BRENTWOOD -- The discovery of three dead birds and a sample group of mosquitoes that tested positive for West Nile virus has prompted a new round of spraying, scheduled for Thursday night, officials said.

The announcement comes only days after Contra Costa County's first human case of West Nile in 2012 was confirmed by a state lab. The Central County woman who has the disease has only mild symptoms and is expected to make a full recovery, officials said.

Two infected crows were found this week in Brentwood, along with a sample group of mosquitoes that were found to have the virus, according to the Contra Costa Mosquito and Vector Control District. One additional infected bird was found in Concord.

The

district plans to fog with trucks Thursday night in Brentwood and Discovery Bay. In central Brentwood, trucks will spray a pesticide known as Pyrocide in an area bordered by Sand Creek Parkway to the north, Sellers Avenue on the east, Balfour Road on the south, and Brentwood Boulevard on the west.

In Discovery Bay, fogging will take place in the area bordered by Marsh Creek Road on the north, Bixler Road on the east, Highway 4 on the south and Coral Street and Anchor Court on the west.

Spraying in both areas will take place from dusk until about 11 p.m., weather permitting.

Pyrocide's active ingredient, called pyrethrins, are compounds derived from chrysanthemum seeds and are considered safe for pets and humans in low

doses.

West Nile virus can cause fever, headache, fatigue and other minor symptoms, and in rarer cases, more serious symptoms, including convulsions, muscle weakness, vision loss paralysis, or death.

There were three human cases of West Nile virus in Contra Costa County in 2011. There have been no fatalities in the county since 2006.

Staff writer Samuel Richards contributed to this story. Contact Daniel M. Jimenez at 510-262-2728. Follow him at Twitter.com/DMJreports.