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West Nile Virus

What is West Nile virus?

West Nile virus (WNV) is the leading cause of mosquito-borne disease in California.  It is most commonly spread to people by the bite of an infected mosquito. Cases of West Nile virus usually occur from summer to fall, when the weather is warmer and certain kinds of mosquitoes are more active. There are no vaccines to prevent or medications to treat West Nile virus in people. Fortunately, most people infected with West Nile virus do not feel sick. About 1 in 5 people who are infected develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness. You can reduce your risk of West Nile virus by using an EPA-registered insect repellent and wearing long-sleeved shirts and long pants to prevent mosquito bites.


How do people get infected with West Nile virus?

West Nile virus is primarily spread by 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. People can also become infected through blood transfusions, organ transplants, and from mother-to-baby transmission during pregnancy.

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.


Who is at risk of getting sick with West Nile virus?

Adults over age 50 are at increased risk for serious illness associated with West Nile virus infection. Children and those with compromised immune systems are also more likely to develop complications when infected.

What are the signs and symptoms of West Nile virus?

Most people who are infected do not develop symptoms, while others may develop mild symptoms like fever, body aches, headaches, nausea, vomiting and sometimes a skin rash. Elderly people and those with certain medical conditions (such as diabetes, hypertension, cancer, and kidney disease) are at higher risk of severe symptoms (including high fever, neck stiffness, vision loss, confusion, neurological damage, paralysis, and coma) and even death.


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. Mosquitofish are available for use 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. You can learn about our West Nile virus and other mosquito-borne disease surveillance programs here.


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 in backyard fish ponds or stagnant pools that are breeding mosquitoes. Contact us for mosquitofish.
  • Fix broken pipes.
  • Use a sump pump to remove standing water from under your home.
  • Request help if there are biting mosquitoes at your home.

Personal protection around your home is also helpful:

  • Keep mosquitoes out of your home by keeping doors and windows closed or tightly screened.
  • Wear an EPA-registered 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.

What insect repellent is best?

EPA-registered insect repellents are proven to be safe and effective. Some of the ingredients these products may contain are DEET, picaridin, oil of lemon eucalyptus, or IR3535 .

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. If you are a horse owner, speak with your veterinarian about whether a West Nile virus vaccine is appropriate for your horse.


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. 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.


What are sentinel chickens?

The District uses sentinel chickens at a few locations in the County to detect West Nile virus and other mosquito-borne diseases. They may serve as an early indicator of disease. You can learn about our sentinel chicken program here.



Page last reviewed: March 5, 2024

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