A dead bird collected in Eastvale has tested positive for West Nile virus, marking the first detection of the season within the Northwest Mosquito and Vector Control District's service area. The finding is pushing local officials to widen surveillance and remind residents of basic precautions as mosquito activity lingers into the fall.
The bird was tested by the UC Davis Center for Vector-borne Diseases, according to the Northwest Mosquito and Vector Control District. The district said that as of its announcement, no mosquito samples collected within its coverage area had tested positive for the virus, even though the dead bird came back positive. In response, crews have placed additional mosquito traps within a half-mile to one-mile radius of where the bird was found, stepping up monitoring in that part of the city.
How Mosquitoes Pick Up and Spread the Virus
West Nile virus is most commonly spread to people through the bite of an infected mosquito, the district explained. Mosquitoes become carriers after feeding on an infected bird, and once infected, they can pass the virus on to both animals and humans. Eastvale sits within the northwest portion of Riverside County, an area spanning more than 240 square miles that the district covers, per the City of Eastvale's description of the arrangement on its website.
Health officials have repeatedly flagged older adults and people with weakened immune systems as those facing the greatest risk of serious neurological illness from the virus, a point the district reiterated in its announcement. Symptoms can include fever, headache, nausea, body aches, skin rashes and swollen lymph nodes, though many infected people never develop noticeable symptoms at all.
Spraying Planned After a Weather Delay
Beyond the trapping effort, MyNewsLA.com reports that the district has scheduled ultra-low-volume insecticide treatments in Eastvale, with crews set to spray between 3 a.m. and 6 a.m. Friday in a residential area bounded by Scholar Way to the east, Wells Springs Street to the west, Oakdale Street to the south and Schleisman Road to the north. The outlet reports the treatment is meant to respond to the West Nile virus presence detected in the area.
The spraying was originally planned for last Friday but was postponed after strong Santa Ana winds lasted two days, according to the same report. The pesticides used are approved by the U.S. Environmental Protection Agency and are dispersed as a fine mist from machines mounted in the beds of pickup trucks. Officials recommend that residents stay indoors during the spraying and keep windows closed until at least 15 minutes after the trucks have moved through.
In 2024, Riverside County Vector Control scheduled a ground application across three Nuevo neighborhoods covering roughly 49.5, 172 and 77 acres after positive mosquito test results, using the EPA-approved product Zenivex E4 RTU and advising residents to stay inside during and for 15 minutes after the application, according to Riverside County.
A Tougher Mosquito Season Across Southern California
Riverside County's broader numbers underscore why the Eastvale finding matters. The county documented four human West Nile virus infections so far in 2026, compared with a single infection in 2025 whose patient recovered, per the MyNewsLA report. Statewide, the California Department of Public Health has reported 378 West Nile virus infections in 2026, including Riverside County cases, with 23 fatal outcomes recorded — though none of those deaths occurred in the Inland Empire, the outlet notes.
Extreme heat has played a role in accelerating mosquito breeding across the region this year. Intense heat shrank breeding cycles in half, according to ABC7, which reported that the combination of heat and humidity drove a spike in mosquito activity even without rainfall. That dynamic prompted emergency mosquito-control treatments in Altadena and Monterey Park, where crews used ultra-low-volume adulticide alongside low-volume larvicide to target mosquitoes at multiple stages of their life cycle.
Orange County has seen an even sharper jump. Vector control officials there reported 718 mosquito nets testing positive for West Nile virus in 2026, compared with just 70 at the same point in 2025, according to CBS News Los Angeles. At least 18 people in the county tested positive for the virus so far this year, compared with zero cases over the same period in 2025. Four vector-control trucks were scheduled to spray a pesticide — applied at roughly two tablespoons per acre — across Anaheim, Costa Mesa and Garden Grove, the outlet reported.
What Residents Can Do Now
Southern California's mosquito season generally runs from May through October, meaning the window for risk hasn't fully closed even as the weather cools. Health officials continue to recommend limiting outdoor activity at dawn and dusk, when mosquitoes are most active, and wearing long sleeves and pants in mosquito-prone areas.
Residents are also urged to use insect repellent, make sure door and window screens are properly fitted, and remove any standing water around their homes, with the exception of properly treated pools. Questions about the Eastvale treatment or general mosquito concerns can be directed to the Northwest Mosquito and Vector Control District through its website or by phone at 951-340-9792.
