Mosquitoes trapped earlier this week near Byron tested positive for West Nile virus, Contra Costa County vector control officials said Friday.
The infected mosquitoes are the first sign of the virus so far this year in the county, where August and September are usually the peak months of West Nile cases in humans, according to the Contra Costa Mosquito and Vector Control District.
Residents are urged to prevent the insects from breeding by dumping and draining any standing water on their property and to report neighborhood mosquito issues including neglected swimming pools.
Using repellents during peak mosquito activity - dawn and dusk - is also recommended.
Officials are asking residents to report sightings of dead birds to the statewide West Nile Virus hotline online or by calling (877) WNV-BIRD (968-2473).
"Dead birds, especially crows and jays, are often a good early indication that West Nile virus is present in a particular area. Birds can be carriers of West Nile virus," said Steve Schutz, the district's scientific programs manager.