The discovery of the West Nile virus in bird and mosquito samples showed that the spread of the virus is not cooling down with the onset of fall, and experts said the early October heat wave didn't help.

Five more birds -- one each in Brentwood, Concord, Discovery Bay, Pleasant Hill and Walnut Creek -- tested positive for the virus, as did a chicken from a sentinel flock in Oakley, the Contra Costa Mosquito and Vector Control District announced Thursday. The findings came on the heels of a heat wave over the past five days that lifted the temperatures into the high 90s and low 100s in parts of the East Bay.

"The mosquitoes that can transmit West Nile virus thrive in warm weather, because higher temperatures allow them to complete their life cycles more quickly," said Steve Schutz, the District's Scientific Program's Manager, in a statement. "Higher temperatures also allow the virus to grow rapidly, increasing the chances that birds or mosquitoes can become infected."

Two Contra Costa residents have tested positive for the virus this year, as have 56 dead birds, 19 groups or mosquitoes and seven chickens. The virus often leads to flu-like symptoms and develops into a severe illness in less than one percent of individuals, the district said.

Thirty-nine people have tested positive for the virus since 2005, but nobody in Contra Costa County has died from the virus since two deaths were reported in 2006, according to the district.

Rick Hurd covers public safety. Contact him at 925-945-4780 and follow him at Twitter.com/3rdERH.