West Nile Virus

What is West Nile virus?
What are the symptoms of West Nile virus?
How can West Nile virus spread?
Is there any treatment for West Nile virus?
How can I prevent West Nile virus?
Resources

Clinical signs and symptoms
Reporting
Laboratory testing
Treatment

Latest Updates

Data as of: 8/5/2026

Current Year Cases	Current Year Deaths	Cases Same Date Last Year	Deaths Same Date Last Year	2025 Total Number of Cases
1	0	0	0	3

For more historical data, please visit our data webpage.

Information for the General Public

West Nile virus (WNV) 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.

What are the symptoms of West Nile virus?

Most people (about 4 out of 5) infected with WNV don't show any symptoms and most likely don't know they have or had WNV. About 1 in 5 people will develop symptoms and may have fever, headache, body aches, joint pains, vomiting, diarrhea, or a rash. Most people with febrile illness due to WNV recover completely, but fatigue and weakness can last for weeks or months. About 1 in 150 (less than 1%) of people infected get severe disease affecting their central nervous system. Inflammation of the brain (encephalitis) or of lining around the brain and spinal cord (meningitis) can cause high fever, confusion, loss of consciousness, paralysis, convulsions, and vision loss. Recovery from severe disease may take weeks to months. About 1 in 10 of those with severe disease die.

How can West Nile virus spread?

WNV almost always spreads through the bite of an infected mosquito. In California, Culex mosquitoes are the carriers 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. Although it is very rare, WNV can also spread through blood transfusions, organ transplants, and from mother to baby during pregnancy. Blood collection agencies have been screening donated blood for West Nile virus since 2003. The risk of getting WNV through blood transfusions is very low and should not prevent people from receiving blood in an emergency or for other medical reasons.

Is there any treatment for West Nile virus?

There are no specific treatments for WNV.

How can I prevent West Nile virus?

Currently, there is no vaccine to prevent WNV. The best way to protect yourself is to avoid being bitten by mosquitos. You can take measures to prevent mosquito bites and you can take measures to control mosquitos in and around your home.

Resources

Information for Healthcare Providers

Clinical signs and symptoms

The incubation period from mosquito bite to onset of WNV disease symptoms is typically 2–6 days but ranges from 2–14 days and may be longer in immunocompromised patients.

If symptomatic, most people develop an acute systemic febrile illness, referred to as non-neuroinvasive disease or West Nile fever. Non-neuroinvasive disease often includes fever, fatigue, headache, myalgia, arthralgia, transient maculopapular rash, or gastrointestinal symptoms like vomiting or diarrhea.

Less than 1% of infected patients develop neuroinvasive disease, which typically manifests as meningitis, encephalitis, or acute flaccid myelitis. Risk of neuroinvasive disease is higher among older individuals and those with immunocompromising conditions.

Reporting

Report all suspected and confirmed cases of WNV to OCHCA Communicable Disease Control within one working day. Reports may be made by fax (714-564-4050) or phone (714-834-8180).

Laboratory testing

Whole blood RT‑PCR

California Department of Public Health’s Viral and Rickettsial Disease Laboratory (VRDL) now offers a whole‑blood RT‑PCR test that detects WNV without requiring confirmatory serology. This assay replaces the discontinued CSF PCR. Collect 5 mL of whole blood in an EDTA lavender‑top tube, keep it refrigerated, and ship it within 14 days of collection. The OC Public Health Laboratory can coordinate shipment to VRDL.

Serology

Order serum WNV IgM and CSF IgM if a lumbar puncture is performed. Because serum IgM can be negative early in illness, repeat serology may be needed if initial testing occurred within 8 days of symptom onset. If serology testing (IgM) is performed at a commercial laboratory, a sample will need to be forwarded to VRDL for confirmation.

Other tests for WNV, such as PCR on serum, CSF, or tissue, and viral culture, have low sensitivity and are not recommended for routine detection.

Treatment

There are no specific treatments for WNV.

Last Updated: 8/5/2026

West Nile Virus

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