Five animals in San Mateo County have been reported with West Nile virus this year, including a squirrel trapped by the county in Menlo Park last month.
West Nile virus is transmitted through bites from infected mosquitoes. The mosquito control district is setting traps in the areas where infected animals have been found and sending mosquitoes to state labs for testing. If tests are positive, the district will "fog" the area the mosquitoes are found in, Mr. Gay said.
Fogging is a "low-volume treatment" that involves releasing a plant-based chemical, pyrethrum, from a truck-mounted apparatus that looks like a large aerosol can, Mr. Gay said. Pyrethrum is a byproduct of the chrysanthemum plant, he said.
The district hasn't fogged an area for about six years, he said, and past fogging has been done to eradicate problems with salt marsh mosquitoes in areas in San Carlos, Redwood Shores, Redwood City and Foster City.
Residents should check for standing water around their homes or workplaces. If they need help getting rid of water or are getting a lot of mosquitoes, they should call the district at 344-8592, says Ms. Shelton.
In humans, West Nile virus symptoms can range from a severe illness affecting the nervous system to flu-like illness with high fever and excessive sleep, according to the mosquito district.
No case of West Nile virus in humans has been reported this year in San Mateo County, Ms. Shelton said. Twenty-three cases of the virus have been reported in the state, mostly in the Central Valley and Los Angeles, she said.
The district asks residents to report dead birds or squirrels, which may be an indication the virus is active in the area, either online at westnile.ca.gov or by calling 877-968-2473.
Although some publications have said people who find dead birds should seal them in plastic bags and leave them on the premises for the district to pick up, Mr. Gay said the district "would rather you not touch the dead animal." When the district is notified, "we will be at your house within 24 hours" to pick it up, he said.
Mr. Gay spoke to the Atherton City Council on Aug. 15, updating the community on the mosquito district's work in the area. Residents have been on edge since word spread that the dead birds tested positive.
Resident John Ruggeiro said he has observed large mosquito populations at home demolition sites, and Pat Dobbie asked Mr. Gay whether residents should be emptying their fountains and ponds.
The answer was a firm "No." That's because the district has been monitoring ponds and other large areas of water, and is using growth regulators — a juvenile hormone incorporated into a small block and dropped into the water — to prevent the mosquitoes from becoming adults and leaving the water.
Councilman Jim Dobbie asked for advice on mosquito bite protection. Later, Mr. Gay sent him an email naming what he believes are the two best products: Sawyer Controlled Release 20 percent DEET lotion; and, for harsher environments and where perspiration is unavoidable, Ultrathon 34 percent DEET lotion.
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