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.

Please click here for a downloadable/printable brochure on WNV. 

General Information

How Do People and Animals Get West Nile Virus?

Infected Mosquitoes

Transmission Cycle of West Nile virus

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. 

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: http://www.cdc.gov/westnile/faq/repellent.html

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.

Birds

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.

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.

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.

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.