On Thursday, more mosquitoes, birds and chickens found in East and Central Contra Costa County tested positive for the virus, Contra Costa County Mosquito and Vector Control District officials announced.
The discovery came on the heels of the county's two reported human cases of West Nile virus this year, affecting two female county residents.
Since 2005, 41 people in Contra Costa County have tested positive for the virus. In 2006, two county residents died from the disease.
The virus tends to cause mild, flu-like symptoms, if any at all, though about 1 percent of those infected experience severe symptoms ranging from high fever, headaches and disorientation to numbness and paralysis.
Mosquito and Vector Control District officials say the virus has hit the eastern part of the county the hardest this year. The latest batch of infected mosquitoes, birds and chicken were discovered in Antioch, Brentwood, Knightsen, Holland Tract and Concord, according to district officials.
Regardless of where they live, county health officials are focusing on getting residents to avoid contracting the virus by preventing mosquito bites and reporting dead birds in their area.
"Our daily surveillance efforts and the public's help in reporting dead birds highlight the areas we need to focus our efforts," district spokeswoman Deborah Bass said.
"The partnership between our District and the public is essential in reducing the risk of West Nile virus infection in our residents," she said.
Crews fogged an area of Antioch Wednesday for adult mosquitoes, but were not planning any other foggings as of today.
County officials continue to remind local residents to protect themselves and their homes against the virus by removing any standing water on or around their property and putting on mosquito repellent before heading outdoors.
More tips to help protect against West Nile virus can be found at www.cchealth.org or by calling (888) 959-9911.