Early Surge in West Nile Virus Cases

The Centers for Disease Control and Prevention (CDC) is seeing the earliest start to the West Nile virus season in the U.S., with the highest number of human disease cases reported by this time of year since 2004. At least 48 cases have been reported as of June 30, and 38 have been cases of severe neuroinvasive disease.

The hardest-hit area in the U.S. this year seems to be Maricopa County in Arizona, with 29 confirmed cases reported, including four deaths. At the same point in 2025, the county had seen just 13 cases and one death, according to the county Department of Public Health.

Since 2004, an average of 10 human disease cases has been reported to the CDC by the end of June. This year’s surge is driven by early circulation of the virus, with 23 states reporting West Nile virus activity this year already—also the highest number recorded over the last 10 years. (The “activity” metric includes positive tests in mosquitoes and other animals.)

CDC recommends taking the following precautions to reduce the risk of mosquito bites:

• Use an insect repellent registered with the Environmental Protection Agency.
• Wear long, loose-fitting shirts and pants, so mosquitoes have a harder time biting your skin.
• Avoid being outside between dusk and dawn when the mosquitoes that spread West Nile virus are most active.
• Use screens on windows and doors, or use air conditioning, if available, to keep mosquitoes outside.

State and local health departments continue to monitor mosquito populations and implement mosquito control measures where appropriate. CDC is working closely with public health partners nationwide to monitor disease activity and provide guidance to help communities reduce the risk of infection.

West Nile virus is the leading cause of mosquito-borne disease in the contiguous U.S., with several thousand cases and about 100 deaths on average reported each year, according to the CDC. Most people infected with West Nile virus do not develop symptoms. However, approximately one in five infected individuals develop a fever and other symptoms, such as headache, body aches, joint pain, vomiting, diarrhea, or rash. Less than 1% of infected people develop severe neurologic disease, including meningitis or encephalitis, which can result in long-term disability or death. Adults aged 60 years and older and people with certain underlying medical conditions are at increased risk for severe illness.