With a historic Pacific hurricane season and string of summer storms, mosquitos have invaded the Golden State, and West Nile virus rates are higher than they've ever been in the two decades since the disease appeared in California. Many of the cases have progressed with neuroinvasive impacts.
The Centers for Disease Control and Prevention reported 1,328 West Nile virus cases nationwide in its latest data, including 310 in California. The figures are preliminary and can change as health departments report additional cases.
West Nile virus is rarely fatal, but it can cause meningitis or encephalitis, a severe, life-threatening brain inflammation, in a small fraction of cases.
Joseph Vasquez, a 13-year-old boy from Long Beach has been hospitalized this month at Children's Hospital Los Angeles with transverse myelitis, a neurological condition affecting the spinal cord. Doctors believe it is a complication from a West Nile infection. Several California counties have reported an uptick in neurological symptoms linked to West Nile Virus this year.
Infectious disease specialist Torrance Memorial Medical Center, told the Daily Breeze that giving a West Nile virus diagnosis is "devastating."
"This diagnosis for some patients is truly catastrophic," he told the paper. "I have to go and tell someone, 'You're never going to talk to your wife again. You had a conversation Monday, and now it's Friday, and I'm telling you I think it's unlikely you'll ever speak to her again.' It's heartbreaking."
The virus is usually spread by infected mosquitoes, and most people who contract it never develop symptoms. About 20 percent develop a fever and other flu-like symptoms, while fewer than 1 percent develop severe illness affecting the brain or nervous system, according to the CDC. About 10 percent of people who develop severe neuroinvasive disease die.
The warning has become especially urgent in Southern California.
Los Angeles County is also experiencing a major increase. The county had reported 100 West Nile cases and five deaths by Sept. 18, compared with 22 cases and two deaths during all of 2025, according to the Los Angeles Times.
A Patch report also found that nearly all of the Los Angeles County patients reported this year had severe illness affecting the brain or spinal cord, with most requiring hospitalization.
The Los Angeles Times reported that unusually warm and humid conditions have helped mosquitoes thrive in Southern California this year. Experts told the newspaper that higher humidity can allow infected mosquitoes to survive longer and potentially spread the virus to more people.
A separate report from KTLA detailed a Long Beach child hospitalized with a rare disease associated with West Nile virus, highlighting the potential severity of infections even among younger patients.
Orange County health officials this month confirmed the county's first West Nile-related deaths of the year. The victims were older adults, and officials did not release additional information about them.
The county has reported 36 infections this year, though health officials said the actual number is likely considerably higher because most infected people do not develop symptoms and many people with milder illness do not seek medical care.
"This year has been particularly intense, with detections of the virus in humans, mosquitoes, and birds," said Orange County Health Officer Dr. Regina Chinsio-Kwong. "The recent fatalities show how serious and harmful WNV can be."
Officials urged residents to eliminate standing water and take steps to avoid mosquito bites.
Health officials are also tracking Eastern equine encephalitis, or EEE, a rare but deadlier mosquito-borne virus, that affects both humans and horses.
EEE in humans is extremely rare, according to the CDC, but human cases have already been reported this summer in Massachusetts and Michigan.
Also, a horse in Michigan was euthanized after testing positive for EEE, and another equine case was confirmed in Georgia.
West Nile virus is transmitted primarily by Culex mosquitoes. Eastern equine encephalitis (EEE) virus circulates between birds and Culiseta melanura mosquitoes in swamps, spreading to humans and horses via other mosquito species.
2026 has been a big year for mosquitoes and the diseases they carry. By the end of June, the United States had recorded its earliest start to the West Nile virus season and the most cases by that point since 2004.
Weather conditions have been ideal for mosquito populations.
June was unusually wet across parts of the South, Midwest and Great Lakes, with Kansas, Illinois, Indiana and Kentucky recording one of their 10 wettest Junes, according to NOAA. Rain fills storm drains, ditches, birdbaths and other places where mosquitoes lay eggs, and some mosquitoes can develop from egg to adult in seven to 10 days, according to the CDC.
A "heat dome" made conditions even more ideal for mosquitoes. July was the warmest month recorded in the contiguous United States since recordkeeping began in 1895, with every state warmer than its 20th-century average, NOAA reported. Warm weather can speed mosquito development, increase biting, and shorten the time West Nile virus needs to become transmissible inside a mosquito.
Warm weather can extend the season. A recent Stanford University analysis of 239 U.S. cities found that 95 percent now average more days warm enough for Culex mosquitoes to transmit West Nile than they did in the early 1970s.
The mosquito season doesn't wind down until the first hard frost. In California, that's typically anywhere from late November to mid December, according to The Old Farmer's Almanac.
The Los Angeles Times reported that Southern California's unusually humid summer has provided another boost to mosquito populations.
Neither the West Nile nor EEE virus has a human vaccine or antiviral treatment, making prevention the only real defense.
The cheapest and most environmentally friendly mosquito control is to make the back yard a lousy nursery. Weekly, empty and scrub birdbaths, flowerpot saucers, toys, buckets and pet bowls; clear gutters; turn over kiddie pools; and cover rain barrels. Mosquito eggs cling to containers, so dumping the water alone may not work.
Mosquitoes rest in cool, damp vegetation. Mow overgrown areas and thin shrubs, and use a fan on the patio; mosquitoes are weak fliers. Screens provide another chemical-free barrier.
Treat water that cannot be drained or covered with a larvicide. Products containing Bacillus thuringiensis israelensis, or Bti, kill larvae while posing little risk to people, pets and most wildlife when used as directed. The EPA recommends targeting mosquitoes at the larval stage, before they take flight.
If mosquitoes remain thick, the CDC advises using an EPA-registered outdoor insecticide where mosquitoes rest, such as beneath decks, shrubs and patio furniture. Follow the label, avoid wind and rain, and keep chemicals away from people, pets, food and water.
Broad spraying can kill bees, butterflies and other beneficial insects, and some insecticides are toxic to fish. Automatic misting systems have not been studied sufficiently to prove they control mosquitoes or prevent disease, the EPA says.
Bug zappers kill harmless insects but few mosquitoes. Citronella candles and repellent plants offer limited protection. For the best results, remove water, treat unavoidable breeding sites and use insecticides sparingly.
Health officials recommend:
For water that cannot be drained or covered, mosquito-control products containing Bacillus thuringiensis israelensis, or Bti, can target mosquito larvae before they become adults. The EPA recommends controlling mosquitoes at the larval stage when possible.
Mosquito season generally continues through the fall. The CDC says West Nile activity typically occurs during summer and fall, with reported cases often peaking from late August into early September.
Residents who develop symptoms such as high fever, severe headache, neck stiffness, confusion, muscle weakness or other neurological symptoms after mosquito exposure should seek medical attention promptly.
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