CA Dept. of Public Health UC Davis Arbovirus Research and Training Mosquito and Vector Control Assoc. of CA

Case Counts by County

CountyDead birds
Alameda-
Alpine-
Amador-
Butte-
Calaveras-
Colusa-
Contra Costa-
Del Norte-
El Dorado-
Fresno-
Glenn-
Humboldt-
Imperial-
Inyo-
Kern-
Kings-
Lake-
Lassen-
Los Angeles-
Madera-
Marin-
Mariposa-
Mendocino-
Merced-
Modoc-
Mono-
Monterey-
Napa-
Nevada-
Orange2
Placer-
Plumas-
Riverside-
Sacramento-
San Benito-
San Bernardino-
San Diego1
San Francisco-
San Joaquin2
San Luis Obispo-
San Mateo-
Santa Barbara-
Santa Clara-
Santa Cruz-
Shasta-
Sierra-
Siskiyou-
Solano-
Sonoma-
Stanislaus-
Sutter-
Tehama-
Trinity-
Tulare-
Tuolumne-
Ventura-
Yolo-
Yuba-
Total5

WNV Activity by County
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2018 | 2017 | 2016 | 2015 | 2014
2013 | 2012 | 2011
2010 | 2009 | 2008
2007

WNV Ecology and Epidemiology

Last Updated: Jul 31, 2007

  • When was WNV first found in the United States?
  • West Nile virus was first detected in the United States in New York in 1999. Since then, WNV has spread to 48 states, and to Canada and Mexico.

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  • What is the history of WNV activity in California?
  • WNV first appeared in California in 2002 with the identification of one human case. In 2003 WNV activity was found in six counties with three human cases. In 2004 , WNV activity was identified in all 58 counties with 779 identified human cases. Out of the 779 human cases, there were 28 fatalities. In 2005 WNV activity was identified in 54 counties with 880 human cases and 19 fatalities. In 2006 54 counties had WNV activity with 278 identified human cases and 7 fatalities. In 2007 WNV activity was identified in 51 counties with 409 identified human cases and 21 fatalities. In 2008 WNV activity was identified in 49 counties with 498 identified human cases and 15 fatalities. West Nile virus is now endemic throughout the state of California and there will continue to be significant numbers of human cases.

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  • How is WNV detected and monitored in California?
  • In 2000, the California Department of Health Services along with other agencies expanded an existing statewide mosquito-borne encephalitis program to include WNV. The state-wide mosquito-borne encephalitis program includes encephalitis case detection, mosquito testing and monitoring of sentinel chickens, dead bird reporting and testing that was introduced in 2000, and dead tree squirrel reporting and testing that was introduced in 2004.

    Human Case Surveillance
    Individuals who are experiencing symptoms of Went Nile virus should contact their health care provider or clinic. The California Department of Health Services and local public health departments provide testing for West Nile virus.

    Mosquito Testing
    Mosquitoes throughout the state are collected and tested for the presence of WNV, western equine encephalomyelitis (WEE), and St. Louis encephalitis (SLE) viruses. Local mosquito and vector control agencies also monitor the abundance and type of mosquitoes.

    Sentinel Chicken Testing
    Over 200 chicken flocks are strategically placed throughout the state and are tested routinely during the mosquito season to detect evidence of infection from WNV, WEE, or SLE viruses. Chickens do not get sick from these viruses.

    Animal Case Surveillance
    Horses with encephalitis are routinely tested for WNV, WEE, and other mosquito transmitted diseases. Animal owners should consult are veterinarian if their animal is ill.

    Dead Bird Surveillance
    California began to test dead crows and related birds for WNV in 2000. Reporting dead crows and other birds helps identify where the virus is active in California. In 2005, WNV positive dead birds were found in 54 counties. State agencies, private organizations, and individuals participate in the surveillance program by reporting dead birds.

    Dead Tree Squirrel Surveillance
    California began to test dead tree squirrels in 2004. Reporting dead tree squirrels, because they do not fly, provides evidence of highly localized WNV transmission to mammals. State agencies, private organizations, and individuals participate in the surveillance program by reporting dead tree squirrels. For more information visit http://www.westnile.ca.gov/wnv_faqs_basics.php?id=56.

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  • How is a sentinel chicken tested for WNV?
  • Some sentinel chicken flocks are kept year round while others are kept through the warmer times of the year when WNV transmission is most likely to occur. Every two weeks, a few drops of blood from each chicken?s comb are placed on filter paper and tested for WNV antibodies.

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Compared to previous year and previous five-year average to date

As of Jun 21, 2019201920185Y AVG
Counties81521
Human cases092
Dead birds527126
Mosquito samples22031180
Sentinel chickens002
YTD - Year to date corresponds to the same time last year or last five years.

- view all 2019 YTD activity -

WNV Reports

Weekly Report

06.21 Arbovirus Bulletin #12
06.21 2019 YTD SLEV Activity Map
06.21 2019 YTD WNV Activity Map

Humans

05.15 2018 Human WNV Incidence Report
05.15 2003-2018 WNV Case Summary
03.29 2017 Human WNV Incidence Report

Dead Birds

06.21Positive Counts by City/County for 2019
06.21Positive Species for 2019
06.21Reported, Tested, Positive 2019

Mosquitoes

06.21 AMOR - EVS Week 24
06.21 AMOR - GRAVID Week 24
06.21 AMOR - NJLT Week 24
- view report archives -