Sacramento County mosquito control officials plan to launch a mass aerial spraying over thousands of homes north of the American River on Monday night to slow the rate of West Nile virus infections, now the highest in California, officials said Thursday.
The 71,000 acres targeted would be the first urban area in California to be sprayed from the air in more than three years of combating the virus, vector control officials said.
Officials said they decided to spray insecticide from planes because the spread of infected mosquitoes has outpaced ground-level extermination.
Dave Brown, general manager of the Sacramento-Yolo Mosquito and Vector Control District, called it a measure of last resort.
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"We're in the middle of a potential epidemic. We want to stop it," Brown said.
State health officials and toxicologists said the insecticide is benign to humans and mammals at the low concentrations that will be used. The active ingredients are pyrethrins, natural insecticides produced by chrysanthemum flowers, that readily degrade in sunlight.
"Nothing is innocuous, but this is about as innocuous as you can get," said Bruce Hammock, UC Davis toxicologist and pesticide expert.
Residents in the targeted area do not need to take special precautions for themselves or pets during the spraying, which is scheduled for 8:30 to 11 p.m. Monday, weather permitting.
The number of reported cases of Sacramento County residents infected with the virus rose to 21 on Thursday, the most of any California county, state health records show. Last year's toll was 3.
Brown said some traps in northern Sacramento County have turned up rates as high as one in 10 mosquitoes infected.
"That's extremely high," he said, noting that state guidelines for West Nile virus surveillance suggest that an infection rate much lower - one in 200 - is very high.
Dr. Glennah Trochet, the county public health officer, said the high insect infection rate means, "we are going to have a lot of cases coming in."
Officials began spraying last month by truck in some residential neighborhoods, but the ground combat wasn't enough to stem the spread of the virus.
"The problem is that it's outpacing our ability to cover the area," he said.
Crews working in Citrus Heights, for example, cover fewer than 5,000 acres at a time, Brown said. The district also has done "fogging" by truck in Rio Linda, Elverta and Galt.
The target area identified for spraying covers 71,000 acres, encompassing most of urban Sacramento County north of the American River.
"It's significant," Brown said. "We've never had to treat that type of acreage at one time, but we've never had West Nile virus here."
The county also has experienced a high number of infected birds, said Vicki Kramer, chief of the vector-borne disease section at the state Department of Health Services.
"All these are high indications that the risk of transmission is increasing." Kramer said. "So it's prudent to initiate large scale control - the best weapon we have right now from preventing people from getting sick."
About 80 percent of people with West Nile virus infections don't feel sick, health officials say.
About 20 percent will get flu-like symptoms, including headache, fever, fatigue and body aches. Less than 1 percent become gravely sick with neurological illnesses such as meningitis and encephalitis. Neuroinvasive cases can be fatal. The virus killed 28 people in California last year.
Ted Toppin, spokesman for the Mosquito and Vector Control Association of California, said he knows of no other mosquito control agency in the state that has resorted to aerial spraying for adult mosquitoes in urban areas.
Jack Hazelrigg, general manager of the Greater Los Angeles County Vector Control District, said aerial spraying is largely unacceptable in his heavily urban district.
"It would be taboo down here," Hazelrigg said. "Not necessarily because it wouldn't be effective and it wouldn't be harmful, (but) there's just too much of a citizen outcry with respect to aircraft flying over houses and spraying of adult flies. You know what happened when they tried to control the Medfly."
Hazelrigg said Los Angeles County had nearly 300 cases of West Nile virus last year, and is seeing mosquito infection rates comparable to Sacramento's.
"We've done ground fogging, but not aerial," he said. "I'd probably have so many lawsuits at my doorstep. It's just not worth all the headache you'd have to go through to try to get it done."
Steve Zien, an organic landscape maintenance consultant in Citrus Heights, said he has doubts about the safety of the spraying.
"If it's strong enough to kill mosquitoes, I don't know why it wouldn't kill other critters," he said.
Clarence Nappier, a Citrus Heights mortgage broker, said he is in favor of the spraying.
"I grew up in Florida, where they sprayed every day," said Nappier.
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- Bee staff writer Gwendolyn Crump contributed to this report. The Bee's Chris Bowman can be reached at (916) 321-1069 or firstname.lastname@example.org.