The Centers for Disease Control and Prevention has now counted 504 human West Nile virus cases across 41 states this year, and 363 of them, about 72 percent, are the neuroinvasive form in which the virus reaches the brain or spinal cord.
Those figures come from the CDC's ArboNET surveillance system, current through Sept. 8 and compiled in a survey report from the Annenberg Public Policy Center at the University of Pennsylvania. The agency refreshed its current-year West Nile dashboard again on Sept. 15. The severity share is higher than at the same point last year, when 490 of 771 reported cases, roughly 64 percent, were neuroinvasive.
The second number in that comparison is the one that changes the shape of this season's story. After an opening stretch the CDC described as the earliest start to a West Nile season on record since 2004, the national case count is now running well below where it stood at this point in 2025. The year that looked like it might break records on volume has instead become a season defined by how severe the reported illnesses are.
For households, that distinction has a practical edge. A smaller national total does not lower the risk for the people most likely to end up hospitalized, and the calendar window in which they are exposed is still open.
Reading the 72 Percent Correctly
The severity share reflects who gets tested as much as how the virus is behaving. The CDC states plainly in its data notes that mild, non-neuroinvasive illness is more likely to go unreported than severe disease, and that non-neuroinvasive counts should not be used to compare activity between places or over time.
Most people infected with West Nile never develop symptoms at all. About one in five develop a fever and other symptoms, and less than one percent develop severe neurologic disease. The 363 neuroinvasive cases therefore describe the tip of a much larger and largely uncounted pool of infections, not a change in the virus itself.
The CDC also counts cases by county of residence rather than by where a bite occurred, and its figures are preliminary. Because reporting lags illness by weeks, totals will keep climbing into October even after mosquito activity fades, which is why any single week's number should be treated as provisional.
In its Independence Day mosquito prevention statement, issued in early July, the CDC reported at least 48 cases as of June 30, including 38 neuroinvasive, against an average of about 10 cases by the end of June since 2004. CDC medical epidemiologist Dr. Erin Staples said at the time that "these findings serve as an important reminder that mosquito season is well underway." That framing has held up better than the record-year projections that followed it.
California Sets a Five-Year High as Cases Cluster
State dashboards often run ahead of the federal count and show where the burden actually sits. California's West Nile tracking site listed 147 human cases across 21 counties and 10 deaths as of Sept. 18, with Los Angeles County reporting 44 cases, Fresno 16, Butte 14, Sacramento 11 and Orange 10. Twenty-five new cases were added in a single reporting week.
The California Department of Public Health described the state in August as seeing its highest West Nile activity in five years, when 30 cases and three deaths had been reported across 13 counties as of Aug. 14. "West Nile virus can be fatal, particularly for the elderly and immunocompromised," said CDPH Director and State Health Officer Dr. Erica Pan, who urged residents to take precautions during the August and September peak.
Environmental surveillance supports that assessment. As of Sept. 18, 3,544 mosquito samples from 29 California counties had tested positive this year, along with 623 dead birds across 24 counties.
In the Midwest, Chicago recorded its first West Nile death of 2026 on Sept. 1 alongside the city's first two human cases, a development MedicalDaily covered in its report on Chicago's first West Nile death this year. City officials pointed to an unusually rainy summer and said they have treated more than 80,000 catch basins with larvicide and sprayed insecticide in 15 community areas.
Older Adults Still Carry the Late Season Risk
Risk is concentrated, not universal. The CDC identifies adults 60 and older and people with conditions including cancer, diabetes, high blood pressure, kidney disease or an organ transplant as facing the highest risk of severe disease. People taking immunosuppressive medications also face elevated risk. Outdoor workers, evening exercisers, and residents near standing water or neglected pools simply have more exposure.
Symptoms typically appear within two weeks of a bite and can include fever, headache, body aches, joint pain, vomiting, diarrhea, or rash. Severe headache, stiff neck, confusion, vision loss, sudden muscle weakness, or seizures after mosquito exposure need emergency evaluation. There is no human vaccine and no specific antiviral, so care for severe disease is supportive, and neuroinvasive illness often requires hospitalization.
Knowing the advice is not the same as following it. The Annenberg survey of 1,904 adults, fielded Aug. 4 to 17, found 82 percent understood that preventing bites is the best defense, but only 31 percent knew to apply sunscreen first and let it dry before repellent, and just 18 percent knew to look for an EPA-registered product. "Taking precautions to prevent mosquito bites and control mosquitoes is important," said Ken Winneg, the center's managing director of survey research.
CDC guidance points to repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol or 2-undecanone, and notes that DEET concentrations above 50 percent generally do not add protection. Repellent should not be used on infants younger than two months, so cover carriers with netting instead. Emptying and scrubbing anything that holds water once a week removes breeding sites. Neglected pools belong to local mosquito and vector control agencies, which take reports.
Canceling outdoor plans is not warranted for most people. Shifting walks and yard work away from dawn and dusk is a reasonable adjustment for older adults.
The CDC updates ArboNET every one to two weeks through December, and California posts weekly. What remains unresolved is how many additional deaths this season's high severe share will ultimately produce.
Key Questions Answered
How many West Nile cases has the U.S. reported in 2026? The CDC counted 504 cases across 41 states as of Sept. 8, including 363 neuroinvasive cases. The data are preliminary.
Is this a record year? Not by case count. The season started earlier than any year since 2004, but the Sept. 8 total was below the 771 cases reported at the same point in 2025.
Why is the severe share so high? Severe cases are more likely to be tested and reported than mild ones. The 72 percent figure describes reported cases, not all infections.
Who faces the greatest risk? Adults 60 and older, people with cancer, diabetes, high blood pressure, kidney disease or a transplant, and people on immunosuppressive medication.
Which symptoms require emergency care? Severe headache, stiff neck, confusion, vision loss, sudden muscle weakness, or seizures following mosquito exposure.
Is there a vaccine or treatment? No human vaccine exists, and there is no specific antiviral. Treatment for severe illness is supportive.
When does the season end? Transmission typically drops after sustained cold weather, but reported case counts keep rising for weeks because of reporting delays.
