California had confirmed 147 human West Nile virus cases across 21 counties by September 18, 2026, according to the state’s West Nile surveillance website, already more than the 117 cases the state counted in all of 2025. The state reported 10 deaths from the virus over the same period, one fewer than the 11 recorded last year, so the case count has passed last year’s while the death toll has not yet.
The figures are a snapshot, and the state posts them weekly. Newer county reports show the picture is still moving, which means the September 18 numbers are best read as a floor for how far the season has already run.
The state’s count as of September 18
The California Department of Public Health’s West Nile virus site listed 147 human cases in 21 counties as of September 18, 2026, along with 10 deaths, 623 positive dead birds from 24 counties, 3,544 positive mosquito samples from 29 counties, 14 horses and 141 positive sentinel chickens. The page said the state added 25 human cases, 48 positive dead birds and 120 positive mosquito samples in the most recent week alone. Los Angeles County led with 44 cases, followed by Fresno County with 16. The department summarized the year in one sentence on that page: “CDPH is seeing the highest levels of West Nile virus activity in five years.”
The comparison year matters. Medical Daily’s report on Glenn County’s second death put 2025’s full-year total at 117 human cases and 11 deaths and said 2026 had already passed it. The state’s own arbovirus surveillance bulletin dated November 21, 2025 listed 102 cases and 9 deaths at that point in the season, so whichever 2025 figure is used, the 147 cases reported by mid-September 2026 exceed it.
Thirty cases in mid-August, 147 a month later
The climb was quick. Medical Daily reported that by August 14 the state had 30 cases and 3 deaths, and that by September 4 it had 86 cases and 6 deaths, including 22 new infections in a single reporting week. Five counties, Madera, Merced, San Bernardino, Stanislaus and Sutter, reported their first cases in that week’s update. Two weeks later the count stood at 147.
“August and September are peak periods of West Nile virus transmission in the state,” Dr. Erica Pan said in the health department’s early September update, urging residents to protect themselves from bites. The pace of reporting suggests the peak has not passed: the state’s own dead-bird and mosquito tallies were still rising by the week when the 147-case figure was published.
Ten deaths, all in Central Valley counties
Medical Daily’s tally of the 10 state-listed deaths puts all of them in Central Valley counties: Butte with three, Glenn, Sacramento and Yolo with two each, and Kings with one. Glenn County’s second death was a resident aged 70 to 80, the outlet reported. Older adults are at the highest risk of severe neuroinvasive disease, along with people who have diabetes, kidney disease, high blood pressure or weakened immune systems, according to county health officials cited in the same reports.
The state list is not the whole story. Los Angeles County announced an August death that does not appear on the state’s official list, and the county’s own arbovirus report number 8, dated September 25, counted 135 human infections and 5 deaths within its borders, 130 of the infections symptomatic. The report added that the number of people infected this season is “greater than the 5-year average during the same period of time.”
A Stanislaus death and the gaps in the state list
After the state’s September 18 update, Stanislaus County announced its first West Nile death of the season on September 25, a resident under 50 who developed the neuroinvasive form. CBS Sacramento reported the county had 11 human cases as of September 22 and quoted County Public Health Officer Dr. Thea Papasozomenos: “West Nile virus can cause serious illness in people of any age.” That death was not part of the state’s 10 as of September 18.
Because of those lags, the deaths comparison with 2025 is unsettled. The state’s listed 10 sits one below last year’s 11, but a Stanislaus death and Los Angeles County’s own count both fall outside that figure, so the next weekly update could close or reverse the gap. The cases comparison is not in doubt, since the 147 already exceeds either 2025 total and can only rise.
Roughly 80 percent of infections cause no symptoms, Glenn County noted. The number to watch in the state’s next posting is whether the death count reaches 11 or more, the point at which 2026 would also pass last year on deaths.
Prevention advice from the state and county health departments has not changed: use an EPA-registered repellent containing DEET or picaridin, wear long sleeves and pants at dawn and dusk when mosquitoes are most active, and drain standing water around homes. Residents can report dead birds to the state’s hotline at 1-877-968-2473, one of the surveillance inputs behind the 623 positive birds counted through September 18.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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