With West Nile virus outbreaks at record levels across the country and warning signs popping up in the Bay Area, some counties are stepping up efforts to control virus-carrying mosquitoes.

In Contra Costa and Santa Clara counties, some cities were fogged with pesticide in recent weeks to combat a swell in the number of adult mosquitoes found carrying the virus. Officials are also treating water in catch basins under sidewalk storm drains, which collect water as residents water lawns or wash cars, creating a fertile breeding ground for swarms.

"Year to date, we're seeing four to five times as many mosquitoes carrying the virus as we have in any recent years," said Deborah Bass, spokeswoman for the Contra Costa Mosquito and Vector Control District.

Members of the Contra Costa Mosquito & Vector Control District park along Fulton Shipyard Road on Aug. 15, 2012, in Antioch.
Members of the Contra Costa Mosquito & Vector Control District park along Fulton Shipyard Road on Aug. 15, 2012, in Antioch. (Susan Tripp Pollard/Staff)

The agency tracks local outbreaks and has fogged or planned fogging in parts of Byron, Brentwood and Antioch with pyrethrin, a chemical derived from chrysanthemum flowers and deemed in low doses to pose minimal risks to human health and the environment. Mosquitoes carrying the virus were discovered in Pleasant Hill last week, but no fogging is scheduled for that city.

Neglected pools have been a growing concern since the national foreclosure crisis began in 2007, as thousands of former homeowners abandoned their property, said Russ Parman, manager of the Santa Clara district. He said Santa Clara officials have found more than 4,000 such pools since the foreclosure wave began, and in almost a fifth of those mosquitoes were already breeding.


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Birds are also a big concern, Parman added -- crows, magpies and jays in particular.

"Birds, when they get sick with it, make lots and lots of virus in their bloodstream, so that just about every mosquito that lands on that bird becomes a carrier," Parman said.

Nationwide, 693 human cases have been reported this year, the highest annual number reported through the second week of August since West Nile virus was first detected in the United States in 1999, according to the Centers for Disease Control and Prevention.

Outbreaks have been worst by far in Texas, where the mayor of Dallas declared a state of emergency Wednesday and called for an aerial spraying of the entire city.

In California, the numbers are highest in Sacramento and Kern counties. Statewide, 23 human cases of the virus have been reported in 2012, and none of them have been in the Bay Area. But local experts say that's to be expected. Seasonally, humans in the Bay Area are most at risk to contract the virus in August and September, and discovery of a person catching it can be delayed as much as a month after that.

The weather this year has likely played a big role in the increased number of cases, said the Contra Costa district's lab director, Steve Schutz. A mild winter allowed more mosquitoes than usual to survive, and a dry, hot summer has residents watering their plants and keeping water in their pools more often, creating prime breeding grounds for the insects, he added.

Most of the Bay Area's risk has historically been seen in Contra Costa, where three to nine human cases are typically reported each year, and where two people died of the virus in 2006. Risks are growing elsewhere, however. The Santa Clara district found its first sample of mosquitoes carrying the virus this year in Los Altos on Aug. 1. The Alameda County district reported Friday that it discovered its first virus-infected dead bird of the year.

Samples of the virus have also been found in birds and mosquitoes in San Mateo, Solano and Sonoma counties.

The virus is normally not fatal but is still to be taken seriously, Schutz said.

"There is a minority of cases where it can cause extremely severe symptoms, including permanent paralysis and other disabilities," Schutz said. "It's not something to be taken lightly. Generally, it's most dangerous for older people, or those with weaker immune systems."

One or two mosquito bites are probably not dangerous, as even in swarms where the virus is present, few individual mosquitoes are usually carrying it, Schutz said.

A person who has contracted the virus might first notice fairly typical flu symptoms, such as fever, muscle aches and chills, he added. In seeing a doctor for those symptoms, he said, patients should tell their doctors if they've been bitten recently.

The breeds of mosquito most likely to carry it tend to be most active around dawn and dusk, he added. Residents who will be outdoors in those hours can protect themselves by wearing mosquito repellent that includes the chemicals DEET, picaridin or oil of lemon eucalyptus.

The latter, especially, is effective for several hours, Schutz said.

To help prevent the spread of mosquitoes, Bass said, residents are encouraged to be careful of leaving standing water on their property, such as in out-of-use swimming pools or dishes under potted plants.

"It's not a question of the volume of water," Bass said. "Mosquitoes breathe at the surface, so that's what we are concerned about. You can literally find a million mosquito larvae in, surprisingly, a partially-filled neglected swimming pool."

State officials are tracking the spread of the virus in part through testing for it in dead birds. Residents who spot a dead bird are encouraged to report it by calling 877-968-2473. Details are at http://westnile.ca.gov.

Additionally, Parman called on residents to watch for Asian tiger mosquitoes, which are smaller than common mosquitoes and colored black with white striping.

The breed has thrived in Southern California but has not taken hold in the Bay Area, he said.

"If it gets here, it will be a game changer," Parman said. "These are day biters, not just active at dawn and dusk, and they can breed in a bottle cap or a thimble of water. So we're wanting early detection of any 'hitchhiker mosquitoes' that come up here in cars and trucks."

Contact Sean Maher at 925-943-8013. Follow him at Twitter.com/oneseanmaher.