BRENTWOOD -- County officials say the discovery of two more birds infected with West Nile virus this week has prompted a new round of spraying, planned for Thursday evening.

Across the county, the most recent results show two groups of infected mosquitoes in Brentwood, three chickens in the Holland Tract, and seven birds in Brentwood, Antioch, Pleasant Hill and Walnut Creek since Aug. 13. Four of the seven birds were found in Brentwood.

Officials from the Contra Costa Mosquito & Vector Control District said the latest group of mosquitoes found with the disease were a species that typically breeds in backyards. Most of the infected mosquitoes found this year are of a type that breeds underground.

The backyard species presents a dual problem, according to Deborah Bass, public affairs manager for the mosquito district. Backyard mosquitoes can breed in spaces as small as a snail shell and as large as a pool, and have a range several times larger than underground-breeding mosquitoes.

"We need the public's help" in eradicating standing water that can serve as a breeding space, Bass said. One neglected pool can produce up to 1 million mosquitoes, infecting people as far as 5 miles away.

While mosquito control workers can focus on the underground sources of water, with only 11 technicians, the district has no hope of finding every backyard mosquito habitat across the county, she said.


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Residents should dump or drain standing water on their property, and wear mosquito repellent when they are outdoors, especially at sunrise or sunset. Neglected swimming pools can be reported anonymously by calling 925-771-6195 or visiting www.ContraCostaMosquito.com.

West Nile virus can cause fever, headache, fatigue and other minor symptoms for several days to several weeks. About one in 150 people can develop more serious symptoms, including convulsions, muscle weakness, vision loss or paralysis. The virus is fatal in rare cases.

So far, there have been no humans infected with the virus this year in Contra Costa County.

Alameda County officials said Monday that a state lab had confirmed that a sparrow found in Castro Valley had the virus -- the second infected bird found in the county this year -- but there have been no human cases so far.

Nationwide, 2012 is the worst year for West Nile virus in the United States since the illness was first detected in 1999, according to the Centers for Disease Control. As of Tuesday, 1,118 cases had been reported in 47 states, with 41 fatalities from the disease.

The fogging will take place Thursday evening from about 8 to 11 p.m., using a mosquito-targeting insecticide called Pyrocide 7396. Pyrocide's active ingredient, called pyrethrins, are compounds derived from chrysanthemum seeds and are considered safe for pets and humans in low doses.

The fogging zone is bordered by Wilkins Way on the north, Sellers Avenue on the east, Armstrong Road and East Contra Costa irrigation canal to the south, and Walnut and Brentwood boulevards to the west.

Contact Daniel M. Jimenez at 510-262-2728. Follow him at Twitter.com/DMJreports.