West Nile virus caused the overwhelming majority of reported arboviral disease in the United States in 2024, while Powassan virus, spread by ticks rather than mosquitoes, reached its highest annual case count on record, according to a CDC surveillance report published in Morbidity and Mortality Weekly Report.
Forty-eight states and the District of Columbia reported 1,955 confirmed and probable arboviral disease cases with illness onset in 2024, including 189 deaths. West Nile accounted for 1,808 of those, or 92%. Powassan was second with 60 cases, up from the previous record of 49 reported the year before.
This is a look backward, not a description of the current season. The report summarizes 2024 data and should not be read as telling you how much virus is circulating in your county this August. It is useful for a different reason: it shows which diseases carry the highest risk of severe illness once infection occurs, and where.
The Full 2024 Picture
After West Nile and Powassan, the remaining cases were La Crosse virus with 37, Jamestown Canyon virus with 27, eastern equine encephalitis with 19, and St. Louis encephalitis with 2.
West Nile activity in 2024 was actually below its 10-year median of 2,162 cases. Of 3,143 counties across the reporting jurisdictions, 591, or 19%, reported at least one West Nile case. Texas reported the most infections at 456, roughly one quarter of the national total and more than double the state's 2023 count.
Severity was high among those who were diagnosed. Three-quarters of West Nile cases involved neuroinvasive disease, meaning the virus reached the brain or spinal cord. Seventy-seven percent of patients were hospitalized, and nearly 10% died.
Demographics followed a consistent pattern across arboviral disease overall. A majority of patients were aged 60 or older, 63% were male, and 82% had illness onset during July through September. The full report is available from CDC in Morbidity and Mortality Weekly Report, and follows earlier annual arboviral surveillance summaries using the same methods.
The Tick-Borne Virus Rising Quietly
Powassan's 60 cases look small next to West Nile's 1,808. The severity profile is what makes it significant.
Every reported Powassan patient in 2024 was hospitalized with neuroinvasive disease, and nine died. There is no vaccine and no specific antiviral treatment. Nationally, Powassan cases have increased an average of 17.5% annually over the past 21 years, and Minnesota and Wisconsin together saw a 160% relative increase from 2023 to 2024, from 10 cases to 26.
The report's authors are careful about interpretation, noting the rise might reflect a true increase in incidence, better clinician awareness, wider availability of diagnostic testing, or expansion of the tick's range.
As CIDRAP noted in its summary of the report, Powassan is spread by blacklegged ticks, the same species that transmits Lyme disease, along with groundhog and squirrel ticks. Unlike Lyme, which generally requires a tick to remain attached for a day or more, Powassan can transmit in a much shorter window, which limits how much protection prompt tick removal provides.
The number of cases is small enough that year-to-year changes should be interpreted cautiously. A record of 60 cases in a country of more than 340 million people is not a public health emergency. The 21-year trend is the more meaningful signal.
The Children Who Get a Different Disease
Arboviral illness in children looks different from arboviral illness in adults, and that distinction matters for parents.
La Crosse virus disease occurred almost exclusively among children, with 35 of 37 cases in people under 18 and a median age of 8. Nearly all of those patients (97%) were hospitalized, though none died.
La Crosse virus circulates mainly in the Appalachian and Midwestern regions and is transmitted by tree hole mosquitoes that breed in containers and tree cavities. Cases were reported across 24 counties in eight states, with North Carolina and Tennessee accounting for 75% of them. Illness can range from mild fever to seizures and encephalitis.
Other arboviruses were rarer but severe. All 19 eastern equine encephalitis patients were hospitalized with neuroinvasive disease and five died. Of 27 Jamestown Canyon cases, 21 were hospitalized, and two died.
Where This Fits with the Current Season
MedicalDaily has been tracking the 2026 West Nile season separately, including Arizona's early case burden and deaths in Maricopa County. Current-year national surveillance data are provisional and updated regularly through the summer.
August and September are historically the peak transmission months across most of the continental United States. That is the practical reason a 2024 report published now still matters: it identifies who gets seriously ill and where, just as the highest-risk weeks arrive.
Local mosquito control districts and county health departments publish current-season data that is more timely than national figures.
Reducing Risk During Peak Weeks
There is no vaccine and no specific treatment for any of these viruses in humans. Preventing bites is the entire strategy.
Use an EPA-registered insect repellent containing DEET, picaridin, IR3535, or oil of lemon eucalyptus, and follow label instructions, including age restrictions. Wear long sleeves and long pants during dawn and dusk hours when the Culex mosquitoes that carry West Nile are most active.
Empty standing water weekly from flowerpot saucers, buckets, tires, gutters, birdbaths, and pool covers. Container water is where the relevant mosquito species breed, and a bottle cap of water is enough. Repair torn window and door screens.
For ticks, walk in the center of trails, treat clothing and gear with permethrin, and do a full body tick check after being outdoors, including the scalp, behind the ears, the waistband, and the backs of the knees. Shower within two hours of coming indoors.
Adults 60 and older accounted for a majority of reported patients and face a higher risk of severe disease. Anyone who develops fever with severe headache, neck stiffness, confusion, sudden weakness, or seizures after outdoor exposure during mosquito or tick season should seek urgent medical evaluation. Mild fever and body aches without neurologic symptoms warrant a call to a clinician rather than an emergency visit.
Frequently Asked Questions
What year does this report cover? 2024. It is a retrospective surveillance summary and does not describe current-season transmission.
What is an arbovirus? A virus spread to people by the bite of an infected mosquito or tick. Nationally notifiable domestic arboviruses include West Nile, Powassan, La Crosse, Jamestown Canyon, eastern equine encephalitis, and St. Louis encephalitis.
Why does Powassan matter with only 60 cases? Every reported patient in 2024 was hospitalized with neuroinvasive disease, and nine died. Cases have risen an average of 17.5% annually over 21 years.
Which state had the most West Nile cases? Texas, with 456 cases, roughly one quarter of the national total and more than double its 2023 count.
Who is most at risk of severe illness? Adults 60 and older accounted for a majority of patients. La Crosse virus is the exception, occurring almost entirely in children.
Is there a vaccine? No human vaccine exists for any of these viruses, and there is no specific antiviral treatment. Preventing bites is the only reliable protection.
When should I seek urgent care? Fever with severe headache, neck stiffness, confusion, sudden weakness, or seizures after outdoor exposure during mosquito or tick season warrants urgent evaluation.
