West Nile Virus and Other Mosquito- and Tickborne Viral Diseases in the U.S., 2024

Arthropodborne viruses, or arboviruses, are mostly spread in the United States by the bites of infected mosquitoes and ticks, and rarely through blood transfusion or organ transplantation. Most infections cause no symptoms, but illness can range from a mild fever to severe, sometimes fatal neuroinvasive disease — meningitis, encephalitis or paralysis. Older age, some chronic conditions such as cancer, diabetes, high blood pressure and kidney disease, and a weakened immune system raise the risk of severe disease. There are no vaccines or specific treatments, so surveillance guides prevention. CDC's annual summary covers cases with illness onset in 2024.

The year in numbers

States reported 1,955 cases to ArboNET, the national arbovirus surveillance system — 594 confirmed and 1,361 probable — from 48 states, the District of Columbia and 647 of the country's 3,143 counties. There were 189 deaths (10%), and 1,485 cases (76%) were neuroinvasive. Most patients were 60 or older (58%) and male (63%), and 82% fell ill between July and September.

West Nile virus

West Nile virus caused 92% of cases. The median patient age was 64, 63% were male, 75% had neuroinvasive disease and 77% were hospitalized. Among patients with neuroinvasive disease, 13% died. Two patients died after being infected through kidneys transplanted from the same deceased donor.

Cases came from 591 counties in 48 states and D.C. Texas, California and New York together accounted for 40% of neuroinvasive cases. Texas reported 456 West Nile cases in all, more than double its 163 in 2023 and a quarter of the national total — an example of how regional outbreaks drive high counts. The national incidence of neuroinvasive disease was 0.40 per 100,000, highest in North Dakota (2.76), Nebraska (2.49) and Mississippi (1.43), and among adults 70 and older (1.30). The year's total was still below the 10-year median of 2,162.

CDC map: incidence of neuroinvasive West Nile virus disease by state, 2024.

Powassan virus: a new record

Powassan virus, spread by ticks, rose to 60 cases, a record, up from the previous high of 49 in 2023, and well above its 10-year median of 23. Every patient was hospitalized with neuroinvasive disease, and 15% died. Cases occurred year-round, with nearly half beginning in April–June, and came from 45 counties in 10 states; incidence was highest in Maine, Minnesota, New Hampshire and Wisconsin. Cases in Minnesota and Wisconsin rose from 10 to 26. Over the past 21 years, reported cases have grown by an average of 17.5% a year, which may reflect a real increase, greater awareness among clinicians, more testing, or a wider range for the ticks.

A disease of children: La Crosse virus

Of 37 La Crosse cases, 35 were in children under 18 (median age 8). Nearly all patients were hospitalized with neuroinvasive disease, and none died. North Carolina (15 cases) and Tennessee (12) accounted for 75% of neuroinvasive cases, up from five each in 2023, though the national total was below the 10-year median. La Crosse caused 54% of the 63 neuroinvasive arboviral cases in children, followed by West Nile (35%) and Powassan (6%).

Other viruses

• Eastern equine encephalitis: 19 cases, the most since 2019 and the third-highest since 2003, all neuroinvasive, with a case-fatality rate of 26% — the highest of any domestic arbovirus. New Hampshire (five) and Massachusetts (four) reported the most.
• Jamestown Canyon virus: 27 cases, similar to the 10-year median, mostly from Wisconsin, Michigan and Minnesota.
• St. Louis encephalitis: 2 cases, both survived — the fewest since 2013.

Missing cases

ArboNET relies on people seeking care, clinicians testing and states reporting, so it undercounts. Based on historical ratios, 30 to 70 non-neuroinvasive West Nile cases occur for every reported neuroinvasive one, suggesting 40,410 to 94,290 such cases in 2024, of which only 461 were reported. Cases can also be misclassified, and the Powassan growth rate describes the past, not necessarily the future.

Prevention and what is needed

With no vaccines or antiviral treatments, prevention relies on insect repellent registered by the Environmental Protection Agency, protective clothing, mosquito and tick control at home and in communities, and screening blood and organ donors for West Nile virus. Unlike blood donors, organ donors were not universally screened until a policy requiring seasonal West Nile screening of all potential living and deceased organ donors (July 1 to October 31) took effect in June 2026.

Clinicians should consider arboviral testing for patients with fever or neurologic illness, including transplant and transfusion recipients, especially when mosquitoes and ticks are active. Ticks are active earlier and later in the year than mosquitoes, whose activity peaks in July–September, so public messaging can be timed to local viruses and their vectors. Some 25 years after West Nile virus was first detected in the United States, the authors call for human vaccines, preventive or therapeutic monoclonal antibodies for people at high risk, and rapid diagnostic tests.

Sources

• Kimberly E. Mace, Daniel Jacobs, Carolyn V. Gould, J. Erin Staples and Shelby L. Lyons, "West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2024," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/75/wr/mm7529a1.htm