West Nile virus cases surge in US, prompting mosquito warnings from Arizona to California and Minnesota

Health officials in Arizona’s Maricopa County reported 25 locally acquired West Nile virus cases and two deaths through June 18, 2026—more than double the same period last year and the first fatalities recorded in the county. The county also noted detection of virus‑positive mosquito pools and the presence of St. Louis encephalitis‑positive mosquitoes. Nearby Pima County confirmed its first human case of the season.

In California, a dead crow in Menlo Park tested positive for West Nile virus, marking the first animal detection in San Mateo County for 2026. The San Mateo County Mosquito and Vector Control District is expanding traps to monitor mosquito infection. In the San Gabriel Valley, a mosquito caught in Alhambra tested positive, and Long Beach recorded its first human case, a neuroinvasive illness that required hospitalization. "West Nile virus is a recurring concern in our region because it is endemic to our area," said Tristan Hallum, director of the San Gabriel Valley Mosquito and Vector Control District.

Minnesota’s Metropolitan Mosquito Control District detected West Nile‑positive mosquitoes in three counties on June 17, the first such findings of the year, though no human cases have been linked yet. Officials noted that the massive 2025 outbreak—over 120 cases—was driven by high Culex tarsalis populations, which are currently lower due to below‑average rainfall.

Additionally, mosquito surveillance in three Las Vegas ZIP codes identified West Nile‑positive pools, prompting public warnings about eliminating standing water and using EPA‑registered repellents.