Nevada County has recently reported their first human case of West Nile virus (WNV) for 2025. As of September 5, 2025, the California Department of Public Health (CDPH) had received reports of 31 human cases this season, with most cases being reported from the southern Central Valley. WNV is a disease that is transmitted by the bites from infected mosquitoes. WNV first appeared in California in 2003, and infections occur most often in the summer and early fall when mosquitoes are active. WNV activity is at its annual peak in California, and the state expects to see a continued rise in human cases through the early fall. As we move into fall, residents should continue to take precautions against mosquito bites.
For most people, the risk of serious WNV illness is low; however, some people (less than one percent) can develop a serious neurologic illness such as meningitis or encephalitis (brain inflammation). People 50 years of age or older, who are immunocompromised, or have diabetes or hypertension, have the greatest risk of complications.
To reduce the risk of being infected, residents are encouraged to:
• Remain indoors at dawn and dusk when the mosquitoes that can transmit WNV are most active, or
• Wear long sleeves and pants during dusk and dawn.
• Defend yourself by using an EPA-registered insect repellent and following the label directions. https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/EPA[1]Registered-Repellent-Ingredients.aspx
• Remove water standing for more than 3 days from bird baths, pet dishes, flowerpots, outdoor stored automotive tires or neglected swimming pools.
• Keep door and window screens in good working condition.
California's WNV website (westnileca.gov) includes the latest information on WNV activity in the state. Californians are encouraged to report all dead birds through the website. Dead birds can also be reported by calling toll-free 1-877-WNV-BIRD (968-2473).
Additional information about WNV can be found in the following resource:
Nevada County Mosquito and Vector Control website: https://www.nevadacountyca.gov/2204/Mosquito-and-Vector-Control
Centers for Disease Control and Prevention West Nile Virus: https://www.cdc.gov/west-nile[1]virus/symptoms-diagnosis-treatment/index.html