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When temperatures rise above 70 degrees, tiny West Nile viral particles begin to rapidly replicate in the gut of Culex mosquitoes, which then go hunting for their next “blood meal,” injecting virus-laden saliva into their prey when they feast.
This year, California’s warm, wet summer and fall produced the most West Nile activity in five years. As of Nov. 6, the virus has seriously sickened 528 people and killed 32, according to the California Department of Public Health.
Most people who are infected never feel sick, but about 1 in 5 may develop fever, headache and body aches. For the rare few — 1 in 150 — things get serious: The virus can hitch a ride to the brain, causing inflammation, neurological problems and even death. There is no cure, only supportive medical care.
Southern California bore the brunt of infections, with Los Angeles County accounting for more than half of all reported human cases.
Residents there may soon notice a reprieve from mosquitoes, as cooler nights slow both mosquito breeding and growth of the virus. But experts still worry about what comes next, as climate change and extreme weather create conditions for mosquito-borne illness to surge, particularly along the coast and in floodplains.
“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.”
Whether next year brings relief or another surge depends on a tangle of birds, mosquitoes and weather that even experts struggle to predict.
Coastal California at risk of increased disease
In September, a warm storm swept past Southern California’s coastal and inland regions. The outer tendrils of Hurricane Marie poured an inch-and-a-half of rain across much of the westside of Los Angeles; downtown L.A. broke a rainfall record from 1965. In the following days, temperatures reached a high of 104 degrees, with overnight lows failing to dip below 77 degrees.
Mosquitoes thrived in the following days. The rainfall flushed mosquito-killing treatments out of more than 3,000 miles of underground storm drains, 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.”
The district treats storm drains monthly, but with the larvicide washed out to sea, mosquito populations surged. At one point this summer, Kluh said more than 70% of mosquitoes trapped for sampling carried West Nile virus. That share has fallen in recent weeks to about one-third.
Research suggests that climate change will increase risk of West Nile transmission along Southern California’s coast. A UC Berkeley-led study of Los Angeles-area data from 2006 to 2016 found a narrow tipping point: when average temperatures stayed below about 70 degrees, the virus was much less likely to turn up in mosquitoes, and when they rose above about 73, it became much more likely. Higher temperatures were also linked to more human infections.
Coastal Los Angeles sits right at that edge. Summer averages hover in the high 60s to low 70s, and a few degrees of warming can push it over. That’s precisely what happened this year.
“The longer that those temperatures are appropriate, the more opportunity there can be for human cases,” said Tristan Hallum, scientific director for San Gabriel Valley Mosquito and Vector Control District.
The virus moves in a complex loop driven by birds, mosquitoes and weather conditions. A female mosquito feeding on an infected bird can ingest the virus and subsequently infect other birds and sometimes humans. But birds that have been previously infected and survived become immune to West Nile, and their blood no longer carries virus for feasting mosquitoes to consume.
That immunity tends to produce a natural three- to five-year cycle of West Nile activity, meaning Los Angeles, Orange County and other areas with many cases may get a reprieve next year, but other parts of the state remain vulnerable.
Virus flourishes during extreme weather
Federal forecasters give a very strong El Niño a greater than 90% chance this winter, raising the odds of heavy rain, particularly in Southern California. That worries public health and vector control officials: if floodwaters linger into spring as temperatures rise, mosquitoes could boom.
That’s happened before. In 2023, heavy precipitation and flooding filled the long-dry Tulare Lake basin in the Central Valley, submerging vast stretches of agricultural land and reviving wetland habitats. The lake persisted for roughly a year, temporarily changing bird migration patterns and behavior; it also created prime habitat for the state’s native Culex mosquitoes.
That year health officials recorded 433 human cases of West Nile.
Drought matters too, and in a counterintuitive way. In 2014 and 2015, more than 1,600 people contracted West Nile during some of the driest years on record.
“One hypothesis is that the dry conditions bring birds and mosquitoes into closer contact due to the water being more limited across the landscape,” said Chris Barker, a professor at UC Davis who studies mosquito-borne illnesses and helps run the state’s West Nile surveillance program.
Because West Nile depends on a shifting mix of birds, mosquitoes and weather, cases won’t climb steadily year after year as climate warms, Barker said. Instead, extreme weather — both floods and drought — will create more years when conditions line up for outbreaks, making the virus harder to fight.
“Climate change has the effect of tipping the scales against us just a little bit more,” he added.
Experts seek vaccine
There is no vaccine to protect against West Nile virus. Aaron Brault, a top West Nile expert and researcher with the Centers for Disease Control and Prevention, said in an email that the U.S. needs one.
“Despite West Nile virus being an important cause of illness in the United States for more than 25 years, our overall toolbox is limited,” Brault said.
The CDC accepted only written questions and would not make Brault available for an interview.
Repellent and long sleeves help to decrease risk, but people frequently forgo these measures, Brault said, and those measures “have not adequately reduced the number of West Nile virus disease cases or deaths in the United States.”
The CDC is exploring new ways to combat West Nile and encourage development of a vaccine, new treatments and better diagnostic tests, but it’s difficult work. West Nile outbreaks are unpredictable, making it tough to run the large clinical studies vaccines require.
“Viruses do defy us sometimes, especially the people trying to predict them,” Barker said.
Climate change supercharges that uncertainty and makes it harder for the people trying to keep mosquito numbers low.
Vector control faces new challenges, limited funding
Vector control districts across California spend millions each year trapping and killing mosquitoes. Residents fund the work locally through property taxes, typically $4 to $10 a year, according to the Mosquito and Vector Control Association of California. New pests and longer mosquito seasons challenge their work.
The mosquitoes most Southern California residents swatted and scratched at this year were likely not the West Nile bearing Culex mosquito, but an invasive species called Aedes aegypti.
Native Culex mosquito species bite between dusk and dawn and prefer birds to humans, though they’re opportunistic biters. In contrast, the black-and-white striped Aedes hunts during the day, and its meal of choice is human blood. It doesn’t transmit West Nile Virus, but it can spread other infectious diseases such as dengue, zika, chikungunya and yellow fever.
Aedes is also harder to control. It breeds in as little as a teaspoon of water, and its eggs can also survive months of drought, reviving whenever wet weather returns. Some vector control districts in California have spent hundreds of thousands of dollars releasing sterile male Aedes mosquitoes to reduce the overall number of mosquitoes, taking away from the ability to fight Culex mosquitoes.
Disasters can add to the strain. The Eaton fire, which destroyed 9,000 structures in 2025, left behind 1,400 stagnant swimming pools.
“Budget over time has been stretched really thin by having to respond to these additional mosquitoes and additional causes,” Macedo, with the Mosquito and Vector Control Association of California, said.
State law limits how much local governments can raise fees for these services, though some vector control districts have gone to voters to ask for more money.
Still, Macedo said, no budget can cover 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.”
Supported by the California Health Care Foundation (CHCF), which works to ensure that people have access to the care they need, when they need it, at a price they can afford. Visit www.chcf.org to learn more.
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