West Nile virus was detected in a California scrub jay in Contra Costa County, marking the first positive bird for 2025. Infected birds indicate the virus's presence, with cases also noted in Alameda and Santa Clara counties. As warmer weather approaches, the risk of human infection increases. The CDC states West Nile virus is the leading mosquito-borne disease in the U.S. There is no vaccine or specific treatment, and symptoms vary among individuals. Three human cases were reported in California this year, including one fatality.
According to the U.S. Centers for Disease Control and Prevention, West Nile virus is the leading cause of mosquito-borne disease in the United States. Mosquitoes can spread the virus to humans after feeding on birds that carry the contagion.
According to the CDC, only about 1 out of 150 infected people develop a serious, sometimes fatal, illness. People older than age 60 and those with weakened immune systems are more likely to get seriously sick.
We typically start seeing West Nile-positive birds and mosquitoes in Contra Costa County around this time of year. Our neighboring counties have been reporting positive samples for several weeks now.
There are no vaccines to prevent or medicines to treat West Nile in people, the CDC said, but most infected people do not feel sick and only about 1 in 5 people develop a fever and other symptoms.
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