Paralyzing mosquito-borne illness surges in multiple states… as many patients develop severe brain-swelling form of disease
Health officials in the Southwestern US are on high alert amid a surge of a life-threatening mosquito-borne illness.
Mosquitoes in at least 13 cities in Orange County, California – just south of Los Angeles – have tested positive for West Nile virus, according to the Orange County Mosquito and Vector Control District.
The county’s latest surveillance data shows 38 positive mosquito samples, with the largest concentration of 15 samples in Fullerton.
There have also been seven positive samples in Buena Park, three in Anaheim, three in Santa Ana and two in Huntington Beach.
Brea, Costa Mesa, Cypress, Garden Grove, Newport Beach, Seal Beach, Stanton and Westminster have each reported one positive sample.
Additionally, Ventura County – northwest of Los Angeles – reported its first mosquito samples that were positive for West Nile virus. In northern California, Yolo and Sacramento counties have also found West Nile positive mosquitoes.
California has, however, only reported one human case of West Nile virus so far this year, in Los Angeles County. But in neighboring Arizona, there have been 35 human cases so far, 29 of which were in Maricopa County, which includes Phoenix.
California’s Orange County Mosquito and Vector Control District has said it will implement mosquito control treatments in a 4.5-square-mile area of Fullerton from July 9 through July 11 between 1am and 5am.
West Nile virus is caused by a bite from Culex mosquitoes (pictured above) and is most common in Great Plains states
‘We only treat when there is an abundance of mosquitoes and the presence of disease,’ district officials told KTLA. ‘Then that triggers us to go out with our trucks and knock down the adults [mosquitoes].’
One Fullerton resident told the outlet that the mosquitoes in the area have ‘been getting pretty bad.’
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‘I don’t go out anymore because you go out and then just mosquitoes, like, right in your face,’ they added.
The Sacramento-Yolo Mosquito and Vector Control District said in a press release it will continue to monitor the virus through ‘widespread mosquito trapping and testing’ according to the press release, and the department will also adapt ground treatments in virus-positive areas.
California has reported a total of 261 West Nile-positive mosquito samples in 2026, along with 133 in dead birds and one in a horse, according to the state’s case dashboard.
West Nile virus is caused by a bite from Culex mosquitoes and is most common in Great Plains states like North Dakota, South Dakota, Nebraska and Colorado, though cities like Los Angeles, Chicago and Dallas-Fort Worth also record high levels due to large populations.
Many of these states have also extended mosquito season – which runs from late spring through early fall – due to the increases in mosquito prevalence later in the year.
The disease usually causes no symptoms, but in one in five patients, it can lead to fever, headaches, nausea, diarrhea, joint pain and pain behind the eyes.
And one percent of cases are neuroinvasive, meaning the virus penetrates the blood-brain barrier and travels to the meninges, causing meningitis and potential paralysis.
CDC data shows 56 human cases of West Nile virus so far this year in 15 states, 44 of which were neuroinvasive. Last year there were a total of 2,100, up from 1,800 the year before.
Arizona has reported the most human cases of West Nile virus cases this year so far with 35, according to CDC data
It’s unclear how many of last year’s cases were neuroinvasive, but CDC data shows there were 1,342 hospitalizations for neuroinvasive disease.
There have been no deaths recorded this year and 172 last year.
Older adults, people with weakened immune systems, and those with certain chronic conditions face the highest risk of severe illness due to West Nile.
Even survivors of severe West Nile illness may experience long-lasting complications, including memory problems, chronic fatigue, muscle tremors, or permanent neurological damage.
West Nile virus can be prevented primarily by avoiding mosquito bites.
CDC officials recommend using insect repellents that contain the ingredient DEET, wearing long sleeves and pants outside and staying inside between dusk and dawn during the summer.
