California county records most cases of West Nile virus in nearly a decade… as mosquito-borne diseases spread across US
Los Angeles County has reported 100 cases of the mosquito-borne West Nile virus, the most cases the county has recorded in almost 10 years.
Among the 100 cases, the County of Los Angeles Public Health reported 90 neuroinvasive diagnoses – when the virus penetrates the blood-brain barrier and travels to the meninges, causing meningitis and potential paralysis.
Of the 90 cases, 70 are encephalitis – brain swelling – 12 are meningitis – inflammation of the protective membranes surrounding the brain and spinal cord – and eight are acute flaccid paralysis.
Ninety-four people are hospitalized and five people have died.
This is a drastic increase from last year, which saw just 22 cases of West Nile virus and 17 diagnoses of neuroinvasive disease. Two people died from the virus in 2025.
The last time at least 100 cases were reported in the county was in 2017, which saw 268 cases reported over the entire mosquito season, health officials said.
Los Angeles County Health Officer Dr Muntu Davis said: ‘West Nile virus can cause serious illness, and the number of severe cases and deaths we are seeing this year is concerning. We are still within the peak period for West Nile virus transmission, so it is important that everyone take steps now to protect themselves and their families.’
And San Gabriel Valley Mosquito and Vector Control District Manager Jason Farned added: ‘West Nile virus activity is higher than we typically see, and our teams are actively responding with enhanced surveillance, targeted mosquito control treatments, disease investigations, and increased community outreach and education.’
West Nile is fatal in about one out of 10 people who go on to develop nervous system infections, or about one in 1,500 people with West Nile
Cases of mosquito-borne diseases and mosquitoes carrying dangerous pathogens, including West Nile virus and dengue fever, are increasingly spreading across the US – to both new locations and in record numbers.
Warmer temperatures, rapid urban growth and global travel are all helping the mosquitoes – and the disease – expand into places where they have never been a problem before in numbers larger than typical.
While most patients who contract West Nile virus have few or mild symptoms, some may suffer potentially life-threatening brain swelling, along with infections and paralysis.
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.
About one in 150 cases are neuroinvasive, meaning the virus penetrates the blood-brain barrier and travels to the meninges, causing meningitis and potential paralysis.
West Nile is fatal in about one out of 10 people who go on to develop nervous system infections, or about one in 1,500 people with the virus.
CDC data, which typically lags behind state and local data, shows there have been 813 cases of West Nile virus and 607 cases of West Nile virus neuroinvasive disease.
Forty-three states are reporting cases.
Last year, there were 2,167 cases of West Nile virus in total.
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 were 172 deaths recorded 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.
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.
