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.

Recent WNV activity in Marin and Sonoma counties

2014 WNV Activity

County Humans Horses Dead Birds Mosquito Pools Sentinel Chickens Dead Squirrels
Marin 0 0 2 3 0 0
Sonoma 0 0 8 2 0 0


WNV Positive Mosquito Samples

Date County City Address Species
05/07/2014 Sonoma Sonoma Ramal Road x Poehlman Road Culex tarsalis
06/04/2014 Sonoma Petaluma Ellis Creek Water Recycling Facility Culex tarsalis
06/27/2014 Marin Novato School Road x Atherton Culex tarsalis
07/08/2014 Marin Novato Stonetree Lane x Harbor Drive Culex tarsalis
07/09/2014 Marin San Rafael Deer Valley Road x Smith Ranch Road Culex stigmatosoma


WNV Positive Dead Birds

Date County City Address Species
06/08/2014 Marin Novato Oak Park Drive x Orange Avenue American Crow
06/17/2014 Marin Novato School Road x Atherton Avenue Western Scrub-Jay
06/18/2014 Sonoma Sebastopol Watertrough Road x Pleasant Hill Road Western Scrub-Jay
06/23/2014 Sonoma Santa Rosa Barnes Road x Mirante Road Common Raven
06/26/2014 Sonoma Petaluma Haverfield Lane x Magnolia Avenue American Crow
07/07/2014 Sonoma Santa Rosa Spring Creek Road x Yulupa Avenue American Crow
07/17/2014 Sonoma Rohnert Park Racquet Club Circle x Country Club Drive American Crow
07/17/2014 Sonoma Rohnert Park Racquet Club Circle x Country Club Drive American Crow
07/17/2014 Sonoma Geyserville River Road x Highway 128 American Crow
07/21/2014 Sonoma Healdsburg Young Road x Tremonte Lane Stellar's Jay


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.