Fifteen dead birds infected with the virus have turned up since the beginning of the year, including one in Palo Alto and two at Stanford, according to Santa Clara County Vector Control District manager Russ Parman. Although they have not yet found infected mosquitoes, "other surveillance indicators tell us that the virus is quite active this year," he said.
Recent rainfall has left abundant pools of standing water, creating more breeding habitats for the pests this spring. Although concentrations are only slightly higher than average, they are more evenly spread across the county. "It's very much easier for a bird to go out right now and encounter a mosquito," Parman said. During summer time, when some natural water sources dry up, they could take advantage of stagnant water in human environments.
Vector-control district officials have declared mosquito abatement their top priority this year. Their main mission: curbing the spread of West Nile.
The agency is sampling mosquitoes within a mile of where infected birds died, focusing on a "high-risk area" covering parts of San Jose, Campbell and Cupertino. If a mosquito tests positive for the virus, the district will conduct "limited fogging operations" using Pyrenone 25-5, a low-volume insecticide that is "essentially nontoxic to just about everything except for mosquitoes," Parman said.
"This year appears to be getting off to a much more active start than last year, so the odds are that we will be finding infected mosquitoes and initiating fogging," he predicted. No fogging has taken place in Palo Alto since the county introduced the technique in 2005, a year after West Nile first made an appearance in the area. Last year, Sunnyvale was the only site to be sprayed.
Aerial tactics also include surveying cities to locate poorly maintained backyard swimming pools and ponds. A survey in San Jose this year found more than 800 green or half-empty pools perfect for breeding mosquitoes. The agency plans to fly over Palo Alto in July.
Aerial surveys have logged 3,500 properties with ill-maintained pools since they began in 2007. "There's a lot of work out there, and the pools are very, very significant producers of mosquitoes," Parman said.
He urged residents to clean up stagnant water and notify the district of mosquito problems and abandoned pools. "The more eyes and ears we have, the more ground we can cover," he said.
The district offers free home or yard inspections. They'll even throw in complimentary mosquito fish — guppy-like predators that devour almost anything — to gobble up larvae. Safe for humans, mosquito fish are also available in local nurseries.
Besides obvious habitats such as pools, mosquitoes can breed in water hidden in gutters, buckets or birdbaths. Even large flowers can collect suitable puddles.
"Water can be in the darndest places," noted Dave Peterson, who has staffed the Master Gardener hotline at Gamble Garden for 13 years. "It can be as small as a thimble."
Even that is enough for raising a colony of mosquitoes. Females can lay up to 300 eggs at a time, and the reproductive cycle takes only five days. "You don't have any warning — they show up overnight, practically," Peterson said. When they do, residents are the closest food source.
Peterson suggests using candles containing citronella oil, a plant-based repellent, or other natural methods to ward off the pests. He does not recommend blacklights, which he said are rather ineffective and kill beneficial insects.
Bacterial products also offer natural pest control. Available at local nurseries, they work by affecting the mosquito larvae.
Like Peterson, most experts suggest that residents avoid chemical controls, which can kill helpful critters such as dragonflies, which prey on mosquitoes. More importantly, poisons can work their way up the food chain, including to fish and humans.
"There are just a lot of impacts when you add toxins to the ecosystem, some of which you can predict and some of which you can't," warned Claire Elliot, who directs the stewardship program at Acterra, an environmental nonprofit in Palo Alto.
She recalled a case in the 1950s, when the World Health Organization sprayed DDT to curb a malaria outbreak in Borneo. The pesticide ended up killing wasps that ate thatch-eating caterpillars, and people's roofs collapsed. To boot, geckos ate poisoned insects, cats died off after eating the geckoes, and the rat population exploded. Two new diseases broke out as a result. "There's an example of why you don't want to spray for mosquitoes," Elliot said.
Even with pesticides that target mosquitoes specifically, "Do they know for sure?" Elliot wondered. "Sometimes the chemicals have breakdown products that are more toxic than the chemicals themselves."
Doug Moran, who lives in Barron Park, sticks to natural methods for controlling mosquitoes that fly up from neighboring Matadero Creek. Urban creeks are a known breeding source, and Vector Control surveys them annually. County officials are also partnering with rangers to monitor water levels in the city's flood basin, where three creeks meet, to prevent fluctuations that help mosquitoes multiply.
Still, "we're getting eaten by mosquitoes here," complained Moran, who was president of the Baron Park Association for six years. A computer scientist by day, Moran tests for mosquito larvae by sampling water in a backyard bucket. They look like "little rods that will just wiggle like crazy when you disturb the water," he said. When larvae show up, he e-mails neighbors, encouraging them to hunt for forgotten pools of water.
He also finds that spiders keep the pests at bay. "I'm very nice to my spiders," he said. "It grosses some people out ... (but) I can leave my door open and not worry about mosquitoes."
In addition to the flood zone, catch basins — street-corner inlets that carry rainwater to creeks — are a source of mosquitoes in Palo Alto. Older basins have a "frump" at the bottom that gathers a small pool of standing water. "In the summertime that can get full of water, if people, for example, over-water their lawn," said Joe Teresi, a senior engineer in the public works department. He encouraged people to water lawns sparingly and avoid washing cars in the street.
Besides mosquito-proofing homes, experts are urging people to protect themselves from bites by wearing long-sleeved clothing and using over-the-counter repellants, especially if outside during dawn and dusk. That is feeding time for culex mosquitoes, the species that carries West Nile.
Eighty percent of people infected with the virus do not know they have it, but a majority of those who do develop severe flu-like symptoms and spend an average of 16 days off work. "It's not an insignificant disease," Parman warned. A smaller percentage develop a form of the disease that causes neurological damage or even death.
If infected, older adults and those with a compromised immune system are more likely to develop a dangerous form of the disease, Parman said. Children are not at higher risk.
Although no human cases of West Nile have been confirmed in the county in 2010, Parman's message is clear: "The virus is here throughout the county, and during this time of year all the way through September, there is a risk of acquiring it if you're bitten by an infected mosquito."
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