West Nile Virus FAQs & Basics


West Nile virus is a virus that is spread by mosquitoes. WNV can make people and animals sick and can even cause death. WNV is the main disease spread by mosquitoes in the U.S., including in California. People most often get infected with WNV during the summer and early fall when the mosquitoes that carry WNV are most active.

WNV was first detected in California in 2003, and human cases of WNV have been reported each year in California since then.
Infected mosquitoes. WNV is almost always spread by the bite of an infected mosquito. Mosquitoes are the carriers (or "vectors") of WNV, and they become infected when they feed on birds that are infected with WNV. Infected mosquitoes can then spread WNV to humans and other animals when they bite.

Transfusions, transplants, and mother-to-baby. Although it is very rare, WNV can spread through blood transfusions, organ transplants, and from mother to developing baby during pregnancy. All donated blood in the U.S. is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very low and should not prevent people from receiving blood in an emergency or for other medical reasons. The spread of WNV from mother to baby during pregnancy or transmission to an infant through breastfeeding is extremely rare.

Not through touching. WNV does not spread through coughing, sneezing, or casual contact, such as touching or kissing a person who has WNV. People also can’t get WNV by touching or being around birds or other animals that have WNV.
The main type of mosquitoes that spread WNV are Culex mosquitoes. These mosquitoes are widespread in California and will bite and feed on birds, other animals, and humans. Mosquitoes get WNV by feeding on the blood of a bird that is infected with WNV. Mosquitoes can then spread the virus to humans the next time they bite. Culex mosquitoes usually bite in the morning and evening. Culex mosquitoes can also spread St. Louis encephalitis virus (SLEV) and western equine encephalitis virus (WEEV), but these viruses are not as common as WNV. For more information, see the CDPH Guide to Important Mosquitoes in California.

In California, WNV most often spreads to people from mosquitoes in the summer and early fall when it is hot outside and Culex mosquitoes are most active. However, mosquitoes are active year-round in California. For more information about the risk of mosquito bites throughout the year in California, see the Annual Risk of Mosquito Bites and Disease Transmission in California graphic.
No symptoms in most people. About 4 out of 5 (80%) people who are infected with WNV don’t have any symptoms and most likely don’t know they have been infected with WNV.

Milder symptoms in some people. About 1 out of 5 (up to 20%) people who become infected with WNV will have symptoms that may include fever, headache, body aches, nausea, vomiting, and sometimes a skin rash. Symptoms usually develop 3 to 14 days after being bitten by an infected mosquito. Most symptoms usually last only a few days, although fatigue and weakness can last for weeks or months after being sick.

Serious symptoms in a few people. About 1 out of 150 (less than 1%) people infected with WNV will become very sick. In these cases, the virus affects the brain and/or nervous system and can cause encephalitis or meningitis (infection of the brain or surrounding tissue). Severe symptoms can include high fever, headache, neck stiffness, vision loss, numbness, confusion, muscle weakness, paralysis, and coma. These symptoms may last several weeks, and effects on the brain and nervous system may be permanent. WNV can be fatal.
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. People with certain medical conditions (such as cancer, diabetes, hypertension, and kidney disease) and people who have received organ transplants are also at greater risk for developing severe symptoms from WNV. These groups should be extra careful to prevent mosquito bites and should talk to a healthcare provider if they are worried about WNV or think they have it.
Milder WNV symptoms (such as fever, headache, and nausea) get better without treatment, and people do not necessarily need to seek medical care, although they may choose to do so. If you develop symptoms of severe WNV (such as unusually severe headaches or confusion) you should seek medical care immediately. Pregnant women and nursing mothers should also talk to a healthcare provider if they develop symptoms commonly associated with WNV.
A healthcare provider can order lab tests to look for WNV infection. A healthcare provider will use the information from lab tests and a person’s symptoms to determine if someone has WNV or not.
There is no specific treatment for WNV. Milder WNV symptoms (like fever, headache, nausea, and vomiting) get better on their own without treatment, but over-the-counter pain relievers can be used to help reduce fever and relieve symptoms. In more severe cases, people may need to go to the hospital where they can receive supportive care, such as IV fluids, pain management, and nursing care.
After most people recover from WNV, they are immune for life to future WNV infections. However, this immunity may decrease over time as someone gets older. Immunity may also decrease from health conditions that can weaken the immune system (such as cancer or autoimmune diseases).


The best way to help prevent WNV is to protect yourself from mosquito bites. You can prevent mosquito bites and help control mosquitoes around your home by practicing the "Three Ds":

1. DEFEND - Apply insect repellent containing DEET, picaridin, oil of lemon eucalyptus, or IR3535 to clothes and exposed skin according to label instructions. Repellents keep mosquitoes from biting you. When used as directed, DEET is safe for use on pregnant or breastfeeding women and children. Also make sure doors and windows have tight-fitting screens to keep mosquitoes out of your home. If screens have tears or holes, fix or replace the screens to keep mosquitoes out.

2. DAWN AND DUSK - The mosquitoes that spread WNV bite in the early morning (dawn) and the evening (dusk), so it’s especially important to wear long sleeves, pants, and repellent if you are outside during these times.

