California weather, El Niño conditions fuel West Nile virus fears

When temperatures rise above 70 degrees, tiny West Nile virus particles begin to replicate rapidly in the intestines of Culex mosquitoes, which then go in search of their next “blood meal”, injecting virus-laden saliva into their prey as they feast.

This year, California’s warm and humid summer and fall produced the most activity in West Nile in five years. As of November 6, the virus had seriously ill 528 people and killed 32, according to the California Department of Public Health.

Most infected people never feel sick, but about one in five people may develop a fever, headache and body aches. For a few rare cases – 1 in 150 – things become serious: the virus can spread to the brain, causing inflammation, neurological problems and even death. There is no cure, only supportive medical care.

Southern California has been hardest hit by infections, with Los Angeles County accounting for more than half of all reported human cases.

Residents may soon notice a reprieve from mosquitoes, as cooler nights slow both mosquito breeding and virus growth. But experts still worry about the future, as climate change and extreme weather create conditions for a surge in mosquito-borne diseases, particularly along coastlines and in floodplains.

“We’re all afraid of 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 the prediction of an El Niño year, we’re all getting ready.”

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 through the coastal and inland regions of Southern California. Hurricane Marie’s outer tendrils dumped an inch and a half of rain on much of West Los Angeles; Downtown Los Angeles broke a record for precipitation since 1965. In the following days, temperatures reached a high of 104 degrees, with nighttime lows falling no lower than 77 degrees.

Mosquitoes thrived in the following days. Rainfall cleared mosquito repellent treatments from more than 3,000 miles of underground storm drains, according to the Greater Los Angeles County Vector Control District.

“One of our greatest sources of Culex “Mosquitoes are our underground storm drains,” said Susanne Kluh, the district’s executive director. “It’s about 72 degrees there and it’s humid all year round.”

The municipality treats storm sewers monthly, but with larvicide carried away by the sea, mosquito populations increased. At one point this summer, Kluh said more than 70 percent of mosquitoes trapped for sampling carried West Nile virus. This share has fallen in recent weeks to around a third.

Research suggests that climate change will increase the risk of West Nile transmission along the Southern California coast. A UC Berkeley 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 appear in mosquitoes, and when they rose above about 73 degrees, it became much more likely. Higher temperatures were also linked to more human infections.

The coast of Los Angeles is right on this edge. Summer averages are between 60 and 70 degrees, and a few degrees of warming can set things back. This is precisely what happened this year.

“The longer these temperatures remain appropriate, the greater the risk of human cases there may be,” said Tristan Hallum, science director for the San Gabriel Valley Mosquito and Vector Control District.

The virus moves in a complex loop driven by birds, mosquitoes and weather. A female mosquito feeding on an infected bird can ingest the virus and subsequently infect other birds and sometimes humans. But birds that have already been infected and survived become immune to West Nile, and their blood no longer contains viruses that mosquitoes can 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 high cases could get a reprieve next year, but other parts of the state remain vulnerable.

The virus thrives in extreme weather conditions

Federal forecasters estimate there is a greater than 90% chance of a very strong El Niño this winter, increasing the chance of heavy rains, especially in Southern California. That worries public health and vector control officials: If floodwaters persist into spring as temperatures rise, mosquitoes could explode.

It’s already happened. In 2023, heavy rainfall and flooding filled the long-dry Tulare Lake basin in the Central Valley, submerging large swaths of farmland and reviving wetland habitats. The lake persisted for about a year, temporarily changing migration patterns and bird behavior; it also created a favored habitat for the state’s natives Culex mosquitoes.

That year, health authorities recorded 433 human cases of West Nile virus.

Drought is also important, and in a counter-intuitive way. In 2014 and 2015, more than 1,600 people contracted West Nile virus during some of the driest years on record.

“One hypothesis is that dry conditions bring birds and mosquitoes closer together due to the fact that water is more limited in the landscape,” said Chris Barker, a professor at UC Davis who studies mosquito-borne diseases and helps manage the state’s West Nile monitoring program.

Because West Nile depends on a changing mix of birds, mosquitoes and weather, cases won’t increase steadily from year to year as the climate warms, Barker said. Instead, extreme weather conditions – floods and droughts – will create more years where conditions align for outbreaks, making the virus harder to combat.

“Climate change has the effect of tipping the scales a little more against us,” he added.

Experts search for a vaccine

There is no vaccine to protect against West Nile virus. Aaron Brault, a leading West Nile expert and researcher at the Centers for Disease Control and Prevention, said in an email that the United States needs it.

“Although West Nile virus has been a significant cause of illness in the United States for more than 25 years, our overall toolbox is limited,” Brault said.

The CDC only accepted written questions and did not make Brault available for an interview.

Repellents and long sleeves help reduce risk, but people often forgo these measures, Brault said, and these measures “have not adequately reduced the number of cases or deaths linked to West Nile virus disease in the United States.”

The CDC is exploring new ways to fight West Nile and encouraging the development of a vaccine, new treatments and better diagnostic tests, but it’s hard work. West Nile outbreaks are unpredictable, making it difficult to conduct the large clinical studies required by vaccines.

“Viruses sometimes challenge us, especially those who try to predict them,” Barker said.

Climate change is increasing this uncertainty and making it more difficult for people to try to keep mosquito numbers low.

Vector control faces new challenges and limited funding

Vector control districts in California spend millions each year to trap and kill 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 that most Southern California residents were squashed and scratched by this year probably weren’t West Nile ones. Culex mosquito, but an invasive species called Aedes aegypti.

Native Culex Mosquito species bite between dusk and dawn and prefer birds to humans, although they are opportunistic biters. On the other hand, black and white stripes Aedes hunts during the day and its meal of choice is human blood. It does not transmit West Nile virus, but it can spread other infectious diseases such as dengue, zika, chikungunya and yellow fever.

Aedes is also more difficult to control. It reproduces in just a teaspoon of water and its eggs can also survive months of drought and revive as soon as rainy weather returns. Some vector control districts in California have spent hundreds of thousands of dollars to release sterile males. Aedes mosquitoes to reduce the total number of mosquitoes, preventing them from fighting Culex mosquitoes.

Disasters can make the situation worse. The Eaton Fire, which destroyed 9,000 structures in 2025, left behind 1,400 stagnant swimming pools.

“Over time, the budget has been very reduced because of the need to respond to these additional mosquitoes and these additional causes,” said Macedo, of the Mosquito and Vector Control Association of California.

State law limits the extent to which local governments can raise fees for these services, although some vector control districts have turned to voters to ask for more money.

Still, Macedo said, no budget can cover every yard.

“We can’t go in…