Two dead birds in Alameda and Santa Clara counties have tested positive for West Nile virus, marking the first detection of the virus in the Bay Area this year.
The two confirmed cases were found in American crows — one in Newark, the other in San Jose — that were subjects in a broader monitoring system tracking the virus through bird deaths, according to a bulletin from the California Department of Public Health. West Nile often appears in birds before the human population.
“It’s a virus that mainly, or primarily affects birds,” Alameda County Mosquito Abatement District spokesperson Judy Pierce told SFGATE. “Dead birds are usually the first indicator that it’s circulating.”
The mosquito-borne virus has been established in California for decades and is now considered endemic, persisting at low levels in the environment and cycling between birds and mosquitoes, with horses and humans only incidentally becoming infected through mosquito bites.
“Humans are really getting impacted more on an accidental basis,” Shannon Bennett, chief of science and virologist at the California Academy of Sciences, told SFGATE. “We’re dead ends for the virus.”
There were no reported human cases and no virus found in the thousands of mosquito pools tested statewide so far in 2026, according to the bulletin. However, Bennett told SFGATE that human infections can occur incidentally when mosquitoes that have fed on infected birds bite people.
“Mosquitoes need water for their life cycle. For those two [first] stages, larva and pupa, they need to be in water,” Pierce said. “Mosquitoes need to swim before they can fly.”
Mosquito populations, which drive transmission, are closely tied to environmental conditions. Weather patterns like El Niño could influence how the season unfolds, Bennett noted.
Recent stretches of rain followed by warm temperatures can create ideal breeding conditions for mosquitoes, which rely on standing water to develop. Even short bursts of heat can speed up how quickly mosquitoes become infectious.
“It doesn’t even have to be an average warming temperature,” Bennett said. “If they can get a couple of really warm days mixed in there, that really does help support viral incubation and speed that whole [process] up.”
So far this year, more than 6,000 mosquito samples have been tested statewide with no West Nile–positive pools detected, indicating to researchers that the virus has not yet spread widely among mosquito populations.
West Nile virus in California is primarily transmitted by Culex mosquitoes, which are most active at night and breed in standing water. As mosquito activity picks up heading into summer, Bennett said residents should take simple precautions — especially during evening hours — such as draining stagnant water, wearing long sleeves and pants, and using insect repellent when needed.
