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

Case Counts by County

CountyDead birds
Alameda11
Alpine-
Amador-
Butte22
Calaveras-
Colusa-
Contra Costa33
Del Norte-
El Dorado4
Fresno6
Glenn4
Humboldt-
Imperial-
Inyo-
Kern-
Kings4
Lake5
Lassen-
Los Angeles124
Madera5
Marin5
Mariposa-
Mendocino-
Merced5
Modoc-
Mono-
Monterey-
Napa1
Nevada2
Orange87
Placer30
Plumas-
Riverside16
Sacramento411
San Benito-
San Bernardino5
San Diego262
San Francisco-
San Joaquin37
San Luis Obispo-
San Mateo15
Santa Barbara-
Santa Clara89
Santa Cruz2
Shasta3
Sierra-
Siskiyou-
Solano13
Sonoma8
Stanislaus14
Sutter10
Tehama-
Trinity-
Tulare3
Tuolumne-
Ventura34
Yolo72
Yuba4
Total1346

WNV Activity by County
Printable Version
2016 | 2015 | 2014
2013 | 2012 | 2011
2010 | 2009 | 2008
2007

What is West Nile virus?

Last Updated: Oct 3, 2016

  • What is West Nile Virus?
  • West Nile virus (WNV) is a mosquito-borne disease that was originally found in Africa. It was first detected in the eastern United States in1999; since then the virus has spread across the continental United States and is well established in most states, including California.

back to top

  • 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. During WNV transmission season, 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 from receiving units of blood for medical conditions or for other circumstances (American Red Cross). 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.

back to top

  • What mosquitoes spread WNV?
  • The main species of mosquitoes that spread WNV are in the genus Culex. Culex mosquitoes are widespread in California and will feed on birds, other animals, and humans. Mosquitoes get the virus by feeding on an infected bird, and may transmit the virus to humans the next time they bite. Culex mosquitoes tend to bite in the morning and evening and are not known to spread Zika, dengue, or chikungunya viruses. For a guide to mosquitoes of importance see "Guide to Important Mosquitoes in California."

back to top

  • How is WNV disease diagnosed?
  • Diagnosis is based on a combination of clinical signs and symptoms, in addition to specialized laboratory testing of the blood or spinal fluid. These tests specifically detect known antibodies that the immune system makes against the viral infection.

back to top

  • How can I reduce the risk of being infected with WNV?
  • CDPH recommends that individuals prevent exposure to mosquito bites and West Nile virus by practicing the "Three Ds":
    1. DEET - Apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus, or IR3535 according to label instructions. Repellents keep the mosquitoes from biting. DEET can be used safely on infants and children 2 months of age and older.


    2. DAWN AND DUSK - The mosquitoes that transmit WNV bite in the early morning and evening, so it is important to wear proper clothing and repellent if outside during these times. Make sure doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens with tears or holes.

    3. DRAIN - Mosquitoes lay their eggs on standing water. Eliminate or drain all sources of standing water around homes and properties, including buckets, old car tires, rain gutters, birdbaths, and pet bowls. If a swimming pool is not being properly maintained, please contact the local mosquito and vector control agency.

back to top

  • Is there a vaccine available to protect people from WNV?
  • No. There is no available WNV vaccine to prevent people from becoming ill from WNV.

back to top

back to top

  • How soon do infected people get sick?
  • People typically develop symptoms 3 to 14 days after they are bitten by an infected mosquito. The incubation period could be longer in people with certain medical conditions that affect the immune system.

back to top

  • What are the symptoms of WNV?
  • Serious symptoms in a few people. Less than 1% percent of individuals (about 1 in 150 people) infected with WNV will develop severe neurological illnesses, such as encephalitis or meningitis (inflammation of the brain or surrounding tissue). 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 may include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Acute symptoms generally last for just a few days, although fatigue and weakness can last for weeks or months.

