California West Nile Activity Hits Five-Year High as the latest state surveillance shows the mosquito-borne virus continuing to spread during one of California’s highest-risk periods of the year.
California has reported 86 human cases of West Nile virus across 21 counties so far in 2026, according to the California West Nile Virus surveillance system. Twenty-two new human cases were reported during the most recent week alone, including six in Los Angeles County and two in Orange County.
California West Nile Activity Hits Five-Year High During Peak Season
The California Department of Public Health warned in August that West Nile virus activity had reached its highest level in five years. At the time, state health officials pointed not only to human infections but also to unusually high levels of the virus detected through mosquito and dead bird surveillance.
More than 24% of dead birds tested through the state’s surveillance program had been positive for West Nile virus, compared with a five-year average of 13%. CDPH also reported that about eight out of every 1,000 mosquitoes tested were positive, compared with a five-year average of five per 1,000.
Those warning signs have continued into September. California’s September 4 surveillance update lists 3,231 positive mosquito samples from 29 counties so far this year. During the latest reporting week, another 230 positive mosquito samples were recorded statewide.
Southern California accounted for a substantial share of those new detections. Los Angeles County reported 72 newly positive mosquito samples during the week, while Orange County reported 29. Year to date, the state surveillance system lists 504 positive mosquito samples in Los Angeles County and 702 in Orange County.
State health officials have emphasized that August and September are typically peak months for West Nile virus transmission in California, making the current level of activity particularly important as summer moves toward fall.
Los Angeles County Has Recorded 50 West Nile Infections
Separate Los Angeles County West Nile surveillance data show 50 confirmed West Nile infections in 2026 as of September 3, including 45 hospitalizations and one death. The county figure includes asymptomatic blood and organ donors and excludes Long Beach and Pasadena, so it should not be directly compared with the state’s county case count.
The increase has been significant compared with the same county surveillance system’s full-year total of 22 infections in 2025. Los Angeles County reported its first human case of the 2026 season in July after an Antelope Valley resident developed West Nile encephalitis and was hospitalized.
Most people infected with West Nile virus do not develop symptoms. Some develop fever, headache, body aches, vomiting, rash or other symptoms, while a much smaller share can develop severe neurological illness. Older adults and people with certain underlying health conditions face a greater risk of serious disease.
West Nile virus is transmitted primarily through the bite of infected mosquitoes. Mosquitoes acquire the virus after feeding on infected birds, which are an important part of the virus’s natural transmission cycle.
Mosquito Prevention Matters as West Nile Activity Remains High
With California West Nile activity at a five-year high, public health officials are urging residents to reduce their exposure to mosquito bites and eliminate places where mosquitoes can breed around homes.
That includes emptying standing water from containers such as buckets, flowerpot saucers and other outdoor items, keeping window and door screens in good condition, and using an EPA-registered insect repellent when mosquitoes are active. Even relatively small amounts of standing water can provide mosquitoes with a place to lay eggs.
Residents dealing with persistent mosquito activity can also contact their local mosquito and vector control agency. PestMarket’s mosquito control guide explains how to identify common mosquito problems around a property and reduce conditions that allow them to reproduce.
California’s West Nile season is not over. With positive mosquitoes still being detected across much of the state and new human infections continuing to be reported, surveillance during the coming weeks will show whether activity begins to decline as temperatures cool or remains elevated deeper into the fall.