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

Case Counts by County

CountyDead birds
Contra Costa26
Del Norte-
El Dorado4
Los Angeles113
San Benito-
San Bernardino5
San Diego254
San Francisco-
San Joaquin35
San Luis Obispo-
San Mateo11
Santa Barbara-
Santa Clara85
Santa Cruz2

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

What is West Nile virus?

Last Updated: 07 24, 2012


What is West Nile Virus?
West Nile virus (WNV) is a mosquito-borne disease that was originally found in Africa. In 1999, it was detected in the eastern United States; since then the virus has spread throughout the United States and is well established in most states, including California.

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How do people get WNV?
Infected mosquitoes. Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers ("vectors") that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.

Transfusions, transplants, and mother-to-child. All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. Transmission during pregnancy from mother to baby or transmission to an infant via breastfeeding is extremely rare.

Not through touching. WNV is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.

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How can I reduce the risk of being infected with WNV?
A person can reduce their risk of WNV infection by:

  1. Staying indoors at dawn and dusk when mosquitoes are most active.
  2. Using a repellent containing the active ingredients DEET, Picaridin, oil of lemon eucalyptus, or IR3535 before going outdoors.
  3. Dumping or draining water that has been standing for more than three days, including neglected swimming pools, birdbaths, pet dishes, tire swings, and flower planters.
    These items provide a perfect place for mosquitos to lay their eggs, which increases the risk of being infected with WNV.
  4. Repairing broken or torn screens on windows and doors to keep mosquitoes from entering your home.

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Where can I get information about mosquito repellents?
The Centers for Disease Control and Prevention has a webpage with information on repellents:

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How soon do infected people get sick?
People typically develop symptoms from 3 to 14 days after they are bitten by an infected mosquito.

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What are the symptoms of WNV?
WNV affects the central nervous system. However, symptoms vary:
Serious symptoms in a few people. Less than one percent of individuals (about 1 in 150 people) infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. WNV infection can be fatal.

Milder symptoms in some people. Up to 20 percent of the people (about 1 in 5) who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although even previously healthy people have been sick for several weeks.

No symptoms in most people. Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms.

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Who is at greatest risk of getting severely ill from WNV?
People over the age of 50 have a higher chance of getting sick and are more likely to develop serious symptoms when infected with WNV.

Being outside, especially at dawn or at dusk, increases your risk of being bitten by an infected mosquito. Take precautions to avoid mosquito bites if you spend a lot of time outside, either working or playing.

Risk of transmission through medical procedures is very low. All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it.

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How is WNV infection treated?
There is no specific treatment for WNV infection. 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 care including intravenous fluids, help with breathing, and nursing care.

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What should I do if I think I have WNV?
Milder WNV illness improves without treatment, and people do not necessarily need to seek medical attention for this infection, though they may choose to do so. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.

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If you have had WNV, are you immune to further infections?
It is thought that once a person has recovered from WNV, they are immune for life to future infections with WNV. This immunity may decrease over time or with health conditions that compromise the immune system.

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Can pets get sick with WNV?
Dogs and cats can be exposed to WNV in the same way as humans. However, these animals are very resistant to WNV and rarely become ill. Concerned pet owners should consult with a veterinarian. For more information, on dogs, cats and WNV, please visit the Centers for Disease Control and Prevention website:

Like people, most horses bitten by mosquitoes will not become sick with WNV. However, of those that do become ill, clinical signs may include stumbling, circling, hind leg weakness, inability to stand, muscle tremors, and death. A vaccine to prevent WNV is available for horses and horse-owners should consult with a veterinarian about the WNV vaccine and other vaccines against mosquito-borne viruses, such as western equine encephalitis. For more information on WNV and horses, please visit the California Department of Food and Agriculture website at

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Can other animals get sick from WNV?
An infected mosquito can bite any animal, but not all animals will become sick. The disease most often affects birds but may occasionally cause disease in other animals.

Wild birds serve as the main source of virus for mosquitoes. Infection has been reported in more than 225 bird species. Although many birds that are infected with WNV will not appear ill, WNV infection can cause serious illness and death in some birds. The most severe illnesses are seen among the corvid birds, which include crows, jays, ravens, and magpies.

Tree squirrels with WNV can develop neurological symptoms such as uncoordinated movement, paralysis, shaking, or circling and may die.

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How can I find the contact information for my local vector control agency?
Simply type your home zip code in the box below "Locate Your Local Mosquito and Vector Control Agency." You will find this in the upper right portion of each webpage.

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What should I do if I find a dead bird?
If you find a dead bird, particularly a crow, jay, magpie, raven, sparrow, finch, or raptor, please file an online report at or call toll-free 1-877-968-2473. Dead bird reports are very important because they are usually the first indication that the virus is active in an area, and this allows CDPH to monitor the virus throughout the year.

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I am a physician, where can I go to obtain more information?
Health care providers should contact their local public health department to discuss requirements for testing or visit WNV Resources and look under "Clinician Information" for more information. In addition, please click on the "Physician Info" box near the top of this page.

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How can real estate professionals help with WNV prevention?
Please see Educational Brochures and Flyers here:

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What is the California Department of Public Health doing to reduce the risk of people becoming infected with WNV?
CDPH has coordinated a statewide mosquito-borne encephalitis surveillance program since 1969 to detect western equine encephalitis (WEE), St. Louis encephalitis (SLE), and other viruses. In 2000, CDPH and other agencies expanded the program to enhance the state's ability to detect WNV. The California West Nile virus surveillance system includes human encephalitis case detection, mosquito testing, dead bird testing, and monitoring of sentinel chickens.

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

As of Sep 28, 2016201620155Y AVG
Human cases234233192
Dead birds128310161237
Mosquito samples324927692451
Sentinel chickens296314325
YTD - Year to date corresponds to the same time last year or last five years.

- view all 2015 YTD activity -

WNV Reports

Weekly Report

09.28 2016 YTD WNV Activity Map
09.23 Arbovirus Bulletin #29
09.20 2016 US Arboviral Activity Update


09.14 2016 Human WNV Incidence Report
09.14 2003-2016 WNV Case Summary
03.21 2015 Human WNV Incidence Report

Dead Birds

09.28Positive Species for 2016
09.28Reported, Tested, Positive 2016
09.28Positive Counts by City/County for 2016


09.23 AMOR - EVS Week 37
09.23 AMOR - GRAVID Week 37
09.23 AMOR - NJLT Week 37
- view report archives -