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

back to top

  • Who is at greatest risk of getting severely ill from WNV?
  • People over the age of 60 are at greatest risk of getting sick and are more likely to develop serious symptoms when infected with WNV. Individuals with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants are also at greater risk for developing severe symptoms and illness.

back to top

  • How is WNV infection treated?
  • There is no specific treatment for WNV infection. In cases with milder symptoms, people experience fever and aches that subside on their own. Over-the-counter pain relievers can be used to help reduce fever and relieve associated symptoms. In more severe cases, people may need to go to the hospital where they can receive supportive care, such as intravenous fluids, pain management, and nursing care.

back to top

  • 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, although 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 consult medical attention if they develop symptoms commonly associated with WNV.

back to top

  • 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.

back to top

  • Can domestic 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 CDC website: http://www.vetmed.ucdavis.edu/ccah/health_information/west_nile.cfm

back to top

  • How are horses affected by WNV?
  • Horses can become ill from WNV through the bite of an infected mosquito. Clinical signs of WNV illness in horses may include stumbling, circling, hind leg weakness, inability to stand, drooping lips and lip smacking, hypersensitivity to touch or sound, muscle tremors, and death. The American Association of Equine Practitioners (AAEP) recommends incorporation of a WNV vaccine as an annual core vaccine in equine vaccination protocols. 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 http://www.cdfa.ca.gov/ahfss/Animal_Health/WNV_Info.html

back to top

  • 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 host of WNV for mosquitoes. When some species of birds become infected, they produce high quantities of the virus, which can then be passed on to other mosquitoes that bite them. The virus is maintained in this bird-mosquito cycle. WNV infection has been reported in more than 225 bird species. Not all birds that are infected with WNV will appear ill, but WNV infection can cause serious illness and death in some birds. Signs of WNV infection in birds may include lethargy, uncoordinated movement, and difficulty breathing. The most severe illness and rapid death is seen among 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.

back to top

  • 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.

back to top

  • 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 www.westnile.ca.gov or call toll-free 1-877-968-2473 (1-877-WNV-BIRD). Dead bird reports are often the first indication that the virus is active in an area, and this allows the California Department of Public Health (CDPH) to monitor the virus throughout the year.

back to top

  • Should I be worried about getting WNV from a dead bird I found?
  • It is extremely unlikely that you, your children, or your pets would contract WNV from a dead bird. The virus is transmitted by the bite of an infected mosquito. However, safety precautions are recommended when handling any dead animal. Wear gloves and use a shovel or inverted plastic bag to place the bird into two plastic bags, and tie or seal the bags. Wash your hands afterwards. Dead birds can leak fluids and have sharp beaks and claws.

back to top

  • Can swimming pools be contaminated with WNV from a dead bird?
  • There is no evidence that swimming pools can be contaminated with WNV or that the virus can live in water. However, swimming pools can be a mosquito breeding source if they are not maintained, so it is important to keep your pool clean and chlorinated.

back to top

  • 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.

back to top

  • How can real estate professionals help with WNV prevention?
  • The CDPH requests that real estate professionals that access unoccupied homes report neglected swimming pools to their local mosquito and vector control agency. Reporting neglected swimming pools will reduce the risk of WNV transmission. Please see Educational Brochures and Flyers here: http://www.westnile.ca.gov/resources.php. To find your local vector control agency, simply type the zip code in the box below "Locate Your Local Mosquito and Vector Control Agency" in the upper right-hand corner of this webpage.

back to top

  • What is the California Department of Public Health doing to reduce the risk of people becoming infected with WNV?
  • In 2000, CDPH and other agencies expanded the statewide mosquito- borne virus surveillance program to enhance the state's ability to detect WNV. The California West Nile virus surveillance system includes human case detection, mosquito testing, dead bird testing, and monitoring of sentinel chickens. CDPH works with local vector control agencies and health departments to detect and monitor WNV activity and respond with enhanced mosquito control and public outreach to reduce WNV transmission risk.

back to top

Having problems with mosquitoes?

Locate your local mosquito and vector control agency
Search by Zip Code




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

As of Dec 02, 2016201620155Y AVG
Counties384040
Human cases406675475
Dead birds134613341467
Mosquito samples350933262825
Sentinel chickens343449460
YTD - Year to date corresponds to the same time last year or last five years.

- view all 2015 YTD activity -

WNV Reports

Weekly Report

12.02 Arbovirus Bulletin #38
12.02 2016 YTD WNV Activity Map
11.29 2016 US Arboviral Activity Update

Humans

12.02 2016 Human WNV Incidence Report
11.16 2003-2016 WNV Case Summary
03.21 2015 Human WNV Incidence Report

Dead Birds

12.02Positive Species for 2016
12.02Reported, Tested, Positive 2016
12.02Positive Counts by City/County for 2016

Mosquitoes

11.10 AMOR - EVS Week 44
11.10 AMOR - GRAVID Week 44
11.10 AMOR - NJLT Week 44
- view report archives -