West Nile Virus Cases About 40% Above Normal Across U.S. as Mosquito Season Peaks

West Nile virus infections across the United States this summer are running roughly 40% higher than normal, with about 770 human cases reported as of early September and nearly 500 classified as severe, federal data show.

The Centers for Disease Control and Prevention, which compiles case reports submitted by state health departments, said the numbers are preliminary and subject to change because many mild infections go unreported and there can be a lag before severe cases are confirmed. Still, the agency and state health officials noted the intensity of the outbreak — including an unusually high percentage of mosquitoes testing positive for the virus — is driving heightened concern.

Most West Nile virus infections produce no symptoms or only mild illness such as fever, headache, muscle aches and rash, the CDC said. In a subset of cases the virus invades the nervous system, causing meningitis, encephalitis, paralysis or death. People older than 50 and those with compromised immune systems are at greatest risk for severe outcomes.

State-level reports mirror the national uptick. Kansas reported five serious cases as mosquito season peaked across the state, and public-health officials in several jurisdictions have issued reminders about personal protection and mosquito-control measures.

West Nile virus is typically transmitted to people through the bite of an infected mosquito. The virus’s transmission cycle often involves birds: mosquitoes that feed on infected birds can acquire the virus and later transfer it to humans. Mosquito activity and virus transmission usually peak in late summer and early fall, with most human cases reported in August and September.

Public-health experts cautioned that surveillance data underestimate the true number of infections because many cases are mild and do not prompt medical care or testing. Laboratories and health departments also may take time to confirm and report severe neuroinvasive disease, the CDC said.

The virus was first identified in Africa in 1937 and has caused sporadic outbreaks worldwide since. It was detected in the United States in 1999, when an outbreak centered in New York sent dozens of people to hospitals. Public-health agencies emphasize that preventing mosquito bites and reducing mosquito breeding sites remain the primary defenses.

Health officials recommend wearing long-sleeved, loose-fitting clothing and using Environmental Protection Agency–registered insect repellents, particularly at dawn and dusk when the mosquitoes that spread West Nile virus are most active. Home measures to reduce mosquito populations include removing standing water from buckets, tires, birdbaths and gutters and ensuring window and door screens are intact.

Surveillance will continue through the remainder of the season, and officials urged clinicians and the public to remain vigilant for symptoms of neuroinvasive disease. State and federal agencies will update totals and guidance as additional reports are received and cases are confirmed.