San Diego County's Worst West Nile Virus Season in Years Triggers Mosquito Spraying

San Diego County went through all of 2025 without a single confirmed human West Nile virus case. By August 7, 2026, the county's public health department had already confirmed four cases since early June, one of them locally acquired. By late October, that count had climbed to 11 — a dramatic swing for a virus that, across the entire 2023-2025 period combined, had produced only two human cases county-wide. The response has scaled up accordingly, most visibly with repeated overnight truck-mounted pesticide spraying across the city of Coronado.

Why Coronado specifically

Three of the county's 11 confirmed 2026 cases were Coronado residents, and county vector control identified West Nile virus in mosquito pools at multiple locations in the city in early September, prompting letters to roughly 1,470 residents in the affected area. The response that followed was an ultra-low-volume pesticide mist, applied by truck overnight across roughly 6,000 lots in Coronado in late September, targeting the specific areas where mosquito testing had come back positive. County officials, speaking to KPBS in late October, described this as an ongoing effort rather than a single completed treatment — vector control keeps responding because WNV-positive mosquito pools keep turning up, not because of one bad week.

Truck-mounted spraying wasn't the only tool in use. The county's separate aerial larvicide program ran a series of drops across the broader region through the 2026 mosquito season, roughly every three to four weeks from April through October, targeting mosquito larvae before they reach biting age rather than adult mosquitoes already capable of spreading the virus. Running both programs together — larvicide to reduce the next generation and adult-targeted ULV spraying where active virus has already been detected — reflects a season where neither measure on its own was keeping pace with how much mosquito and virus activity the county kept finding.

How West Nile virus actually spreads

West Nile virus moves through a bird-mosquito-bird cycle: Culex mosquitoes pick up the virus feeding on infected birds, and it builds up in that loop before a mosquito carrying it happens to bite a person. People are a dead-end host — infected individuals can't pass the virus back to a mosquito at a level that sustains further spread, which is also why it doesn't spread directly between people. Most people infected never develop noticeable symptoms, which is part of why confirmed case counts like San Diego's 11 almost certainly understate the true number of infections circulating in the mosquito and bird population locally.

San Diego County's own historical data gives useful context for how serious an active season can get if it isn't brought under control. The county's worst recorded year was 2015, with 44 human cases and 6 deaths; across the full 2015-2024 period, the county logged 82 cases, with 71% classified as the more severe neuroinvasive form of the disease, 78% requiring hospitalization, and 9 deaths total. Eleven cases by late October 2026 is well short of that 2015 peak, but it's a sharp enough jump from zero cases the year before to explain why the county's response escalated as quickly as it did.

• San Diego County had zero human West Nile virus cases in all of 2025; by late October 2026, the count had reached 11, with four confirmed as early as August 7.
• Three of the 2026 cases were Coronado residents, after the virus was detected in mosquito pools at multiple locations in the city in early September.
• The county ran truck-mounted ultra-low-volume pesticide spraying across roughly 6,000 Coronado lots overnight in late September, targeting areas with confirmed positive mosquito pools.
• Aerial larvicide drops ran roughly every 3-4 weeks from April through October across the broader region, targeting mosquito larvae before they reach biting age.
• The county's worst historical year, 2015, saw 44 human cases and 6 deaths — a reminder of how quickly an active season can escalate if mosquito populations aren't brought under control.

West Nile virus, like most mosquito-borne diseases, ultimately traces back to standing water and the mosquito populations it supports — reducing breeding sites around a property is the same practical response whether the local concern is West Nile virus, dengue, or something else entirely. Sevacorp offers a free inspection for anyone wanting a closer look at what's actually breeding mosquitoes on a property.

• Sources:
• KPBS — "Do San Diego County mosquito treatments work? Officials say it's an ongoing effort" (Oct. 27, 2026) — https://www.kpbs.org/news/health/2026/10/27/do-san-diego-county-mosquito-treatments-work-officials-say-its-an-ongoing-effort
• FOX 5 San Diego / KUSI News — "Coronado mosquito spraying" — https://fox5sandiego.com/news/local-news/coronado/coronado-mosquito-spraying/
• San Diego County Health & Human Services Agency — CAHAN public health alert (Aug. 7, 2026) — https://www.sandiegocounty.gov/content/dam/sdc/hhsa/programs/phs/cahan/communications_documents/8-7-2026.pdf
• San Diego County — West Nile Virus aerial mosquito treatment program page — https://www.sandiegocounty.gov/content/sdc/deh/pests/treatment_info/AdultMosquitoTreatmentInformation.html
• San Diego County — West Nile virus epidemiology and historical case data — https://www.sandiegocounty.gov/content/sdc/hhsa/programs/phs/community_epidemiology/dc/wnv.html
• Coronado Times — on West Nile virus detections at Coronado Golf Course and other city sites prompting letters to 1,470 residents (Sept. 3, 2026) — https://coronadotimes.com/?p=258455