As the Central Valley rolls into summer, local health officials are urging caution following recent West Nile Virus (WNV) activity in both Merced and Stanislaus counties.

Merced County has reported two confirmed human cases of WNV so far this season — an early spike compared to surrounding counties. Additionally, one mosquito pool tested positive for the virus, indicating active transmission.

Stanislaus County recorded its first positive mosquito samples of 2025 on June 17, collected by the Turlock and East Side Mosquito Abatement Districts. One human case has also been confirmed. No WNV-positive dead birds have been reported in either county the previous week.

West Nile Virus, which first appeared in California in 2003, spreads to humans through the bite of an infected mosquito. Mosquitoes contract the virus by feeding on infected birds and can then transmit it to humans and animals.

According to the Centers for Disease Control and Prevention (CDC), about one in five infected people will experience symptoms such as fever, headaches, joint pain, rash, or fatigue. Less than 1 percent develop severe neurological complications, including meningitis or encephalitis — conditions that can be fatal, especially in individuals over 50 or those with underlying health conditions.

There is currently no vaccine or antiviral treatment for WNV. Recovery from milder symptoms may take weeks or even months, while neuroinvasive forms of the illness can result in long-term or permanent effects.

Preventing West Nile Virus: Local mosquito abatement districts are continuing surveillance and treatment programs and encourage the public to take the following preventive actions:

• Dump or drain standing water around your home to eliminate mosquito breeding grounds.
• Defend yourself with insect repellents containing DEET, Picaridin, or Oil of Lemon Eucalyptus.
• Avoid being outdoors at dawn and dusk — peak hours for mosquito activity.
• Use tight-fitting screens on doors and windows.
• Report neglected swimming pools or mosquito activity to your local abatement district.
• Vaccinate horses, which are also vulnerable to the virus, under veterinarian guidance.

Residents in Newman and Crows Landing can contact the Turlock Mosquito Abatement District at (209) 634-1234 or visit www.turlockmosquito.org. Those in Gustine and Santa Nella should contact the Merced County Mosquito Abatement District at (209) 722-1527 or www.mcmosquito.org.

To report dead birds — a critical tool in tracking WNV — call 1-877-WNV-BIRD or report online at www.westnile.ca.gov. Species of particular concern include crows, ravens, jays, magpies, and hawks.

As the summer heats up, local agencies stress the importance of awareness and prevention. Even simple actions, like tipping over a birdbath or wearing bug spray, can make a measurable difference in reducing West Nile’s spread.