Six weeks after the year’s first mosquitoes carrying West Nile virus were detected in the Delta west of Stockton, San Joaquin County has confirmed a human infection case.

The patient is a woman in her 60s, San Joaquin County Public Health Services said in a news release. Officials did not disclose where in the county she lives, whether she was hospitalized or her current condition.

The California Department of Public Health last updated its statewide infection tracker on July 31, which showed there had been 11 human case reported in the state. San Joaquin County health officials say their case marks the 12th.

There has been a significant escalation in the spread of West Nile virus this year. There were six new human cases reported in California in the last week alone, according to state data, including two fatalities.

On Tuesday, Glenn County officials confirmed the death of a resident who lived in the southern portion of the county. It was not immediately clear when the person died, and no identifying information was released.

The announcement came one day after a West Nile virus-related fatality was confirmed in Kings County. Officials there said the person’s death marked the first in the state in 2026.

Infections have also rapidly spread in San Joaquin County. State surveillance data last updated July 31 show the county had recorded 51 positive mosquito samples and one infected horse, as well as three dead birds.

Mosquitoes carrying the virus were first detected locally in June at Upper Jones Tract in the Delta islands west of Stockton. At that time, California had not yet recorded any human infections.

Last year’s first publicly reported human case in San Joaquin County did not come until October.

The Centers for Disease Control and Prevention says West Nile virus is most commonly spread through the bite of an infected mosquito. About 80% of infected people develop no symptoms, while roughly 20% experience flu-like symptoms that can include fever, headaches and body aches. 

About 1% of infected people develop a serious illness that can affect the brain and nervous system. Warning signs include a severe headache, stiff neck, high fever, disorientation or confusion.

Adults over 60, people with weakened immune systems and those with diabetes or kidney disease face a higher risk of serious illness.

County Public Health Officer Dr. Maggie Park urged residents to take precautions against mosquito bites, particularly around dawn and dusk, when mosquitoes are most active.

“If you know you have been bitten by mosquitos and are exhibiting flu-like symptoms, you should consult with your healthcare provider, especially if you have an immunocompromised health status,” Park said in a statement.

Health officials recommend draining standing water from yards and other areas around homes. Mosquitoes can reproduce in as little as a tablespoon of water.

Residents also should use an insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus, wear long sleeves and pants outdoors and make sure doors and windows have tightly fitted screens.

Significant mosquito infestations and neglected swimming pools can be reported to the San Joaquin County Mosquito and Vector Control District. Residents can also request property inspections through the district’s website.

Dead birds can be reported to the state by calling 1-877-968-2473. Such reports help health agencies track where the virus may be circulating.