Los Angeles just logged its worst mosquito season in a decade, and public health officials say an incoming El Niño winter could set the stage for an even rougher year ahead.
When temperatures climb above 70 degrees, the West Nile virus multiplies rapidly inside the gut of Culex mosquitoes, which then go searching for their next blood meal — passing virus-laden saliva into birds and people along the way.
This year, a hot, humid summer and fall across California fueled the most intense West Nile activity the state has seen in five years. As of Nov. 6, the virus had sickened 528 people seriously enough to require medical attention and killed 32, according to the California Department of Public Health.
Most people who become infected never show symptoms. But roughly one in five develops fever, headaches and body aches, and in rare cases — about one in 150 — the virus reaches the brain, triggering swelling, neurological damage and sometimes death. There is no cure, only supportive medical care.
Southern California bore the brunt of this year’s outbreak, with Los Angeles County alone accounting for more than half of all reported human cases statewide.
Residents may soon notice fewer mosquitoes as cooler nights slow both reproduction and viral spread. But experts remain uneasy about what comes next, since climate change and increasingly extreme weather are creating conditions ripe for more mosquito-borne illness, particularly along the coast and in flood-prone areas.
“Everybody’s scared about next year,” said Paula Macedo, president of the Mosquito and Vector Control Association of California. “With the amount of activity we’ve already seen this year, and forecasts pointing to an El Niño year, we’re all bracing ourselves.”
Whether next year brings relief or another surge depends on an intricate interplay of birds, mosquitoes and weather patterns that even scientists struggle to forecast with confidence.
Coastal California at heightened risk
In September, a warm storm swept through both coastal and inland Southern California. Remnants of Hurricane Marie dumped an inch and a half of rain across much of the west side of Los Angeles, and downtown L.A. broke a rainfall record that had stood since 1965. In the days that followed, daytime highs reached 104 degrees, with overnight lows staying above 77.
Mosquitoes thrived in the aftermath. According to the Los Angeles County Vector Control District, the rain washed mosquito treatments out of more than 3,000 miles of underground storm drains.
“One of our major sources of Culex mosquitoes is our underground storm drains,” said Susanne Kluh, the district’s general manager. “Down there, it stays around 72 degrees, with moisture year-round.”
The district treats those drains monthly with larvicide, but once it gets flushed away, mosquito populations can explode. At one point this summer, Kluh said more than 70% of mosquitoes collected for testing were carrying West Nile virus. That rate has since dropped to roughly a third.
Research suggests climate change will heighten the risk of West Nile transmission along the Southern California coast. A study led by UC Berkeley researchers, drawing on Los Angeles-area data from 2006 to 2016, identified a clear tipping point: when average temperatures stayed below about 70 degrees, the odds of mosquitoes picking up the virus were much lower, but once temperatures climbed past roughly 73 degrees, transmission rates jumped sharply. Warmer temperatures were also linked to more human infections.
Coastal Los Angeles sits right on that threshold. Average summer temperatures typically hover in the 60s and low 70s, meaning just a few extra degrees can push conditions into dangerous territory — exactly what happened this year.
“The longer those optimal temperatures stick around, the higher the chances of seeing human cases,” said Tristan Hallum, scientific director of the San Gabriel Valley Mosquito and Vector Control District.
The virus moves through a complicated cycle driven by birds, mosquitoes and weather. A mosquito that bites an infected bird can pick up the virus and later pass it to other birds — and occasionally to people. But birds that survive a previous infection typically build immunity, meaning their blood no longer carries virus for mosquitoes to pick up.
That immunity tends to drive a natural three-to-five-year cycle of West Nile activity, which means Los Angeles, Orange County and other hard-hit areas might get something of a breather next year — even as other parts of the state remain vulnerable.
Extreme weather fuels the virus
Federal forecasters are putting the odds of a strong El Niño winter above 90%, raising the likelihood of heavy rain, especially across Southern California. That has public health and vector control officials on alert: if flooding lingers into spring as temperatures rise, mosquito populations could surge.
