UPDATE NOON WEDNESDAY, AUG. 9, 2006: The Sacramento-Yolo Mosquito and Vector Control District carried out aerial spraying for mosquitoes Tuesday night in Davis and Woodland, and planned a repeat application for 8 p.m. to midnight today (Aug. 9).

Wind speed must be 10 mph or less to allow the spraying to go forward.

This morning, Yolo County Public Health Officer Bette Hinton said the county had confirmed seven cases of West Nile virus in people this season. All of the victims are adults: six from Davis and one from Woodland.

People can contract West Nile virus when bitten by infected mosquitoes. The disease can be fatal in people, though most who are infected do not develop any disease at all, state and federal health authorities say. According to the California Department of Health Services, up to 20 percent of people, about one in five, will show mild symptoms such as fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands, or a rash on the chest, stomach and back.

About 80 percent of people who are infected will not have any symptoms, state health authorities say.

In severe cases, affecting less than 1 percent of people who are infected -- about one in 150 -- symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

Among severe cases, fatality rates range from 3 percent to 15 percent, with the highest rate applying to elderly people, according to the U.S. Centers for Disease Control and Prevention.

Yolo County reported 12 human infections and no deaths last year, while Sacramento County reported the most infections in the state, 177, and one death among 19 statewide.

This year, the virus seems to have hit harder in Yolo than in Sacramento. As of July 31, the district had confirmed West Nile virus in 54 samples of multiple mosquitoes trapped in Yolo and Sacramento counties -- with 28 of the positive samples from Yolo County. Also, the district had detected West Nile virus in 26 dead birds and six sentinel chickens, but no horses.

"I'm anxious to see the spraying done, to get this nipped in the bud," Hinton said Friday.

She said the climbing human infection count, particularly in Davis, given the mosquito population is highly infected, and given the city's "highly young and highly outdoor population."

The people now testing positive probably were infected up to two weeks ago, Hinton said, because the virus can take that long to incubate. So, "we still have another two weeks to go" for more infection to turn up in people, "if we spray tonight."

Hinton joined David Brown, the mosquito control district's manager, at district headquarters in Elk Grove on July 31 to announce the spraying over Davis and Woodland.

Brown and Hinton acknowledged that nothing is perfectly safe, but said the risk is minimal from the insecticide being sprayed over Davis and Woodland. Pyrethrins, derived from chrysanthemums, are the active ingredient in the insecticide: EverGreen Crop Protection EC 60-6.

The risk of the disease is "hugely larger" than any risk of the insecticide, said Hinton, answering public criticism of the spraying. "It's a pretty rare person who would have any kind of reaction to this product."

Brown said the insecticide applied to each acre amounts to 0.66 ounce, less than a packet of Sweet'n Low. The application process involves atomizing the insecticide into billions of droplets.

The spray zone encompasses about 15,000 acres in Davis and 13,000 in Woodland.

Brown said the onboard computers that run the aerial drops are programmed to help keep the spray from hitting certified organic farms. Gary Sandy, director of Local Government Relations for UC Davis, said the Yolo County agricultural commissioner had identified one certified organic plot on campus -- the Student Farm.

Sandy said the university hoped to keep the campus's other organic plots out of the spray area, and Brown said the district is working with UC Davis "to make sure we don't compromise any research projects."

The mosquito control district advises people to stay indoors during spraying.

Officials also are advising people not to wash off the spray. However, Brown advised people to wash produce from their gardens -- which is something he said people should do anyway.

People need not turn off air conditioning units, because they draw air from inside a house, not outside.

Ground spraying already is taking place in urban Davis and Woodland, and aerial spraying is taking place in Yolo County's rural territory.

"We are trying to reduce any further amplification" of West Nile virus in the mosquito population, Brown said.

He and Hinton urged people to avoid venturing outdoors at dawn and dusk, when mosquitoes are most active. And people who do go outside should put on mosquito repellent.

"However uncomfortable they might feel, people really need to do that," Hinton said. "It works very well."

The mosquito control district's news releases regarding the Davis-Woodland aerial spraying are available online, http://www.fightthebite.net/mediacorner/releases.php.

UC Davis' Aug. 1 directive on the spraying operation is available at http://directives.ucdavis.edu/2006/06-081.cfm.