CONTRA COSTA COUNTY (BCN) — Two groups of mosquitoes found in the areas of Bethel Island and Discovery Bay have tested positive for West Nile virus, as did a dead bird found recently in Concord, Contra Costa Mosquito & Vector Control District officials said on Thursday.

Fogging from hand-held and truck-mounted sprayers to control the mosquitoes is taking place from 7:30 p.m. to 11 p.m. Thursday in areas including Stone Road in Bethel Island and Wells Lane and Sandmound Boulevard in Oakley, according to the district.

“The risk of getting West Nile virus is elevated when it’s found in mosquitoes,” Steve Schutz, the district’s scientific program manager, said in a statement. “Mosquitoes can transmit the disease through a simple bite — and it only takes one.”

Residents are encouraged to help reduce the risk of contracting the virus by dumping or draining standing water where the insects breed, using mosquito repellents containing DEET, picaridin or oil of lemon eucalyptus, and avoiding the outdoors during dawn and dusk when mosquitoes are typically present.

A map of the area being sprayed tonight and more information about the operation can be found on the district’s website at http://contracostamosquito.com.

Since 2005, 55 people in Contra Costa County have been diagnosed with West Nile virus, which can cause symptoms including fever, headache, body aches, and in rare cases, neurological damage or death.