San Diego County had confirmed 11 human West Nile virus cases this year as of Sept. 16, the most local infections since 2016, according to the County of San Diego's announcement of Coronado mosquito treatments. Three of the patients live in Coronado, where the county scheduled overnight truck treatments on about 6,000 lots on Sept. 25 and Sept. 26 after surveillance found elevated numbers of mosquitoes, some of them carrying the virus.

The jump is sharp. Local news reports say the county recorded no human cases in 2025 and that four of this year's 11 patients were hospitalized. Dr. Mark Beatty of the county's public health services told CBS 8 that the county typically sees one to three cases a year, adding, "it's just September, and we have 11."

For households, the practical message is that the season is not over. County officials have said West Nile risk in the region typically runs from June through October, and local reports say they have flagged a forecast El Niño as a reason standing water and mosquito breeding, could last longer than usual. Nature World News reviewed county and state data on Thursday, Oct. 1 (ET).

Coronado Spraying and What It Means for Residents

The Coronado treatment zone runs from 2nd Street in the north to Ocean Boulevard in the south, and from F Avenue in the west to Glorietta Boulevard in the east. The treatments were scheduled from 1 a.m. to 6 a.m., when most people are indoors. Trucks apply an ultra-low-volume mist of AquaDuet, a pesticide that mimics natural insecticides found in chrysanthemum flowers. The county says the product is approved by the Environmental Protection Agency, does not pose a risk to people and pets as applied, and dissipates in under 30 minutes.

Beekeepers and people who keep insects or fish outdoors were encouraged to cover those habitats during treatments, and beekeepers registered with the county can request advance notice. The county said additional treatments could follow the next week depending on conditions. FOX 5 San Diego reported that the county's next scheduled aerial mosquito treatment will take place Oct. 7 and 8.

State surveillance data show why crews moved in. The California West Nile tracker, last updated Sept. 25, reported 37 new West Nile-positive mosquito samples from San Diego County in the most recent week, bringing the county's 2026 total to 95 positive samples and 47 infected dead birds.

State and county human counts differ. The state dashboard lists three San Diego County human cases, while the county reports 11. The county's own reporting is the most current local count.

Southern California's Heaviest Season in Years

San Diego is not an outlier. Los Angeles County has reported 100 West Nile cases this year, including 94 hospitalizations and five deaths, the highest total since 2017, according to the county Department of Public Health as reported by Good Morning America. The California Department of Public Health says statewide West Nile activity is at a five-year high.

The state tracker lists 210 human cases from 23 counties and 15 deaths this year. In its latest weekly update, Los Angeles County added 42 new cases and Orange County added nine. State totals for individual counties can differ from county health department counts because of reporting timelines.

The National Picture Since Our Last Report

Nature World News previously reported that the Centers for Disease Control and Prevention had counted 504 cases in 41 states as of Sept. 8, with about 72% neuroinvasive, the severe form that affects the brain or spinal cord. By the time of Good Morning America's Sept. 22 report, CDC data listed 671 cases across 42 states.

The CDC refreshed its current-year West Nile dashboard again on Sept. 29. The agency describes all current-season figures as preliminary and notes that state and local health departments may have more up-to-date information.

The CDC also cautions that its data are reported by county of residence, not where a bite occurred, and that milder cases are more likely to go unreported than severe ones. The true number of infections is therefore likely higher than the official count.

El Niño and a Longer Mosquito Season

The Climate Prediction Center has an El Niño Advisory in effect. Its Sept. 10 discussion gave a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter of 2026-27, with the next update scheduled for Oct. 8.

That forecast is probabilistic, not a guarantee of heavy rain in San Diego. The Climate Prediction Center notes that a stronger event makes El Niño-related impacts more likely without guaranteeing them. Still, any fall or winter storms that leave standing water in yards, gutters, and flood-control channels could give mosquitoes new places to breed, which is the concern local officials have raised.

Protecting Your Household Before the Season Ends

There is no licensed vaccine or specific medicine for West Nile virus in people, which makes prevention the main tool. Most infected people have no symptoms. Some develop fever with headache, body aches, joint pain, vomiting, diarrhea, or rash, and in rare cases the virus causes severe illness or death. Older adults and people with certain medical conditions face a greater risk of severe illness, according to the CDC.

The county advises residents to dump or remove anything that holds water, including plant saucers, buckets, garbage cans, toys, old tires, and wheelbarrows. Stagnant pools, rain barrels, ponds, fountains, and horse troughs can be treated with mosquito dunks or free mosquitofish from the county's Vector Control Program. Use repellent containing DEET, picaridin, oil of lemon eucalyptus, para-menthane-diol, 2-undecanone, or IR3535, wear long sleeves and pants outdoors, and check that window and door screens are intact.

Seek medical care for symptoms after mosquito exposure. A severe headache, stiff neck, confusion, or new weakness warrants urgent attention. San Diego residents can report increased mosquito activity, unmaintained green pools, or dead crows, ravens, jays, hawks, and owls to Vector Control at (858) 694-2888.

The next milestones are the county's scheduled aerial treatments, the CDC's next dashboard refresh, and the Climate Prediction Center's Oct. 8 outlook.

What Readers Want to Know

How many West Nile cases has San Diego County reported this year?

The county reported 11 human cases as of Sept. 16, 2026, the most local infections since 2016. Three of the patients live in Coronado.

Why did the county schedule spraying in Coronado?

Surveillance found elevated numbers of mosquitoes in Coronado, some positive for West Nile virus. Truck treatments on about 6,000 lots were scheduled overnight on Sept. 25 and 26.

Is the spray safe for people and pets?

The county says the product, AquaDuet, is EPA-approved and does not pose a risk to people and pets as applied. Beekeepers and people with outdoor fish or insects were asked to cover those habitats.

How bad is the season in the rest of Southern California?

Los Angeles County has reported 100 cases, 94 hospitalizations, and five deaths, its highest total since 2017. California as a whole listed 210 cases and 15 deaths in its Sept. 25 update.

Could El Niño extend the mosquito season?

The Climate Prediction Center gives a greater than 90% chance of a very strong El Niño this fall and winter. Standing water from any storms could prolong mosquito breeding, though heavy rain is not guaranteed.

What is the best way to protect my household?

Remove standing water regularly, use repellent, wear long sleeves outdoors, repair screens, and seek medical care for symptoms such as severe headache, stiff neck, or confusion.