West Nile virus (WNV) was the most commonly reported mosquito-borne disease in the United States in 2024, according to a Centers for Disease Control and Prevention (CDC) report published last week. Powassan virus infections, which are transmitted to humans from ticks, hit a new record high that same year.
The findings, published in Morbidity and Mortality Weekly Report, identified 1,955 cases of confirmed and probable arboviral disease cases in 48 states and the District of Columbia in 2024. Arboviral diseases are infections caused by viruses transmitted to humans through tick and mosquito bites.
West Nile dominates, despite fewer cases overall
Of all the arboviral cases reported that year, WNV accounted for 1,808 (92%), Powassan virus for 60 (3%), La Crosse virus for 37 cases (2%), Jamestown Canyon virus for 27 (1%), eastern equine encephalitis for 19 cases (1%), and St. Louis encephalitis virus for 2 (under 1%).
While WNV was the dominant arboviral disease in 2024, the 1,808 reported cases were below the 10-year median of 2,162. Of the 3,143 counties across 48 states and DC, 591 (19%) reported WNV cases. Texas reported the most infections—456—or one-quarter of all WNV cases nationwide and more than double the state’s total in 2023.
The virus was responsible for severe illness in many patients. Three-quarters of WNV cases involved neuroinvasive disease, 77% of patients were hospitalized, and nearly 10% died. WNV can cause flu-like symptoms or serious brain infections.
Powassan and La Crosse cases continue to climb
The report also highlights the rise of Powassan virus. Sixty cases were reported in 2024, up from the previous record of 49 cases in 2023. All patients were hospitalized with neuroinvasive disease, and nine died.
Nationally, Powassan cases have increased an average of 17.5% annually over the past 21 years. The increase in Minnesota and Wisconsin was especially steep. From 2023 to 2024, the two states experienced a combined 160% relative increase in cases, from 10 to 26.
The reasons for the increase remain unclear. The rise “might be related to an actual increase in incidence, improved health care provider awareness, expanded availability of diagnostic testing at commercial and state public health laboratories, or potential expansion of the vector range,” the researchers write.
From 2023 to 2024, Minnesota and Wisconsin experienced a combined 160% relative increase in [Powassan virus] cases.
Powassan virus is spread by infected blacklegged ticks, groundhog ticks, and squirrel ticks.
In 2024, La Crosse virus primarily affected children. Ninety-five percent of cases occurred in kids, with a median age of 8 years, and 97% of patients were hospitalized. None died, though the researchers note that “arboviral morbidity among children can be severe, including neuroinvasive disease and death.” La Crosse virus is transmitted to humans by mosquitos and can cause brain inflammation.
La Crosse virus infections were reported across 24 counties in eight states, with North Carolina and Tennessee accounting for the largest percentage of cases (75%). Together, the two states represented a relative increase of 170% from the year before.
Less common arboviruses still pose serious risks
Twenty-seven cases of Jamestown Canyon virus disease were reported in 2024, up six cases from 2023. Of all reported cases, 21 patients (78%) were hospitalized, 20 (74%) had neuroinvasive disease, and two (7%) died.
Twenty-three counties in six states reported cases of the mosquito-borne virus, and the highest case numbers were reported in Wisconsin (10), Michigan (eight), and Minnesota (four). Jamestown Canyon virus can cause flu-like symptoms and dangerous brain swelling.
Arboviral morbidity among children can be severe, including neuroinvasive disease and death.
A total of 19 cases of eastern equine encephalitis virus disease were reported in 2024, all in patients hospitalized with neuroinvasive disease. Five patients died. Cases of this rare mosquito-borne disease were reported in 12 counties in nine states, with New Hampshire reporting the highest number (five), followed by Massachusetts (four). Eastern equine encephalitis causes brain inflammation and is often fatal.
Two cases of St. Louis encephalitis virus disease were reported to the CDC in 2024, with one of the patients hospitalized. Both survived. St. Louis encephalitis is transmitted to people by mosquitos and can cause fever, headache, and brain inflammation.
The authors say clinicians should consider arboviral infections in patients with fever or neurologic illness during mosquito and tick season. They also emphasize prevention because no vaccines or antiviral treatments are currently available for these infections.
