CONTRA COSTA COUNTY, CALIFORNIA – West Nile virus is again circulating in Contra Costa County, with local officials confirming the first infected mosquitoes and bird of the year in the 925.
The Contra Costa Mosquito and Vector Control District recently found a West Nile-positive mosquito sample near Pacheco Marsh along the Martinez waterfront. Days later, officials confirmed that a young American crow from El Cerrito had also tested positive for the virus.
Together, the two findings show that West Nile virus is active in the local bird-and-mosquito cycle that can eventually put people at risk. The district’s 2026 surveillance map now lists the Martinez mosquito sample.
That does not mean every mosquito in Contra Costa County is infected, nor does it mean residents should avoid summer evenings outdoors. But it is an early-season reminder that the familiar backyard pest can carry a genuinely serious disease.
West Nile virus is primarily spread when an infected mosquito bites a person after feeding on an infected bird. Most people who contract the virus never develop symptoms.
About one in five experience fever, headache, body aches, rash, vomiting, or diarrhea, according to the Centers for Disease Control and Prevention. In rare cases, however, the illness can affect the brain or nervous system and lead to hospitalization, paralysis, or death. The risk of severe illness rises with age and for people with certain underlying health conditions.
The mosquito district says West Nile virus is commonly detected in local bird and mosquito populations during the summer and can remain a concern until cooler weather arrives in the fall.
The most important thing residents can do
There’s a simple way to stop West Nile: remove places that mosquitos can breed.
Mosquitoes can develop in very small amounts of standing water, including plant saucers, clogged gutters, buckets, outdoor toys, birdbaths, neglected pools, and tarps that collect rainwater. Residents should walk through their yards at least once a week, dump out standing water, and scrub containers before refilling them.
Officials also recommend using an EPA-registered insect repellent when spending time outside, especially around the evening hours. Products containing DEET, picaridin, oil of lemon eucalyptus, or IR3535 are among the active ingredients recommended by the district.
Keeping window screens in good condition, wearing longer sleeves and pants when mosquitoes are especially active, and making sure pools and spas are properly maintained can also make a meaningful difference.
Contra Costa County residents can contact the mosquito district for help with mosquito problems, including neglected swimming pools and persistent breeding areas. The agency says its services are available countywide and funded through local property taxes. There’s a whole website for mosquito control locally.
We even have anti-mosquito drones!
It’s important to have perspective, too. With our strong, local vector control agencies and dry Summer weather, mosquitos aren’t as much of an issue here as in many parts of the country.
Many of our staff hail from the East Coast, where wet summers make mosquitos a daily problem. Here in the West, they’re something to be mindful of, but not a reason to avoid eating or walking outside in the evenings.
We’ll keep following West Nile in the 925. Make sure to join our free 925 News newsletter so we can provide updates.
