaii and Alaska. (Human cases have occurred in Alaska but were not locally acquired.) It is a seasonal epidemic that flares up in summer and continues into fall. Most human infections with WNV are asymptomatic or relatively mild; however some infections result in serious illness.


In 2005, nationally 2949 human infections and 116 fatalities nationwide were reported to the CDC. In California 935 human infections (CDC figure is 873) and 19 fatalities were reported, predominantly in the Sacramento region, and WNV infected birds were found in 52 counties. In 2005, San Francisco had only 3 cases of West Nile Virus. Two of the patients were infected in the Sacramento area and one in Wyoming. 


The 2009 WNV season has already begun. Find out about WNV activity in California and nationally.


How it is Spread to Humans?

West Nile Virus is transmitted to humans and animals through a mosquito bite. Mosquitoes become infected when they feed on infected birds. Human-to-human transmission of does not occur.


Prevention Measures

Individuals can reduce their risk of mosquito-borne diseases by taking these precautions:

  • Report a dead bird on line or call 1-877-WNV-BIRD
  • Avoid outdoor activity when mosquitoes are most active, especially at dawn and dusk.
  • When outdoors, wear protective clothing like long pants and long-sleeved shirts.
  • Apply a repellent containing permethrin, picaridin, oil of lemon eucalyptus, or DEET when mosquitoes are active. For guidance on use of DEET on children, refer to the CDC website below.
  • Install or repair screens on doors and windows.
  • Drain standing water; as little as a tablespoon of water can support mosquito breeding.
  • Contact the Department of Public Health, Mosquito Control program at (415) 252-3806 if there is a significant mosquito problem where you live or work.


Eighty percent of individuals who become infected with West Nile Virus will not experience any illness. Others will have only mild symptoms, such as fever, fatigue, nausea, headache, muscle aches, skin rash, or swollen lymph nodes. However, West Nile Virus can affect the elderly and individuals with compromised immune systems with more severe illness.



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

Control activities in San Francisco

San Francisco is taking several steps to limit risks to residents. The City's mosquito control program includes: collecting birds for testing, monitoring ponds and other possible mosquito breeding sites, trapping to detect high numbers of mosquitoes, treating sewer catch basins to prevent breeding, and educating residents and owners about removing standing water from private property and avoiding mosquito bites. Learn about statewide control activities.


Useful West Nile Virus Links

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