San Diego County is urging residents countywide to guard against mosquitoes after recording 11 human West Nile virus cases through Sept. 16, the highest local count in a decade.
No positive mosquito detections have been recorded in Carlsbad, according to the county's West Nile virus tracking page. Positive results are concentrated in San Diego, El Cajon, Coronado and La Mesa. But the virus has been found in all 58 California counties, according to the county's Vector Control Program, and health officials say the risk extends regionwide.
The County of San Diego reported the 11 cases as the most local West Nile infections since 2016. The affected individuals range in age from 22 to 89. Four have been hospitalized. No deaths have been reported.
"Typically we have 1-3 cases a year, and it's just September, and we have 11," Dr. Mark Beatty of San Diego County Health told CBS 8 on Sept. 24.
Three of the 11 cases involve Coronado residents. The county detected at least 10 locations on Coronado with infected mosquitoes and sent warning letters to nearly 1,500 homes there in the past month, according to The Coronado News.
Spraying and surveillance
County Vector Control crews sprayed approximately 6,000 properties in Coronado with a truck-mounted pesticide mist early Friday, Sept. 25, between 1 a.m. and 6 a.m. A second round was scheduled for Saturday, Sept. 26, during the same hours, according to the county's treatment information page. The treatment area stretches from Second Street south to Ocean Boulevard and from F Avenue east to Glorietta Boulevard.
The county used AquaDuet, a pesticide that mimics natural insecticides found in chrysanthemum flowers. The product is approved by the U.S. Environmental Protection Agency and dissipates within 30 minutes, the county said. Additional treatments may follow depending on conditions.
Countywide, 99 batches of mosquitoes and 51 birds have tested positive for West Nile virus this year. The county maintains 91 traps across the region. By comparison, 2025 saw 182 positive mosquito batches and 34 positive birds but zero human cases.
CBS 8 reported that county officials warned the upcoming rainy season, potentially worsened by El Niño, could increase mosquito breeding.
How Carlsbad residents can protect themselves
Most people infected with West Nile virus show no symptoms. About 20% develop mild flu-like illness. Less than 1% develop severe neurological conditions such as encephalitis or meningitis. There is no treatment for the virus.
The county's "Prevent, Protect, Report" guidelines recommend:
- Dump standing water from plant saucers, buckets, garbage cans, toys, old tires and wheelbarrows. Mosquitoes can grow from eggs to biting adults in less than a week.
- Protect yourself by wearing long sleeves and pants or using insect repellent containing DEET, picaridin or oil of lemon eucalyptus.
- Report increased mosquito activity, unmaintained green swimming pools or dead birds to the County Vector Control Program at (858) 694-2888 or [email protected].
The county also provides free mosquitofish for backyard water sources such as stagnant pools, rain barrels, ponds and fountains.