3. DRAIN - Mosquitoes that spread WNV lay their eggs on standing water. To keep mosquitoes from breeding around your home, get rid of any sources of standing water around your home and property. Dump and drain any containers that hold water, including buckets, old car tires, birdbaths, and pet bowls. Swimming pools that aren’t maintained can also be a breeding source for mosquitoes. If you notice a swimming pool in your neighborhood that is not being kept clean, please contact your local mosquito and vector control agency.
No. There is no WNV vaccine to prevent people from getting WNV. The best way to prevent WNV is to prevent mosquito bites.

Pets and Other Animals

Dogs and cats can get WNV from the bite of an infected mosquito, but it is rare for pets to get sick. Pet owners who are worried about WNV should talk to a veterinarian.
Horses can also get WNV from the bite of an infected mosquito. Like in humans, WNV can cause severe disease and even death in horses. Signs of WNV in horses may include stumbling, weakness in the hind legs, inability to stand, drooping lips, sensitivity to touch or sound, muscle tremors, and death. The American Association of Equine Practitioners (AAEP) recommends that horses be vaccinated for WNV every year. Horse-owners should talk to a veterinarian about the WNV vaccine for horses, as well as other vaccines for horses that protect against mosquito-borne viruses, such as western equine encephalitis virus. For more information on WNV and horses, please visit the California Department of Food and Agriculture Equine WNV website.
A mosquito infected with WNV can bite any animal, but not all animals will get sick from WNV. WNV most often affects birds but may sometimes cause disease in other animals, such as squirrels.

Birds. Wild birds are the main source of WNV for mosquitoes. When certain birds become infected with WNV, they will have WNV in their blood. If a mosquito bites an infected bird and feeds on their blood, the mosquito can become infected and pass the virus on to people or other animals that they bite. Not all birds that are infected with WNV will get sick, but WNV can make some birds very sick and even cause death. Signs of WNV in birds may include uncoordinated movement, a lack of energy, and difficulty breathing. Corvid birds (such as crows, jays, ravens, and magpies) are the most likely to get sick and die from WNV. Unlike other birds, chickens usually do not get sick from WNV and do not pass the virus on to mosquitoes.

Squirrels. Tree squirrels with WNV can have signs of brain or nervous system infection, such as uncoordinated movement, paralysis, shaking, or circling. Squirrels can die from WNV.
If you find a dead bird (especially a crow, jay, magpie, raven, sparrow, finch, or hawk), please report the dead bird online or call toll-free 1-877-968-2473 (1-877-WNV-BIRD). Dead bird reports are often the first sign that WNV is active in an area, and the reports help the California Department of Public Health track WNV throughout the year.
It’s extremely unlikely that you, a child, or a pet would get WNV from a dead bird. WNV spreads to people from the bite of an infected mosquito, not from dead birds. However, you should always be careful to protect yourself when touching or handling any dead animal. Dead birds can leak fluids and have sharp beaks and claws. Wear disposable gloves and use a shovel or plastic bag turned inside-out to scoop the bird into a plastic bag. Tie or seal the bag and wash your hands with soap and water when you are done. When you report a dead bird, someone will tell you if you should save the bird for pickup and testing, or if you should throw it away.
If you find a dead bird in your pool, you should not be worried about the pool water being contaminated with WNV. 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 source for mosquito breeding if they are not maintained or cleaned, so it’s important to keep your pool clean and chlorinated.
Sentinel chickens are used to help local vector control agencies track WNV activity in both urban and rural areas of California. Chickens are helpful because the mosquitoes that spread WNV prefer to bite birds, including chickens, rather than people and other animals. Chickens attract mosquitoes that can be carrying WNV, but they don’t get sick from WNV and can’t spread it to people or other mosquitoes. About every two weeks during WNV activity season, a small blood sample is collected from each sentinel chicken and sent to the California Department of Public Health for testing. The blood is tested for antibodies to WNV and shows whether the chickens have been bitten by infected mosquitoes. If the chickens test positive for WNV antibodies, this alerts local vector control agencies that WNV is being spread by infected mosquitoes in that area, and the local agency will increase mosquito monitoring and control. In this way, sentinel chickens act as an alert system for WNV activity in many regions of California. At the end of the WNV activity season, the chickens are “retired” and spend the rest of their lives as backyard or farm chickens.

More Information

You can search for your local mosquito and vector control agency information on the Westnile.ca.gov homepage. Simply click the “Look Up Local Vector Agency” button at the top of the page and search using your ZIP code.

Healthcare providers should contact their local public health department to discuss requirements for WNV testing. You can also find information on the CDPH WNV Information for Physicians webpage or the CDC WNV Information for Healthcare Providers webpage.
In 2000, CDPH and other agencies in California 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 and horse case detection, mosquito testing, dead bird testing, and monitoring of sentinel chickens. CDPH works with local vector control agencies and health departments throughout California to detect and monitor WNV activity and respond with enhanced mosquito control and public outreach to reduce the risk of WNV transmission.
Updated March 2024