This is the first time since 2017 that mosquitoes in Placer County have tested positive for SLEV

ROSEVILLE, Calif., September 13, 2023 – The Placer Mosquito and Vector Control District detected St. Louis Encephalitis virus (SLEV) in a mosquito sample near the Lincoln Airport. This is only the second time SLEV has ever been detected in a mosquito in Placer County although surrounding counties including Yolo, San Joaquin and Stanislaus have detected SLEV this summer. In 2017, SLEV was detected east of Sheridan in Placer County.

SLEV can be transmitted to humans through the bite of an infected mosquito just like West Nile virus (WNV). According to the California Department of Public Health, human cases of SLEV are very rare, although there is concern of increasing incidence in the past decade.

“Placer Mosquito will continue to monitor SLEV presence in mosquitoes in Placer County. We plan to increase trapping, enhance surveillance and make appropriate control responses near the area of Lincoln where SLEV was detected,” said District Manager, Joel Buettner.

Mosquito samples in Placer County are routinely tested for WNV, SLEV and Western equine encephalitis virus (WEE) because they all share the same mosquito vector species and have similar symptoms in humans. For more information about SLEV visit: or

The best way to avoid WNV or SLEV is to prevent mosquito bites with an EPA-registered repellent, dump and drain standing water on your property to prevent mosquito breeding and if you think you have a mosquito problem, submit a problem report on the District website.

For more information, contact the Placer Mosquito and Vector Control District at (916) 380-5444 or Follow the District on FacebookTwitterInstagram and TikTok: @placermosquito. For a list of EPA-registered repellents, please visit or