West Nile virus cases top 400 nationwide as Arizona outbreak drives an early, active season

The national tally of human West Nile virus infections has climbed past 400 for the year, according to federal surveillance data, with the disease's traditional peak weeks of late summer still ahead. The Centers for Disease Control and Prevention's tracking dashboard, current as of September 1, shows 407 reported cases across 40 states, of which 285 involved the virus's most severe, neuroinvasive form.

Arizona has recorded more cases than any other state, driven almost entirely by an outbreak in metro Phoenix that began unusually early in the year. California and Texas follow, and health officials in several states say the coming weeks, when Culex mosquito populations and virus transmission typically peak, could push the season's totals substantially higher.

The numbers

CDC's ArboNET surveillance system, which compiles case reports from state and local health departments, lists Arizona with 88 human cases as of the latest update, ahead of California's 66 and Texas's 46. North Dakota, Nebraska and Utah each report totals in the high teens despite far smaller populations, reflecting the patchy, weather-dependent nature of mosquito-borne outbreaks. Of the 407 cases logged nationally, 285, or about 70 percent, are classified as neuroinvasive disease, meaning the virus reached the brain or spinal cord and caused conditions such as encephalitis, meningitis or acute flaccid paralysis.

That proportion sounds alarming on its own, but it reflects how West Nile virus is actually detected rather than how dangerous a typical infection is. According to CDC's own guidance on symptoms and severity, roughly 80 percent of people infected with the virus never develop any symptoms at all and are never tested or counted. About one in five develop a milder, flu-like illness with fever, headache, body aches and sometimes a rash, and many of those cases also go undiagnosed. Fewer than 1 percent of all infections progress to neuroinvasive disease. Because mild and asymptomatic infections are rarely captured by surveillance, the cases that do get reported to health departments skew heavily toward the severe end of the spectrum. Among people who do develop neuroinvasive disease, CDC estimates about 10 percent die, and survivors frequently report lasting fatigue, weakness or cognitive effects.

By comparison, the record U.S. season remains 2003, when CDC recorded 9,862 human cases nationwide, the tail end of the virus's initial spread across the country after it was first detected in New York in 1999. Case totals have fluctuated since, generally landing between roughly 1,000 and 3,000 a year over the past decade, with 2025 finishing at 2,146. Where this year's national total ultimately lands will depend heavily on the next several weeks, since West Nile transmission typically peaks in late August and September as mosquito populations build through the summer.

How Arizona became the epicenter

Maricopa County, home to Phoenix and roughly 4.5 million residents, has reported 58 confirmed human cases and five deaths this year, according to county figures cited by local outlet azfamily.com. What distinguishes this season from prior years is timing: mosquitoes testing positive for the virus showed up months earlier than usual.

"We started off early. We were getting positive mosquitoes for West Nile in February this year," John Townsend, Maricopa County's vector control division manager, told azfamily.com, adding that in a typical year the county doesn't see a surge until "September into October."

County crews have logged more than 180 mosquito samples that tested positive for the virus and now run upwards of 850 surveillance traps a week, deploying overnight pesticide treatments and larvicide in standing water where mosquitoes breed. Activity has also shifted geographically within the county, moving from the traditional hot spots in the southeast valley toward Peoria and Glendale, complicating efforts to target control measures. Recent monsoon rains, which leave behind pools of standing water ideal for mosquito breeding, have added to the concern heading into September.

The county's first fatality of the year was reported in mid-June, an older adult with underlying health conditions, according to the Maricopa County Department of Public Health. By June 18, the county had logged 25 locally acquired cases and two deaths, compared with just 10 cases and no deaths over the same stretch in 2025, a pace that has continued through the summer.

Who is most at risk

People over 60 and those with weakened immune systems, chronic conditions such as diabetes or hypertension, or a history of organ transplant face the highest risk of severe illness if infected, per CDC's guidance. There is no vaccine for West Nile virus in humans and no antiviral treatment; care for neuroinvasive cases is limited to supportive measures such as intravenous fluids, pain management and respiratory support in a hospital setting. That leaves prevention, primarily avoiding mosquito bites, as the only real defense available to the public.

Everyone living in or traveling through affected areas is potentially exposed, since the Culex mosquitoes that carry the virus are common across much of the country and bite mainly between dusk and dawn. Outdoor workers, gardeners and anyone spending evenings outside without repellent face elevated exposure risk regardless of underlying health.

What officials are telling residents

Local health departments have been reiterating basic mosquito-control guidance as case counts climb. Melissa Kretschmer, an epidemiology supervisor with the Maricopa County Department of Public Health, urged residents in June to treat prevention as a routine part of summer life rather than an afterthought.

"As mosquito activity increases across Maricopa County, we encourage residents to take prevention seriously by using insect repellent, eliminating standing water around their homes," Kretschmer said, according to a report from the public health newsletter Outbreak News Today.

The county promotes a "4 Rs" framework for residents: repel mosquitoes with EPA-registered repellent, remove standing water around the home, repair torn window and door screens, and remind neighbors to do the same. CDC's national prevention guidance largely mirrors that advice, recommending repellents containing DEET, picaridin, IR3535 or oil of lemon eucalyptus; long sleeves and pants during dusk and dawn hours; window and door screens or air conditioning; and draining standing water from birdbaths, flowerpots, buckets, pet dishes and gutters every two to three days, since mosquitoes can complete their breeding cycle in that span.

What comes next

With transmission typically peaking over the coming weeks, health officials in Arizona and nationally are bracing for the case count to keep rising before mosquito activity tapers off with cooler weather, usually by late October or November depending on the region. CDC updates its national case dashboard roughly every one to two weeks through the fall, and county health departments update local figures on a similar cadence, though officials caution that reporting delays mean the most current true case count in any given week is likely somewhat higher than what shows up in public dashboards.

For now, the practical message from health departments has changed little from earlier in the summer: use repellent, cover up outdoors during peak mosquito hours, and clear standing water at home. With five weeks or more of active mosquito season still ahead in much of the country, officials say the choices individual households make about basic prevention will do more to shape the rest of this season's toll than anything happening at the surveillance level.