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Local West Nile virus activity confirmed in Stanislaus and Merced counties
mosquito spraying
A twin-engine aircraft contracted by the Merced County Mosquito Abatement District flies over Gustine on Oct. 2, 2025, during an aerial treatment to reduce adult mosquito populations. The evening operation targeted Los Banos and Gustine between 6:45 and 11:45 p.m. as part of countywide efforts to curb the spread of West Nile virus. - photo by Photo Contributed

West Nile virus activity has been confirmed in both Stanislaus and Merced counties in 2025, prompting increased mosquito control efforts and public health advisories.

In Stanislaus County, the Turlock Mosquito Abatement District reported the first detection of the virus in mosquito samples on June 17. As of late September, at least two human cases have been confirmed, according to county health officials. No fatalities have been reported.

The early detection triggered expanded mosquito surveillance and control measures, including larvicide applications, adult mosquito spraying and increased testing of mosquito pools for arboviruses.

In Merced County, the first positive mosquito sample was confirmed June 30 in Delhi by the Center for Vector-borne Diseases at UC Davis. The county’s first human case of neuroinvasive West Nile virus was announced Sept. 8 and involved a resident of Atwater. No additional details were released due to medical privacy laws.

To reduce mosquito populations ahead of the Fourth of July holiday, the Merced County Mosquito Abatement District conducted aerial spraying July 3 over communities along the Highway 99 corridor. Additional aerial treatments were scheduled for Oct. 2 over Los Banos and Gustine, with ground spraying in Merced, Atwater, Livingston, Hilmar and Dos Palos.

Health officials in both counties urge residents to take precautions, including using EPA-registered insect repellents, wearing long sleeves and pants during peak mosquito hours, and eliminating sources of standing water around homes.

Residents can report mosquito concerns or neglected swimming pools to their local abatement districts. In Merced County, services are free and anonymous reports are accepted at mcmosquito.org or by calling (209) 722-1527. Stanislaus County residents can contact the Turlock Mosquito Abatement District at (209) 634-1234 or the East Side Mosquito Abatement District at (209) 522-4098.

California has reported more than 50 human cases of West Nile virus in 2025, with five deaths statewide. Nearly 80% of confirmed infections have occurred in the San Joaquin Valley.