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 most common and serious disease spread by mosquitoes in the U.S., including in California. People most often get WNV during the summer and early fall when the mosquitoes that carry WNV are most active.

How do people get WNV?

WNV almost always spreads through the bite of an infected mosquito. In California, Culex mosquitoes are the carriers (or "vectors") of WNV, and they become infected when they bite and feed on birds that are infected with WNV. Infected mosquitoes can then bite and spread WNV to humans and other animals.​

What are the symptoms of WNV?

No symptoms in most people. 

About 4 out of 5 people who are infected with WNV don't have any symptoms and most likely don't know they have or had WNV.

Milder symptoms in some people. 

About 1 out of 5 people who are infected with WNV will have symptoms including:

• Fever

• Headache

• Body aches

• Nausea

• Vomiting
  

• Skin rash

Symptoms usually start 3 to 14 days after being bitten by an infected mosquito. Most symptoms get better on their own and 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 people (less than 1%) infected with WNV will become very sick. In these cases, the virus affects the brain 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

• Coma

These symptoms may last several weeks, and effects on the brain and nervous system may be permanent. WNV can be fatal, and each year in California, people die from WNV. About 1 out of 10 people who develop severe WNV disease affecting the brain and nervous system die.

There is no specific treatment for WNV. 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.

Learn more about symptoms, diagnosis, and treatment of WNV

Who is at greatest risk of getting very sick from WNV?

People 55 years of age and older are at greatest risk of getting sick from WNV and are more likely to develop serious symptoms. 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 of developing severe symptoms from WNV. People in 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.

​How can WNV be prevented?

​The best way to prevent WNV is to protect yourself from mosquito bites:​

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. 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.​​

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.​​​

Dra​in

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. 

What is the California Department of Public Health (CDPH) doing about WNV in California?

CDPH and other agencies in California manage a West Nile virus surveillance system. This system uses human and horse case detection, mosquito testing, dead bird testing, and monitoring of sentinel chickens to understand and track WNV activity in California. CDPH also works with local vector control agencies and health departments throughout California to control mosquitoes and raise awareness about WNV in California.​

Where can I find current data about WNV in California?

For the latest information about WNV activity in California – including human case counts for the current season – please visit the California West Nile Virus website, which is a resource maintained through a partnership with the University of California, Davis Arbovirus Research and Training Laboratory and the Mosquito and Vector Control Association of California. ​