Four human West Nile virus cases recorded in San Diego County this summer remain the basis for a renewed mosquito warning issued Friday. Health officials used the Labor Day period to ask residents to remove standing water, prevent bites and report signs of local virus activity while warm-weather mosquito exposure continues.
The reminder did not announce a fifth patient. The county's most recent case summary, released Aug. 4, covered four residents whose symptoms began or whose tests turned positive between June 5 and July 14. One person required several days in a hospital, one had a mild illness that did not require admission, and two infections were detected during blood-donation screening.
One of those infections was acquired locally. No county resident had died from West Nile virus in 2026 as of the August update. The distinction between detected and symptomatic cases matters because most infections cause no symptoms. Others can bring headache, fever, nausea, fatigue, rash or swollen glands, while a small share progress to severe neurologic illness or death.
Bird and mosquito testing provides a separate view of circulation. The county reported virus in 23 dead birds and 10 mosquito batches so far in 2026.
During 2025, when no human cases were recorded locally, 34 dead birds and 182 mosquito batches tested positive. Those figures show surveillance findings and human illness do not move in lockstep from one season to the next.
Transmission follows a specific route: mosquitoes acquire the virus after biting infected birds and can then pass it to people. Human cases are not the virus's only signal. An increase in infected birds and mosquito pools can precede more recognized illness, which is why reports from yards and neighborhoods feed the county's monitoring program.
The easiest intervention begins at home. Plant saucers, clogged gutters, buckets, toys, old tires and wheelbarrows can hold enough water for mosquitoes to breed. Rain barrels should be screened, and mosquito dunks can be used where appropriate. The Vector Control Program also provides mosquito-eating fish for ponds, fountains and unused swimming pools.
Personal protection requires products with evidence behind them. County guidance names repellents containing DEET, picaridin, oil of lemon eucalyptus or IR3535. Long sleeves and pants reduce exposed skin, while intact window and door screens make it harder for mosquitoes that live near homes to move indoors.
Residents can report increased mosquito activity, green pools and certain dead birds to SDVector.com or 858-694-2888. Crows, ravens, jays, hawks and owls are among the birds the county asks people to report. A dead-bird report is not a diagnosis, but it can direct sampling toward places where the virus may be circulating.
The September notice is a prevention update, not evidence that the four known patients share one exposure site or neighborhood. Its message is distributed because the risk is distributed: a neglected container can create a breeding place on one block, and a repaired screen can interrupt contact inside one home. Those small actions are the practical edge of a surveillance system that otherwise works through laboratory results reported weeks later.