LOS ANGELES, CA — California just came through its most active West Nile year in five years, and Los Angeles County took the hardest hit. By Nov. 6, the virus had seriously sickened 528 people statewide and killed 32, according to the California Department of Public Health.
More than half of all reported human cases were in Los Angeles County. Public health and mosquito control officials say cooler nights may bring some temporary relief, but they are already watching the weather closely because next winter could line up for another surge.
The concern is not just the number of mosquitoes, but the conditions that help the virus spread. Warm nights, standing water and shifting bird populations can all push West Nile activity higher, and experts say climate change is making those swings harder to predict.
Warm rain and hot nights helped mosquitoes explode across Southern California
One major outbreak driver this year was a stormy stretch that hit Southern California in September. A warm system dropped about an inch and a half of rain across much of the Westside of Los Angeles, and downtown L.A. set a rainfall record that had stood since 1965.
That was followed by extreme heat. Temperatures climbed to 104 degrees, while overnight lows stayed above 77 degrees. Those conditions are ideal for mosquito growth and for West Nile virus to replicate inside Culex mosquitoes.
Susanne Kluh, general manager of the Greater Los Angeles County Vector Control District, said the rain washed mosquito-killing treatments out of more than 3,000 miles of underground storm drains. The district treats those drains monthly, but the larvicide was carried away before it could do its job.
Storm drains became a major breeding ground after treatments washed away
Underground storm drains are a major source of Culex mosquitoes in the region, Kluh said, because they stay around 72 degrees and humid throughout the year. When the district’s treatments were flushed out, mosquito numbers jumped.
At one point this summer, more than 70% of mosquitoes trapped for sampling carried West Nile virus, Kluh said. That share has since fallen to about one-third, but officials say the summer showed how quickly conditions can turn dangerous when rain and heat arrive in the wrong sequence.
Tristan Hallum, scientific director for the San Gabriel Valley Mosquito and Vector Control District, said the longer temperatures stay in the virus-friendly range, the more chances there are for people to be exposed. Coastal Los Angeles, where summer averages hover in the high 60s to low 70s, sits close to that tipping point.
Public health experts say coastal Southern California sits near a tipping point
Research has linked a small temperature shift to a much higher chance of West Nile transmission along the coast. A UC Berkeley-led study of Los Angeles-area data from 2006 to 2016 found that when average temperatures stayed below about 70 degrees, the virus was much less likely to appear in mosquitoes.
Once temperatures rose above about 73 degrees, detection became much more likely, and higher temperatures were also tied to more human infections. That helps explain why a few degrees of warming can matter so much in places that already sit near the edge of the virus’s preferred range.
The system is driven by birds, mosquitoes and weather working together. Mosquitoes feed on infected birds, then spread the virus to other birds and sometimes to humans. Birds that survive a prior infection can become immune, which can help slow activity for a few years at a time.
El Niño raises fears of floodwater and a new outbreak next spring
Federal forecasters say a very strong El Niño has a greater than 90% chance of forming this winter, which raises the odds of heavy rain in Southern California. Public health and vector control officials worry that if floodwaters linger into spring, mosquito populations could surge again as temperatures rise.
California has seen that pattern before. In 2023, heavy precipitation and flooding filled the long-dry Tulare Lake basin in the Central Valley, submerged farmland and recreated wetland habitat that favored native Culex mosquitoes. Health officials recorded 433 human West Nile cases that year.
Drought can also fuel outbreaks. In 2014 and 2015, more than 1,600 people contracted West Nile during some of the driest years on record. That history has left experts thinking in terms of extremes, not simple wet-versus-dry seasons.
Officials urge basic prevention as mosquito control costs climb
State public health officials say simple steps can lower the risk of infection. They recommend dumping standing water at least once a week, treating ponds or other permanent water with larvicides, and using EPA-registered repellents that include DEET, picaridin or oil of lemon eucalyptus.
People are also advised to wear long sleeves and long pants outdoors between dusk and dawn, and to report dead birds such as ravens, crows and California Scrub-Jays at westnile.ca.gov/report. Anyone with a severe headache, stiff neck, confusion or muscle weakness should seek medical care, especially adults over 60.
There is no cure for West Nile virus, only supportive medical treatment once a person becomes seriously ill. Most infected people never feel sick, but about one in five can develop fever, headache and body aches, and about one in 150 can suffer brain inflammation, neurological problems or death.
Vector districts warn limited budgets cannot cover every backyard
Across California, vector control districts spend millions of dollars each year trapping and killing mosquitoes. Those efforts are funded locally through property taxes, usually about $4 to $10 a year, but officials say the work has become harder as mosquito seasons stretch longer and new pests spread.
Many of the mosquitoes people noticed this year were likely not the West Nile-carrying Culex species. Instead, officials pointed to Aedes aegypti, an invasive day-biting mosquito that prefers human blood and can spread dengue, zika, chikungunya and yellow fever.
Aedes is difficult to eliminate because it can breed in as little as a teaspoon of water, and its eggs can survive months of drought. Some districts have spent hundreds of thousands of dollars releasing sterile male mosquitoes to reduce Aedes populations, pulling money and staff away from Culex control.
Paula Macedo, chair of the Mosquito and Vector Control Association of California, said the strain has been compounded by disasters such as the Eaton fire, which destroyed 9,000 structures in 2025 and left behind 1,400 stagnant swimming pools.