Current Geographic Distribution of West Nile Virus in the US
West Nile virus has been detected in all 48 contiguous states and the District of Columbia, with the highest concentration of human cases historically in the central and southern plains, the Southwest, and parts of California, according to the CDC and state health departments. The virus is primarily maintained in bird-mosquito cycles, and surveillance data show that counties in Texas, California, Arizona, Colorado, and the Mississippi River basin consistently report the largest share of neuroinvasive disease cases and positive mosquito pools each year.
Public health agencies track the virus through ArboNET and state-level reporting systems, and recent maps from the CDC highlight clusters in both urban and rural areas where mosquito control programs and case counts are closely monitored. The CDC West Nile Virus page provides updated maps and surveillance summaries that show where human cases, animal infections, and positive mosquito pools are concentrated across the country.
Risk Factors, Case Counts, and Transmission Patterns
Most people infected with West Nile virus do not develop symptoms, but about 1 in 5 develop fever and other symptoms, and roughly 1 in 150 develop severe neuroinvasive illness such as encephalitis or meningitis, with older adults and immunocompromised individuals at highest risk. CDC data show that the number of reported neuroinvasive cases can vary widely by year depending on weather patterns, bird immunity, and local mosquito populations, and recent summaries from state health departments highlight spikes in counties with high mosquito activity and older populations.
Transmission risk peaks during mosquito season, typically from late spring through early fall, and local health departments use trapping and testing data to identify high-risk zones and issue alerts. The EPA and CDC provide guidance on insect repellents and mosquito control methods that can reduce exposure, and some local governments partner with private pest control companies to conduct aerial and ground spraying in areas with high infection rates.
Prevention, Surveillance, and Public Health Response
Prevention focuses on avoiding mosquito bites through repellent use, wearing long sleeves, and removing standing water where mosquitoes breed, while surveillance programs track bird die-offs, mosquito pools, and human cases to guide local response efforts. The CDC and state health departments publish weekly and seasonal West Nile virus updates that include case counts, geographic spread, and risk assessments, and these reports help local governments decide when to implement mosquito control measures and public awareness campaigns.
Health departments also collaborate with universities, laboratories, and federal agencies to sequence viral strains and monitor for changes in transmission patterns, and some jurisdictions use data-driven models to prioritize spraying and outreach in high-risk neighborhoods. The EPA maintains a database of approved insect repellents and mosquito control products, and local agencies often refer to these resources when selecting treatments that are effective and safe for human health and the environment.