Riverside County Vector Control Reports First West Nile Virus Detections

Riverside County Vector Control this month reported its first two confirmed detections of West Nile Virus in unincorporated Riverside County for the 2026 mosquito season.

“The first positive mosquito pool was identified during routine surveillance in Nuevo,” the department said in a news release. “The second detection was confirmed through laboratory testing of a dead bird found near Spencer’s Park in the unincorporated area of Murrieta.”

In response, the district said it was increasing surveillance efforts in Murrieta and would be conducting mosquito treatment in three areas of Nuevo, which was scheduled to take place last Tuesday.

Riverside County Director for Disease Control Barbara Cole said Riverside University Health System – Public Health was investigating two potential cases of West Nile Virus in people, but noted there have not been any confirmed cases this year.

“The final status of whether they’re confirmed or not is pending,” she said in an interview with The Riverside Record. “There’s a process and criteria that has to be met to call them confirmed, so right now they’re under review.”

West Nile Virus spreads through the bite of an infected female mosquito that acquired the virus from an infected bird. Although most people will experience mild or no symptoms, some individuals may develop a serious illness requiring immediate medical attention. Older adults and people with weakened immune systems are at greater risk for severe complications.

Last year, Cole said the county only had one confirmed case. Over the past several years, she said the number of confirmed cases has ranged between one and five. The typical season in Riverside County for vector-borne diseases, which are human illnesses spread by vectors like mosquitos, ticks and fleas, generally runs from about May to October.

“Some seasons, we have seen a few cases earlier, like in April,” she said. “And in some seasons we’ve seen cases later in the year, like November.”

To reduce the risk of contracting West Nile Virus, RUHS-Public Health recommends draining standing water weekly, using EPA-registered mosquito repellent when outdoors, wearing long sleeves, pants and light-colored clothing during peak mosquito times, installing or repairing screens on windows and doors, avoiding activities at dawn or dusk when possible, reporting dead birds to local vector control agencies and supporting local mosquito control efforts.

Cole said her department keeps in close communication with the county’s vector control agencies to ensure they’re aware of any potential cases so they can assess whether there’s an environmental concern, like a mosquito pool, that needs to be addressed.

Riverside County Vector Control is just one of multiple agencies providing vector control services throughout the county, serving mid-western Riverside County and the Palo Verde Valley Area outside of Blythe. It also provides services to the contract cities of Banning, Beaumont, Calimesa, Hemet, Menifee, Moreno Valley, Murrieta, Perris, San Jacinto, Temecula and Wildomar.

The Northwest Mosquito Vector Control District, which provides services to the northwest area of the county including Canyon Lake, Corona, Eastvale, Jurupa Valley, Lake Elsinore, Norco and Riverside, reported its first detection in July after a dead bird was found in Eastvale

“While this is the first local detection this season, West Nile virus is endemic in California, meaning it is established throughout the state and returns each year,” the agency said in a release announcing the detection. “West Nile virus-positive mosquito samples, infected birds, and human cases have already been reported in other parts of California.”

The district said it would increase mosquito surveillance in the area, which would help guide any additional mosquito control measures if necessary.

The Coachella Valley Mosquito and Vector Control District, which serves the desert areas of the county from Palm Springs to Chiriaco Summit and including the Salton Sea, reported its first detection back in May. The samples were collected in the community of North Shore near the intersection of Avenue 72 and Cleveland Street

“While no human cases have been reported at this time, the detection serves as an important reminder that West Nile virus activity is endemic in the region and that preventive action is critical during the warmer months when mosquito activity increases,” the district said in a release announcing the detection.

As a result of the detection the district increased surveillance and mosquito control efforts in the surrounding area, including the use of targeted larval treatments and adult mosquito monitoring.