A Bay Area county has undertaken a second round of pesticide spraying this year after finding mosquitoes infected with the West Nile virus, according to public health officials. San Mateo County's Mosquito and Vector Control District used truck-mounted foggers to spray pesticide across about a square mile near the city of San Mateo Wednesday night. A previous spraying took place on July 21.
The pesticide applications took place after five dead birds found in the county tested positive for West Nile virus, a potentially debilitating disease that also infects humans. Public health workers also discovered virus-laden mosquitoes in the area.
West Nile virus is a growing problem in San Mateo County, which is just north of Santa Clara County, where at least 474 birds have been found dead of the disease so far this year. That county has conducted at least nine spray operations since may, with another planned for the Sunnyvale area the night of August 5.
That makes Santa Clara County the state's hotspot for West Nile, with Sacramento County a distant second at 162 birds dead so far this year.
Infection by West Nile virus poses a relatively low risk to humans; with almost all patients recovering in a short time and many thinking they've merely had a bad cold. But around one in a hundred people infected with the virus can develop far more serious symptoms, including meningitis and encephalitis. People over 50, or who suffer from diabetes or high blood pressure, are at particular risk of more serious symptoms.
The pesticide being sprayed in San Mateo, trade-named Zenivex E4, has the synthetic pyrethroid insecticide etofenprox as its sole active ingredient. Exposure to etofenprox causes insects' nervous systems to stop working properly. The pesticide is toxic to a wide range of insects other than mosquitoes, some of which may well be of ecological importance. Though Zenifex E4's manufacturer maintains, for instance, that its formulation dries to a form that isn't toxic to honeybees, any bees that are directly sprayed with etofenprox are likely to suffer ill effects. The pesticide is also considered highly toxic to fish and other aquatic organisms.
Still, the formulation being used in San Mateo is in many ways less toxic than other common mosquito-killing products, and night spraying may well have reduced the likelihood of collateral damage to bees -- though not necessarily to night-flying insects, or the bats and other animals that eat them. San Mateo vector control officials are encouraging residents in the affected areas to wash garden produce thoroughly before eating it, and to dump unattended sources of standing water where mosquitoes may breed.
They're also asking residents to notify them of any dead birds or squirrels -- which are also susceptible to West Nile -- at www.westnile.ca.gov or by calling (877) WNV-BIRD, a.k.a. (877) 968-2473.