West Nile Virus

Overview & History: 

Although West Nile virus (WNV) is primarily a disease of birds, humans and other mammals may incidentally become infected when bitten by an infected mosquito. The virus was first discovered in 1937 in the blood of an infected individual in Uganda (Africa), and was introduced to New York City in 1999. WNV was able to spread rapidly across the United States primarily because it often affects highly mobile, migrating birds. In 2002, the virus arrived on the west coast of the United States, and by 2007, had spread as far south as Argentina. Currently, WNV is found in parts of Africa, Asia, Europe, North and South America.

2018 WNV activity in Marin and Sonoma counties / Year-end 2018 Vector Surveillance Report

2017 WNV activity in Marin and Sonoma counties / Year-end 2017 Arbovirus Surveillance Summary

2016 WNV activity in Marin and Sonoma counties / Year-end 2016 Arbovirus Surveillance Summary

2015 WNV activity in Marin and Sonoma counties  / Year-end 2015 WNV Report

2014 WNV activity in Marin and Sonoma counties 

Impact on human health: 
  • Approximately 80% of human cases of WNV do not show any symptoms.
  • Approximately 20% of people infected with WNV experience mild symptoms that may include fever, headache, body aches, nausea, rashes, swollen lymph nodes and vomiting. 
  • Less than 1% of people infected with WNV develop serious illness. These cases may last for extended periods of time, result in permanent neurological damage and may be fatal.
What you can do to protect yourself: 

Reduce outdoor exposure to mosquitoes:

  • Avoid being outdoors during peak mosquito activity (dawn and dusk).
  • Properly use mosquito repellents containing DEET, picaridin or oil of lemon eucalyptus (always read the label carefully).
  • Wear protective clothing.

Reduce indoor exposure to mosquitoes:

  • Keep doors and windows closed whenever possible.
  • Check and maintain all window and door screens.

Prevent mosquitoes from breeding near your home:

  • Flip, dump or drain all temporary sources of standing water.
  • Call Marin/Sonoma Mosquito & Vector Control District if you have a mosquito-related problem that you cannot resolve on your own.

Report dead birds:

  • Reporting dead birds can help the District detect West Nile virus activity. 
  • Click here to watch a short video about what to do if you find a dead bird. 
  • Residents who find dead birds are encouraged to report them to the West Nile virus hotline at 1-877-WNV-BIRD or online at www.westnile.ca.gov.
Impact on animals/wildlife: 
  • WNV infection may be deadly to over 200 species of birds
  • The virus is dangerous to horses, but a vaccine is available (consult a veterinarian).
  • Dogs and cats are very resistant to WNV and rarely become ill.