Southern California just endured its buggiest West Nile virus season in half a decade, and public health officials say the region may not be out of the woods yet.
As of early November, state health officials had confirmed 528 serious human cases of West Nile virus statewide this year, with 32 deaths — the highest totals California has seen since 2020. Los Angeles County alone accounted for more than half of those infections, making it the epicenter of this year’s outbreak.
The culprit is the Culex mosquito, a species that thrives once temperatures climb past 70 degrees. In that warmth, the virus multiplies rapidly inside the insect’s gut before it’s passed along through a bite. Most people who get infected never notice symptoms, but roughly one in five develop fever, headaches and body aches. For a small fraction — about one in 150 — the virus crosses into the brain, causing dangerous swelling and neurological damage. There’s no cure, only supportive treatment.
Now, as nights cool and mosquito activity naturally tapers off, experts are turning their attention to what next year might bring — and they’re not entirely optimistic.
“We’re all scared of next year,” said Paula Macedo, chair of the Mosquito and Vector Control Association of California. “With the amount of activity that we already saw this year and having the forecast of being an El Niño year, we are all bracing ourselves.”
A storm that changed everything
The roots of this year’s outbreak trace back to September, when the remnants of Hurricane Marie dumped an inch and a half of rain across the Los Angeles basin, breaking a rainfall record that had stood since 1965. Days later, temperatures spiked to 104 degrees, with nighttime lows staying above 77 — ideal conditions for mosquitoes to flourish.
The storm also washed away mosquito-killing treatments from more than 3,000 miles of underground storm drains across Los Angeles County, according to the Greater Los Angeles County Vector Control District.
“One of our biggest sources for Culex mosquitoes are our underground storm drains,” said Susanne Kluh, the district’s general manager. “It’s pretty much 72 degrees down there and humid all year round.”
With the larvicide gone, mosquito populations exploded. At the height of summer, Kluh said more than 70% of trapped mosquitoes tested positive for West Nile virus — a share that has since dropped to about a third.
Research backs up what officials saw play out this year. A UC Berkeley-led study examining a decade of Los Angeles data found a striking threshold: when average temperatures stay below about 70 degrees, the virus rarely shows up in mosquito populations, but once temperatures climb past 73 degrees, infections spike sharply. Coastal Los Angeles typically hovers in that danger zone during summer, meaning even modest warming can tip the balance.
“The longer that those temperatures are appropriate, the more opportunity there can be for human cases,” said Tristan Hallum, scientific director for the San Gabriel Valley Mosquito and Vector Control District.
The virus spreads through a complicated relationship between birds and mosquitoes. A mosquito that bites an infected bird can pick up the virus and pass it along to other birds — and occasionally, people. But birds that survive infection typically develop immunity, meaning their blood no longer carries the virus. That dynamic tends to create natural three- to five-year cycles of West Nile activity, suggesting Los Angeles and Orange counties might see somewhat calmer conditions next year, even as other parts of the state remain at risk.
El Niño adds uncertainty
Federal forecasters are giving strong odds — more than 90% — to a powerful El Niño pattern this winter, which typically means heavier rainfall for Southern California. That has vector control officials watching closely, since lingering floodwater combined with rising spring temperatures could set the stage for another mosquito boom.
It’s happened before. In 2023, flooding revived the long-dormant Tulare Lake basin in the Central Valley, creating sprawling wetlands that altered bird migration and gave native Culex mosquitoes prime breeding grounds. That year, the state recorded 433 human cases of West Nile virus.
Oddly enough, drought can be just as dangerous. During the dry years of 2014 and 2015, more than 1,600 Californians were infected — one theory being that scarce water forces birds and mosquitoes into closer contact as they compete for the same limited sources.
“Climate change has the effect of tipping the scales against us just a little bit more,” said Chris Barker, a UC Davis professor who helps run the state’s West Nile surveillance program. Rather than producing a steady year-over-year increase in cases, he said, extreme weather — whether flood or drought — is simply creating more opportunities for outbreak conditions to align.
No vaccine in sight
Despite West Nile virus circulating in the U.S. for more than 25 years, there remains no vaccine to prevent infection. Aaron Brault, a West Nile researcher with the Centers for Disease Control and Prevention, said in a written statement that the nation’s defenses against the virus remain limited.
Repellents and protective clothing can reduce risk, Brault noted, but inconsistent use means those measures alone haven’t meaningfully cut down on cases or deaths nationwide. The CDC is pursuing new treatments, diagnostic tools and a potential vaccine, but the unpredictable nature of outbreaks makes it difficult to conduct the large clinical trials needed to bring one to market.
“Viruses do defy us sometimes, especially the people trying to predict them,” Barker said.
Stretched thin
Local vector control agencies, funded largely through small annual property tax assessments — typically $4 to $10 per household — are facing mounting pressure from multiple directions.
Much of what Southern Californians battled this year wasn’t actually the West Nile-carrying Culex mosquito but an invasive species, Aedes aegypti. Unlike Culex, which bites mainly at dawn and dusk and prefers birds, the black-and-white striped Aedes bites during the day and specifically targets humans. It doesn’t spread West Nile but can carry other diseases including dengue, Zika, chikungunya and yellow fever.
Aedes is notoriously difficult to control — it can breed in a teaspoon of water, and its eggs can survive months of drought before hatching once rain returns. Some districts have spent hundreds of thousands of dollars releasing sterile male Aedes mosquitoes to curb their numbers, diverting resources from Culex control efforts.
Disasters compound the challenge. The 2025 Eaton fire, which destroyed roughly 9,000 structures, left behind some 1,400 stagnant swimming pools — ideal mosquito breeding sites.
“Budget over time has been stretched really thin by having to respond to these additional mosquitoes and additional causes,” Macedo said.
State law caps how much local agencies can raise fees, though some districts have asked voters directly for additional funding. Even with more resources, Macedo said, vector control agencies can’t reach every backyard.
“We cannot enter every property. We cannot treat every mosquito source,” she said. “We need people to understand how they can prevent the situation.”
Health officials recommend residents take simple precautions: dump standing water from pet dishes, plant saucers, toys and birdbaths at least weekly; treat ponds or other permanent water features with larvicide; use EPA-registered repellents containing DEET, picaridin or oil of lemon eucalyptus; wear long sleeves and pants during dawn and dusk hours; and report dead birds, especially crows, ravens and California scrub-jays, to the state health department at westnile.ca.gov/report.
Anyone experiencing a severe headache, stiff neck, confusion or muscle weakness — particularly those over 60 — should seek medical attention promptly.
Original source: CalMatters
