Seven birds and one mosquito group have tested positive for West Nile virus in Contra Costa County so far in 2026, the Contra Costa Mosquito and Vector Control District confirmed Saturday, Aug. 8. It's the highest level of virus activity in the county all year, and it arrives during a heat advisory that could push Walnut Creek temperatures into the low 100s Saturday and Sunday, Aug. 8–9.
The timing matters. Hot, stagnant conditions accelerate mosquito breeding, and the National Weather Service advisory covering East Bay interior valleys runs through 10 p.m. Sunday, Aug. 9. Residents spending more time outdoors in the heat face greater exposure to mosquitoes that carry the virus.
"Finding four West Nile virus-positive dead birds in El Cerrito is significant because we have not previously detected this level of activity in the area," said General Manager Paula Macedo.
Four of the seven infected birds were found in El Cerrito, which had recorded only one positive bird since the District first detected West Nile virus in the county, in 2023. District crews are now setting traps and treating known breeding sites in that area. So far, the traps have not collected infected mosquitoes, but surveillance continues.
What Walnut Creek residents should do
The District recommends three steps:
- Apply repellent at dawn and dusk, when virus-carrying mosquitoes are most active. Effective ingredients include DEET, Picaridin, Oil of Lemon Eucalyptus (repellent version) and IR3535.
- Dump standing water weekly. Walk your front and back yards and empty anything holding water. An unmaintained pool can produce more than 1 million mosquitoes, according to the District's services page, and those mosquitoes can travel up to five miles.
- Request a free inspection. County residents can call (925) 685-9301 or submit a request online. The service is funded through property taxes at no additional charge.
District spokesperson Nola Woods told the Mercury News on July 29, when the Bay Area's first human cases were reported in Santa Clara County, that even a water-bottle cap can hold enough water to breed mosquitoes.
Regional surge
West Nile virus activity is elevated across the Bay Area in summer 2026. Alameda County had 32 positive birds by late July, up from just six in all of 2025. Santa Clara County reported the region's first two confirmed human cases of 2026 on July 29. One person in Contra Costa County has died from the virus in the last two years, according to state data.
People over 60 and those with weakened immune systems face higher risk of serious illness. Less than 1% of those infected develop severe neurological symptoms, and there is no vaccine or specific treatment.
Residents can report dead birds to the California West Nile Virus Call Center at (877) 968-2473 or online at westnile.ca.gov. The District has not announced a date for its next surveillance update.
