CDC Reports Record Powassan Virus Activity as West Nile Cases Decline in 2024

West Nile virus (WNV) activity in the United States remained below its 10-year median in 2024, while Powassan virus reached a record number of reported cases, according to new surveillance data from the Centers for Disease Control and Prevention (CDC).

The findings, published in the Morbidity and Mortality Weekly Report, summarize cases of nationally notifiable arboviral diseases reported to CDC's ArboNET surveillance system during 2024. In the continental U.S., the most common nationally notifiable arboviral diseases are transmitted by mosquitoes, including West Nile virus, La Crosse virus, Jamestown Canyon virus, eastern equine encephalitis virus and St. Louis encephalitis virus, while Powassan virus disease is transmitted by ticks.

State and local health departments in 48 states and the District of Columbia reported 1,955 confirmed and probable arboviral disease cases, including 189 deaths. WNV remained the most commonly reported arboviral disease, accounting for 1,808 cases (92%) and 173 deaths.

Among reported WNV cases, 1,396 patients (77%) were hospitalized and 1,347 patients (75%) developed neuroinvasive disease, including meningitis, encephalitis or acute flaccid paralysis. The case-fatality rate among those with neuroinvasive disease was 13%. Texas reported the highest number of WNV cases, followed by California and New York, with the three states accounting for 40% of all reported neuroinvasive WNV cases.

CDC also reported record Powassan virus activity, with 60 cases identified in 2024 compared with 49 in 2023. All reported Powassan virus cases involved neuroinvasive disease, and the case-fatality rate was 15%. The report also documented 19 cases of eastern equine encephalitis, the highest annual total since 2019, with a case-fatality rate of 26%, the highest among nationally notifiable arboviral diseases. In contrast, St. Louis encephalitis activity declined to its lowest level in more than a decade.

Although arboviruses are transmitted primarily through mosquito and tick bites, the report stated that transmission can also occur through blood transfusion and solid organ transplantation. Investigators described two deaths from neuroinvasive WNV infection among kidney transplant recipients who received organs from a single deceased donor.

The authors also cautioned that passive surveillance substantially underestimates the true burden of arboviral disease because patients with milder illness are less likely to seek care, undergo diagnostic testing or be reported to public health authorities. According to the authors, the 461 reported non-neuroinvasive WNV cases in 2024 suggest that approximately 40,000 to 94,000 infections occurred nationwide.

The report noted that no approved human vaccines, specific therapeutics or rapid diagnostics are currently available for arboviral diseases. The authors recommended that clinicians consider arboviral disease in patients with unexplained fever or neurologic illness during periods of arboviral transmission, particularly among organ transplant and blood transfusion recipients. They also emphasized that mosquito control, personal protective measures and blood and organ donor screening for WNV are essential to reducing arboviral disease morbidity and mortality.

Additional Information for Blood Collectors

AABB's West Nile Virus Biovigilance Network collects voluntary reports of suspected WNV-positive blood donors from facilities across the United States. The network provides a dashboard of reported activity and email alerts that can help blood collection facilities assess local WNV activity and support decisions about transitioning from minipool nucleic acid testing (MP-NAT) to individual donor nucleic acid testing (ID-NAT), consistent with FDA guidance.

Additional information about WNV and other nationally notifiable arboviruses is available in AABB’s emerging infectious diseases fact sheets.