Health officials in the Southwestern United States are on high alert due to a surge in a mosquito-borne illness, with West Nile virus detected in multiple states. In California's Orange County, mosquitoes in at least 13 cities have tested positive for the virus, according to the Orange County Mosquito and Vector Control District. The county has reported 38 positive mosquito samples, with the highest concentration found in Fullerton.
Ventura, Yolo, and Sacramento counties in California have also reported West Nile-positive mosquito samples. So far this year, California has recorded one human case in Los Angeles County. Neighboring Arizona, however, has reported 35 human cases, with 29 concentrated in Maricopa County, which includes Phoenix.
Nationally, the Centers for Disease Control and Prevention (CDC) data shows 56 human cases of West Nile virus across 15 states this year, with 44 of those being neuroinvasive cases, indicating the virus has affected the brain and spinal cord. Last year, the US saw a total of 2,100 West Nile virus cases.
West Nile Virus Transmission and Symptoms
West Nile virus is transmitted through the bite of infected Culex mosquitoes. While many infections cause no symptoms, approximately one in five individuals develop fever, headaches, nausea, diarrhea, joint pain, or pain behind the eyes. In about one percent of cases, the virus becomes neuroinvasive, leading to inflammation of the brain or spinal cord, potentially causing meningitis or paralysis.
Older adults, individuals with weakened immune systems, and those with chronic health conditions are at the highest risk for severe illness. Survivors of severe West Nile illness may experience long-lasting complications, including memory problems, chronic fatigue, muscle tremors, or permanent neurological damage.
Public Health Response and Prevention
In response to the rising detections, California's Orange County Mosquito and Vector Control District plans to implement mosquito control treatments in a 4.5-square-mile area of Fullerton from July 9 through July 11. These treatments involve using trucks to reduce adult mosquito populations when disease presence and mosquito abundance trigger the need.
The Sacramento-Yolo Mosquito and Vector Control District stated it will continue monitoring the virus through widespread mosquito trapping and testing, adapting ground treatments in virus-positive areas as needed.
The CDC recommends preventive measures to avoid mosquito bites, including using insect repellents containing DEET, wearing long sleeves and pants when outdoors, and limiting outdoor activity between dusk and dawn during warmer months. The mosquito season typically runs from late spring through early fall. This year, no deaths have been recorded, compared to 172 in the previous year. Last year, there were 1,342 hospitalizations for neuroinvasive disease.