Mosquitoes carrying West Nile virus are back in San Joaquin County.

The Mosquito and Vector Control District said it has detected mosquitoes carrying the potentially deadly virus in the Delta islands west of Stockton. The sample is the first positive test in the county this year.

The district said the virus has been detected here every summer for more than 20 years — but its prevalence makes it no less dangerous. Last year, three of the four state deaths attributed to the virus through October were in the San Joaquin Valley. In all, 11 people died of the virus in California in 2025.

“As temperatures continue to rise this summer, residents should take steps to reduce mosquito development around their homes and protect themselves from mosquito bites,” district spokesperson Sumiko De La Vega said in a statement.

She urged residents to eliminate standing water around their yards where mosquito eggs can hatch and to use insect repellant to prevent bites.

While mosquitoes carrying West Nile virus were found in San Joaquin County around the same time last year, it was months before anyone was infected locally. A report in October indicated a patient identified only as an older man had become infected. By then, human cases had been reported in 13 other California counties.

So far this year, there have been no reported human cases in the state, but there have been 49 dead birds linked to the disease — 15 of them in Sacramento — and one horse.

West Nile virus has been detected as far south as the Mexican border. Besides San Joaquin, the disease has shown up in 16 other California counties so far this year.