A dead crow that tested positive for West Nile virus in Menlo Park recently prompted alarm amid a spike of infected birds across California
West Nile has only sickened two Californians so far in 2026, one in Long Beach and another in Sacramento County, but it’s just the start of the typical summer-to-fall season for the illness. Signaling this year could be worse for residents, the number of infected birds — the primary animals that host the mosquito-spread disease — has already surpassed those recorded for all of 2025 statewide
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Nationally, the Centers for Disease Control and Prevention warned of a “surge” of West Nile on July 1, reporting the highest number of human cases by this time of year since 2004
“The bird is a wake-up call to West Nile season starting,” Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, told SFGATE about the Menlo Park crow. “Seeing the birds is the beginning, then you start seeing the mosquitoes and then you start seeing humans.”
West Nile is the most common and serious mosquito-borne disease in California, leading to thousands of human cases and killing more than 400 people since it turned up in the state in 2003. Although most people with West Nile virus remain asymptomatic, about one in five people develop flu-like symptoms. In rare cases, people develop a severe illness that affects the central nervous system. There is no vaccine
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West Nile had an early peak in California in 2005 — when 21 residents died out of almost 900 recorded human cases. Since then, the numbers have mostly trended down but fluctuated year over year. In 2025, 11 people died out of the 117 human cases; those numbers corresponded with 154 dead birds and almost 3,000 sampled mosquitoes. In 2026, the state has already recorded 166 dead birds.
The Long Beach Department of Health and Human Services reported the first symptomatic human case statewide on June 26, sharing that “the person was hospitalized with neuroinvasive illness and is currently recovering at home.” Sacramento County Public Health and the Sacramento-Yolo Mosquito & Vector Control District recently reported the second case, a woman in her 60s who is “recovering.”
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“When West Nile virus was first described in the U.S. and now every year, clusters of dead birds are early warning signs of increased activity,” Dr. Monica Gandhi, infectious disease expert and professor of medicine at UCSF, wrote to SFGATE. “So, just because there hasn’t been an increase in human cases yet, we have to be very vigilant to any dead bird with the virus and monitor for other birds in the area and human cases.”
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Across the U.S., the CDC reported 48 cases by June 30, including 38 cases of people with severe neuroinvasive disease. That’s many more people than the average of only 10 human disease cases recorded by the end of June since 2004. “The [CDC] is seeing the earliest start to the West Nile virus (WNV) season in the United States with the highest number of human disease cases reported by this time of year since 2004,” a news release stated on July 1. “… This surge is driven by early circulation of the virus, with 23 states reporting West Nile virus activity — also the highest number recorded over the last 10 years.”
Although the CDC did not offer a reason for the surge, Chin-Hong speculated that weather patterns are likely playing a role. “When you have higher temperatures, you get more replication of the virus in mosquitoes and also more mosquitoes,” he said. “And second, when you have more rain, then you have more breeding places for mosquitoes. … In California, you have less water in the summer, and then the mosquitoes and the birds come around to feed in the limited water supply areas.”
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On the San Francisco Peninsula, the San Mateo County Mosquito and Vector Control District responded to the dead crow incident in Menlo Park’s popular Nealon Park. It’s not the first time the district has found an infected bird, though none tested positive last year. Although the crow could have flown some distance before dying in Nealon Park, the district investigated the area
“We set additional mosquito traps in the nearby area where the bird was reported, tested those mosquitoes and did not find any evidence of West Nile virus in the mosquitoes in that area,” Rachel Curtis-Robles, a spokesperson for the district, told SFGATE. “So that’s good news, because it means that we didn’t detect the risk for human infection there. But we always do want to remind the public that West Nile virus is a threat.”
The district has recommended precautions, including wearing long sleeves and pants in the early mornings, evenings and nights when the mosquitoes are out. They also advise removing standing water, using insect repellent and keeping mosquitoes out of homes with window and door screens. The district also encourages California residents to report biting mosquitoes and dead birds
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