Mosquito sample with West Nile virus found in Oakley, Calif.

West Nile virus is a potentially deadly illness that is spread through mosquitoes. Its symptoms in humans are often mild, ranging from fever to diarrhea, body pains and vomiting. Birds are also frequent victims and may carry the virus.

Workers collected the sample this week in the area of West Cypress Road and O'Hara Avenue, said Deborah Bass, who is with the county Mosquito and Vector Control District.

A dead crow infected with the virus was found July 1 near Treat Boulevard and Bancroft Road in Walnut Creek, Calif., the vector district's website said. But no mosquito sample was discovered in that incident, Bass said.

"Dead birds can provide a reservoir for the virus," she said. The latest find "came after the discovery of a dead bird led to a search of mosquitoes in the area."

Late summer generally is when mosquitoes with West Nile virus are found, Bass said. But the milder summer weather has slowed the spread of the disease because mosquitoes multiply faster in high temperatures, she said.

To reduce the risk of West Nile virus, Bass urged people not to let water sit in potted plants or to leave ponds or swimming pools untreated. One neglected pool can produce more than a million mosquitoes and affect a five-mile radius, she said.

"Unfortunately, with the many foreclosures, you have many pools that have standing water," she said.

West Nile virus originated in East Africa in 1937 and arrived in the United States in 1999. It first appeared in California in 2003 and can be found in all 58 of the state's counties, according to the state Department of Public Health.

According to the mosquito district, 23 people in Contra Costa County have been diagnosed with West Nile virus since 2006, including two who died from the disease that year.

In California overall there have been six deaths from the disease.

(c) 2011, Contra Costa Times (Walnut Creek, Calif.).
Distributed by McClatchy-Tribune Information Services.