Mosquitoes in Southern California Test Positive for St. Louis Encephalitis Virus

Mosquitoes sampled in Brawley, California, have tested positive for St. Louis encephalitis virus (SLE), public health officials reported, marking another detection of the mosquito-borne pathogen as surveillance continues across the western and central United States.

The mosquito pool that tested positive was identified near Palm Avenue in a residential area east of Brawley's city center. Brawley, a city of more than 28,000 residents about 130 miles east of San Diego, is part of an area where public health agencies monitor mosquito populations for several disease-causing viruses. St. Louis encephalitis circulates primarily between Culex mosquitoes and wild birds and can infect humans who are bitten by infected mosquitoes.

Most people infected with SLE do not develop symptoms, health officials said. When illness occurs, it typically begins with fever, headache, nausea and fatigue. In about 30 percent of human cases, according to the agencies reporting the latest detection, the infection can progress to severe neuroinvasive disease, including brain swelling, vomiting, seizures and death.

The Imperial County Public Health Department confirmed two human cases of St. Louis encephalitis in the county in 2019. In recent years, the virus has also been detected in mosquito pools in other California counties, including Fresno, Kings and Madera.

This summer, surveillance programs have reported numerous detections of SLE in mosquito populations across multiple states, including California, Arizona, Utah, Nebraska and Louisiana. Public health and vector-control programs collect mosquitoes in groups, or "pools," that are tested in laboratories to identify the presence of viruses; a single positive pool indicates that at least one mosquito in that group carried the virus.

State and local health agencies use those findings to inform mosquito-control efforts and public information campaigns, aiming to reduce human exposures. Control measures typically focus on reducing mosquito breeding sites, targeted insecticide applications and public guidance about protective measures such as using EPA-registered repellents, wearing long sleeves and avoiding outdoor activity at dawn and dusk when Culex mosquitoes are most active.

St. Louis encephalitis is one of several mosquito-borne viruses monitored by U.S. health authorities; others include West Nile virus and, in certain regions and seasons, La Crosse encephalitis and eastern equine encephalitis. Continued detection of SLE in mosquito populations underscores the role of routine surveillance in identifying risk to communities and guiding public-health responses.

Local residents with concerns or symptoms consistent with mosquito-borne illness were advised to contact their health care provider or local public health department for guidance and testing. Health officials emphasized that most infections do not cause serious illness but warned that severe cases require prompt medical attention.