It’s happened before. In 2023, torrential rain and flooding refilled the long-dry Tulare Lake basin in the Central Valley, submerging vast stretches of farmland and temporarily reviving wetlands. The lake persisted for about a year, disrupting migratory bird patterns and creating prime habitat for native Culex mosquitoes.
That year, health officials recorded 433 human cases of West Nile virus.
Drought matters too, in a counterintuitive way. In 2014 and 2015 — among the driest years on record — more than 1,600 people contracted West Nile virus in California.
“One hypothesis is that dry conditions push birds and mosquitoes into closer contact around shrinking water sources,” said Chris Barker, a UC Davis professor who studies mosquito-borne diseases and helps lead the state’s West Nile virus surveillance program.
Because West Nile depends on such a shifting mix of birds, mosquitoes and weather, Barker said cases won’t necessarily climb steadily as the climate warms. Instead, extremes in both directions — flooding and drought — are likely to produce more years with conditions favorable to outbreaks, making the virus harder to predict and control.
“Climate change has the effect of tilting the scales a little further against us,” he said.
Scientists push for a vaccine
No vaccine currently exists to protect against West Nile virus. Aaron Brault, a leading West Nile researcher with the Centers for Disease Control and Prevention, said in an email that the country needs one.
“Despite West Nile virus being a significant cause of illness in the U.S. for more than 25 years, our toolkit remains limited,” Brault wrote. The CDC only accepted written questions and did not make Brault available for an interview.
Repellents and long sleeves help reduce risk, Brault said, but people often don’t follow through on those precautions, and such measures “have not adequately reduced the number of West Nile virus cases or deaths in the United States.”
The CDC is exploring new strategies and encouraging development of a vaccine, better treatments and improved diagnostic tests, but the work is slow going. West Nile outbreaks are notoriously unpredictable, making it difficult to run the large-scale clinical trials vaccines require.
“Viruses sometimes outsmart us, especially the people trying to predict them,” Barker said.
Climate change only deepens that uncertainty, complicating efforts to keep mosquito populations in check.
Vector control grapples with funding limits
California’s vector control districts spend millions each year trapping and eliminating mosquitoes, work funded locally through property assessments that typically run $4 to $10 annually, according to the Mosquito and Vector Control Association of California. But new invasive pests and longer mosquito seasons are straining those budgets.
The mosquitoes most likely to have bitten Southern California residents this year probably weren’t the West Nile-carrying Culex mosquito at all, but an invasive species called Aedes aegypti.
Native Culex mosquitoes bite between dusk and dawn and generally prefer birds over humans, though they’ll bite people occasionally. The black-and-white striped Aedes mosquito, by contrast, hunts during the day and feeds almost exclusively on human blood. It doesn’t spread West Nile virus, but it can transmit other diseases including dengue, Zika, chikungunya and yellow fever.
Aedes mosquitoes are also notoriously harder to control. They can breed in as little as a teaspoon of water, and their eggs can survive months of drought, hatching as soon as rain returns. Some California vector control districts have spent hundreds of thousands of dollars releasing sterile male Aedes mosquitoes to shrink the overall population — work that pulls resources away from fighting Culex mosquitoes.
Disasters can make matters worse. The Eaton Fire, which destroyed 9,000 structures in 2025, left behind roughly 1,400 pools filled with stagnant water.
“Over time, budgets have been stretched thin by the need to respond to these additional mosquito threats, among other factors,” Macedo said.
State law limits how much local governments can charge for these services, though some vector control districts have asked voters to approve additional funding.
Even so, Macedo said no budget can cover every backyard.
“We can’t get onto every property. We can’t treat every mosquito breeding site,” she said. “We need people to understand how they can help prevent this themselves.”
This program is supported by the California Health Care Foundation, which works to ensure people have access to the health care they need, when they need it, at a price they can afford. Visit www.chcf.org for more information.
Original source: CalMatters
