Orange County Mosquito and Vector Control District
This website includes CSS elements that your browser does not support. Please upgrade your browser to a current version, then come back and try again.

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..

General Information

How Do People and Animals Get West Nile Virus?

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. 

Transmission Cycle of West Nile Virus

What are the Symptoms of West Nile Virus in People?

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. 

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.

Where can I get information about repellents?

The Centers for Disease Control and Prevention has a webpage with information on repellents:

How soon do infected people get sick?

People typically develop symptoms from 3 to 14 days after they are bitten by an infected mosquito.

Which Animals Get West Nile Virus?

An infected mosquito can bite a variety of animals, and many animals will become infected, but only a few species exhibit disease. Disease is seen in a several species of birds, in horses, and man. Cats, dogs, bats, chipmunks, skunks, squirrels, and domestic rabbits have been shown to be infected without exhibiting disease symptoms.


An infected mosquito can bite a variety of animals, and many animals will become infected, but only a few species exhibit disease. Disease is seen in a several species of birds, in horses, and man. Cats, dogs, bats, chipmunks, skunks, squirrels, and domestic rabbits have been shown to be infected without exhibiting disease symptoms.

[Learn more about WNV from the California Department of Public Health.]

What We Do

WNV exhibits similar ecological requirements as other North American mosquito transmitted arboviruses, such as St. Louis encephalitis (SLE) virus. Studies of the SLE virus vectors and reservoir hosts observed during the SLE outbreak (26 reported human cases) in southern California in 1984 have provided information that can be applied to the WNV cycle when it enters southern California.

The District's experience with SLE has demonstrated that mosquitoes capable of transmitting virus develop readily in 1) residential back yards (e.g., sprinkler filled containers and runoff) and in 2) surface drainage found in curb gutters, catch basins, underground drains, and storm channels. The Orange County Mosquito and Vector Control District (District) controls mosquito breeding in the latter accessible public areas throughout Orange County (OC). However, the District cannot adequately control mosquito production in water holding containers in the more than 730,000 individual back yards in Orange County. The extent to which WNV transmission occurs in OC will depend largely on the degree to which citizens respond to backyard mosquito control advisories from the District's Communications Department, the effectiveness of mosquito (larva) control around identified WNV activity foci, and the success of the underground catch basin and manhole chamber mosquito control program.

What is the California West Nile Surveillance Program?

The first human case of WNV was confirmed in California in September 2002. The California Department of Health Services (DHS) has overseen a statewide mosquito-borne encephalitis virus surveillance program since 1969 for western equine encephalitis (WEE), St. Louis encephalitis (SLE), and other viruses. In 2000, DHS, the District, and other agencies expanded their programs to enhance the ability to detect WNV. A protocol to report and test dead birds has been added to the existing encephalitis case surveillance system, that also includes mosquito testing and monitoring of sentinel chickens.

Encephalitis Case Surveillance

DHS tracks cases of human, horse, and ratite (e.g., emu, ostrich) encephalitis. The routine testing of encephalitis cases for WNV will assist in the early detection of the virus in California. Human and animal encephalitis cases are also routinely tested for WEE and SLE viruses.

Mosquito Testing

Mosquitoes throughout the state are sampled for the presence of WEE, SLE, and now WN viruses. Local mosquito and vector control agencies also monitor the abundance and type of vector mosquitoes.

Small Bird Surveillance

The District and a number of other vector control districts in the state also conduct SLE, WEE, and WNV surveillance using small birds (e.g., house sparrows, house finches) as indicators of virus transmission. Based upon studies of the SLE virus transmission cycle (SLE is another flavivirus, closely related to WNV) established flavivirus enzootic cycles are maintained in discrete small bird/mosquito interactions. Virus enzootic cycles must become well developed before non-reservoir animals (e.g., crows, humans, horses) become infected when they inadvertently contact the virus/mosquito/small bird cycle. By tapping into an enzootic cycle (e.g., mosquito sampling, small reservoir bird bleeding), detection of virus activity (i.e., virus or antibodies) provides the earliest possible awareness of its presence.

Dead Bird Surveillance

California began to test dead crows and related birds for WNV in 2000. Now that the virus has been reported in California, monitoring dead crows and other corvids will help identify other virus foci. State agencies, private organizations, and individuals participate in the surveillance program by reporting dead bird sightings. The District arranges for collection, assessment, and testing for WNV at the District laboratory.

What Do I Do if I See a Dead Bird?

Please call the District to report a dead bird.
You can obtain our contact information by clicking here.

The District WNV Defense Program

Since 1947 the District has provided an effective county-wide mosquito control program to eliminate pest and pathogen carrying mosquitoes. The mosquito control program, still in operation, consists of spraying accessible mosquito sources (e.g., manhole chambers, street gutters, catch basins, flood control channels) with environmentally safe pesticides. Mosquito control of backyard sources (approx. 40% of all mosquito production sites) is accomplished through public awareness announcements to the property owners of Orange County by pamphlets, television, radio, and newspaper.

Biological Controls

Click Here

What You Can Do

The best way to prevent West Nile Virus infection is to avoid being bitten by mosquitoes.

A West Nile Virus Vector
Culex quinquefasciatus
Southern House Mosquito
Whenever You Are Outdoors
  • Avoid spending time outdoors at dawn or dusk, when mosquitoes are most active.
  • Wear loose fitting and light colored clothes to protect yourself from mosquito bites.
  • Use DEET or Picaridin based mosquito repellent or Lemon Oil of Eucalyptus, according to label instructions. Please visit the Centers for Disease Control website for more information.
Around The House
  • Eliminate any source of standing water in your yard.
  • For larger water sources that cannot be eliminated (i.e., fountains, ornamental ponds), the District provides mosquito larvae eating fish free of charge.
  • Make sure window screens are in good repair and fit properly.
  • Report any large breeding sites in your neighborhood to the District:
    • Abandoned swimming pools. Click here to see a video of mosquito larvae in an unused pool.
    • Accumulations of trash and containers capable of holding water.

In addition to eliminating mosquito-breeding sites around your home, you can help Vector Control in its battle against larger breeding areas by preventing excess water from running off your property.

  • Make sure sprinklers are positioned properly and in good repair. Adjust sprinkler timers to prevent over watering. If water is running off your lawn onto the sidewalk or curb, your lawn is getting too much water. Over-irrigation is the primary source of water in our gutters.
  • Check rain gutters for any leaves or debris that could be blocking the flow of water. Click here to see a video of mosquito larvae in a rain gutter.
  • Take your car to a carwash if possible. When washing your car at home, turn the hose off while you are soaping and washing, and divert the water to a vegetated area such as the lawn.
  • Sweep your driveway instead of hosing it down. Collect leaves and landscape debris and put them in your green waste cart. Just a few leaves in the gutter can cause water to back up and form a breeding site for mosquitoes.
  • If you notice standing water in the gutter between street sweeping days, please sweep it out to minimize mosquito-breeding areas.
An abandoned swimming pool is a breeding ground for mosquitos.
Vector Reduction Manual For the District's specific guidelines on how to avoid or reduce mosquitoes, rats, red imported fire ants, and flies please refer to recommendations in the Vector Reduction Manual: Procedures and Guidelines.
 Orange County Mosquito and Vector Control District