As another heatwave overtakes Southern California, the presence of West Nile Virus has reached record levels.
While West Nile Virus has always been present in the state, this year’s numbers have far exceeded the five-year average in many regions, from the San Fernando Valley to Long Beach to Orange County to the Inland Empire.
West Nile Virus originates in wild birds and is transmitted through mosquitoes. So, if a mosquito bites a bird who has West Nile Virus, the mosquito will then spread the disease to the next warm-blooded creature – often humans.
In California, West Nile Virus is endemic, meaning that for the past 23 years, at least one human has contracted the virus each year. Since the first detection in 2003, over 8,000 people have contracted the virus, with over 400 deaths.
According to the CDC, around 80% of people who contract West Nile Virus will be asymptomatic. The severity of symptoms of the remaining 20% can vary, with some having flu-like symptoms and others developing a severe illness, leading to meningitis or encephalitis, causing swelling of the brain and spinal cord. This can lead to brain damage, lifelong neurological delays, paralysis and even death.
Since many people do not develop symptoms, West Nile Virus itself is not necessarily rare, but for the few who develop the neuroinvasive illness, it is deadly. There is no vaccine or treatment for the virus, only hospitalization to administer fluids and pain medication.
Because of how dangerous the virus is, Southern California has taken a proactive approach to detecting and eliminating the presence of the disease, mostly by targeting female culex mosquitoes – the type of mosquito that contracts West Nile Virus – from around May to November, when the mosquitoes are the most active.
This year, however, vector control agencies have upped the ante as they try to keep up with the jump in dead birds, mosquitoes and humans carrying West Nile Virus. Agencies from around the region are warning residents to be vigilant to protect themselves.
“The California Department of Public Health (CDPH) is seeing the highest levels of West Nile virus (WNV) activity in five years,” the CDPH said in a Thursday, Aug. 20, press release.
As of Friday, Aug. 21, there have been 11 human cases in the Southern California region and one fatality.
According to the Los Angeles County Department of Public Health, the person who died was a San Fernando Valley resident who was hospitalized with a neurological illness caused by West Nile Virus. At the time of publication, no other information about the individual had been released.
“This tragic loss is a reminder of the serious threat posed by mosquito-borne diseases, including West Nile virus and dengue,” said Muntu Davis, Los Angeles County health officer.
San Fernando Valley has long had one of the highest rates of West Nile virus in the area. In fact, Los Angeles County has the third-highest West Nile virus presence of all California counties, with 294 mosquito samples testing positive for West Nile virus so far this year. This is over double the amount for the entire 2025 season.
“After experiencing the warmest winter on record, West Nile virus amplification and consequently detection in Los Angeles County started almost two months early this year, leading to widespread activity countywide,” said Susanne Kluh, general manager of the Greater Los Angeles County Vector Control District.
There are three ways vector control agencies test for West Nile virus: collecting and testing dead birds, taking blood samples from sentinel chickens and testing mosquito samples. Only a handful of agencies have a sentinel chicken program, but all agencies will collect dead birds if reported and test mosquito samples.
According to Kluh, testing mosquitoes is the best way to determine which areas need to be treated in order to protect humans since a mosquito bite is the only way a human can contract West Nile virus.
To do this, agencies will set mosquito traps throughout their district and collect the trapped mosquitoes as often as daily during the busy season. The mosquitoes are then sorted based on type, isolating female culex mosquitoes. That day’s “pool” of female culex mosquitoes will then be ground up and tested for West Nile virus. If it comes back positive, the agency counts that as one positive sample of West Nile virus and will treat the area.
While high levels of West Nile virus can be expected in the San Fernando Valley, Kluh has seen high levels in the coastal region, which is very unusual, she said.
“It’s just really unusual for these coastal cities to have early activity and activity this high,” Kluh said, “because most years they really don’t experience a lot of West Nile virus down there because the temperatures stay cool longer, the nights are cooler, but this has been our warmest winter on record and it has gone all the way to the coast.”
Kluh said that when it comes to the coastal areas, they are seeing the most West Nile virus activity in Long Beach, Carson, Torrance and Venice.
Other regions of Southern California that have seen an increase in West Nile virus include Orange County, the Antelope Valley and San Bernardino County.
So far this year, Orange County has the most West Nile virus – positive mosquito samples of all California counties at 476. This is a massive increase, according to Orange County Mosquito and Vector Control District public information officer Brian Brannon, because for the entire year of 2025, Orange County had 186 positive mosquito samples.
“We are currently seeing a large abundance of West Nile virus mosquito activity throughout the northwestern area of Orange County,” Brannon said, noting that hotspots at this time are Anaheim with 89, Buena Park with 41 and Cypress with 31. “We did a recent treatment for mosquitoes and numbers went down for a week and then the next week they came back from approximately 5,000 in the week that we sprayed, to 7,000 two weeks later.”
There are multiple ways vector control agencies treat the areas where West Nile virus-positive mosquitoes are found. First of all, if there are adult female culex mosquitoes, agencies will kill the mosquitoes with an insecticide. However, according to John Franklin, general manager of the Compton Creek Mosquito Abatement District, the best way to protect humans from West Nile virus is to use larvicide, killing the mosquito larvae while they are still in the water.
Mosquitoes breed and lay their eggs in warm pools of standing water, so agencies will use the larvicide in areas where they know that exists, Franklin said. In Compton, that would be the creek and flood channels.
Other culprits, however, can be found in someone’s backyard.
“Unmaintained swimming pools remain one of the main breeding sources of West Nile virus mosquitoes,” reads an Aug. 6 Antelope Valley Mosquito and Vector Control District press release. “Hot weather allows mosquitoes to breed quickly. Stagnant water turns green more quickly and mosquitoes can go from egg to adult in just five days.”
Brenna Bates-Grubb, community outreach specialist with the Antelope Valley Mosquito and Vector Control District, said that they rely heavily on residents in the region to maintain their own properties and remove standing water.
“Even the tiniest little source can harbor hundreds of larvae and create an issue,” Bates-Grubb said.
Aaron Arugay, executive director of the Los Angeles County West Vector Control District, emphasized this sentiment.
“The smallest amount of water can have mosquito breeding,” Arugay said. “Just the tiny bit of water in the saucer under a plant – that’s enough to breed mosquitoes.”
Arugay also warned residents to make sure all doors and windows have screens if they are open and that they are fitting properly. And, vector agencies throughout all of Southern California recommend that residents wear mosquito repellent or long sleeves and pants between dusk and dawn.
According to Franklin, it is also recommended that bushes and grass be maintained as that is where culex mosquitoes hide during the day.
Another West Nile virus abatement technique being used in Orange County is to release sterilized males throughout the county with the idea that they will outcompete the non-sterilized males, preventing further breeding.
While West Nile virus is present in the areas of Compton Creek, Riverside County and the San Gabriel Valley, they have not seen as dramatic of an increase as other parts of Southern California. Their respective vector control agencies, however, still advise residents to be aware and protect themselves.
“There’s West Nile virus in your vicinity,” Brannon said. “It’s getting pretty crazy out there and we do everything that we can to take care of it, but we need people’s help. Our slogan this season is ‘together we fight.’ In a world that’s slightly disjointed, hopefully everyone can come together and agree that mosquitoes suck.”
All vector control agencies encourage residents to report dead birds, unmaintained swimming pools and any mosquito problems in their area. California residents can track the current West Nile virus presence in their area and find their regional vector control agency at westnile.ca.gov.