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West Nile Virus

What is West Nile virus?

West Nile Virus is a mosquito-borne virus that was first discovered in Africa. It first appeared in the United States in 1999 and it moved west until it reached California in 2003. It is present throughout the year, although illness is often associated with peaks during hot summer months.

How do you get West Nile virus?

West Nile virus is transmitted through the bite of an infected mosquito. Very few mosquitoes are actually infected with West Nile virus, but it only takes one infected mosquito to bite and transmit West Nile virus. Blood transfusions and organ donation account for a small number of West Nile virus cases nationwide.

Do all mosquitoes have West Nile Virus?

Not all mosquito species carry West Nile virus, and even among mosquitoes capable of carrying the virus, most are NOT infected. Being bitten by a mosquito does not automatically mean you will be infected with West Nile virus.

What are the signs and symptoms of West Nile virus?

Illness associated with West Nile virus varies by the severity of symptoms: 1) Severe illness associated with nervous system malfunctions 2) An extended flu-like illness with high fever and excessive sleep 3) No illness at all.

  1. Serious Symptoms: Less than 1% (1/150) of those infected with West Nile virus will have more severe symptoms that may include: high fever, headache, neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.
  2. Milder Symptoms: About 20% (1/5) of those infected display milder symptoms which can include: fever, headache, body aches, nausea, vomiting, and a skin rash. Symptoms generally last for a few days, although some people have been sick for several weeks.
  3. No Symptoms: About 80% (4/5) of those infected with West Nile virus have no symptoms and feel nothing at all. In this group, the body produces an antibody that attacks the virus and the individual never gets sick.

Adults over age 50 are at increased risk for serious illness associated with West Nile virus infection.

What should I do if I think I have West Nile Virus?

If you believe you have West Nile virus, or any other medical problem, consult a medical professional.

Is there West Nile virus in San Mateo County?

Yes, West Nile virus has been found in San Mateo County. For current information, visit this page.

What is the District doing to protect me?

San Mateo County Mosquito and Vector Control District has an extensive preventative mosquito control and disease surveillance program. Mosquito fish are also available at no additional cost to be used in backyard fish ponds and swimming pools. At the District, we are monitoring and testing dead birds, mosquitoes, and sentinel chickens for the presence of West Nile virus.

How can I protect myself and my family?

Mosquitoes develop in standing water. Eliminating standing water around your home will reduce your risk of getting West Nile virus. Here are some examples of how:

  • Turn over buckets and other containers.
  • Check your sprinkler system for leaks.
  • Use mosquitofish, at no additional charge from the district, in backyard fish ponds or stagnant pools that are breeding mosquitoes.
  • Fix broken pipes.
  • Use a sump pump to remove standing water from under your home.
  • Request help if you are breeding mosquitoes.

Personal protection around your home is also helpful:

  • Keep mosquitoes out of your home by keeping doors and windows closed or tightly screened.
  • Wear insect repellent when outdoors, especially at dusk and dawn when the mosquitoes that transmit West Nile virus are active. Wear long sleeves and pants to protect yourself from bites.

Can I use natural or DEET-free insect repellent?

Products containing DEET, Picaridin, oil of lemon eucalyptus, or IR3535 are recommended for prevention of mosquito bites. Both oil of lemon eucalyptus and IR3535 are considered biopesticide repellents by the US EPA.

Some ‘home remedies’ or products marketed as insect repellents may not contain active ingredients that have been tested for safety and effectiveness. If you choose to use a product containing another active ingredient, be aware that it may not be effective at preventing mosquito bites or may need to be reapplied much more often.

Is there a vaccine for West Nile Virus?

There is not a human West Nile virus vaccine, but there is a vaccine available for horses.

How do you monitor West Nile virus?

You can learn about our West Nile virus and other mosquito-borne disease surveillance programs here.

Why report dead birds?

Dead birds, especially corvids (crows, ravens, and jays) are often a sign that West Nile virus is present in an area. Testing dead birds for West Nile virus is one of the most important disease surveillance tools we have.

A dead bird may indicate the presence of West Nile virus in a specific area. The California Department of Health Services uses these reports in a risk model that assist in determining areas of high risk to humans. Testing is performed on dead birds that meet the following criteria:

  • Birds that are dead for less than 24 hours.
  • The chest cavity is intact.
  • We will test any species, but are most interested in Crows, Ravens, Scrub-jays, Stellar Jays, Magpies, Red-tailed Hawks, Sparrows and House Finches.
  • Birds should not show any signs of decomposition, such as ants or maggots.

To report a bird, call call 877-WNV-BIRD or report it online at

Can my cat or dog get sick from West Nile virus?

Cats rarely get sick from West Nile virus and dogs have been known to get sick but few cases have been detected. For more information regarding West Nile virus and cats and dogs click here.

Can I get West Nile virus from birds or animals?

There has never been any documented case of West Nile virus transmission to humans from an animal other than a mosquito.

What are sentinel chickens?

The District uses sentinel chickens in rural parts of the county to detect West Nile virus and other mosquito-borne diseases. They serve as an early indicator of disease in areas where dead birds are less likely to be seen and/or reported.

You can learn about our sentinel chicken program here.

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