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Should local residents be worried about West Nile virus?

Those of you near Stanford may have heard the local news that Santa Clara County is spraying pesticide to control an unusually large hatch of summer salt marsh mosquitoes in and around the Palo Alto Baylands park. This saltwater marsh at the south end of the San Francisco Bay is providing an unusually good hatching ground for the mosquitoes because of a breach in a tide wall that normally controls marsh water levels, as a press release from the county's Vector Control District explains:

A breached tide wall in the Palo Alto baylands has created ideal conditions for the breeding of the mosquitoes by allowing water levels in the basin to rise and fall. SCCVCD has been closely monitoring the development of mosquito larvae, and current field conditions are producing continued egg-hatch. Recent adult “fly-offs” have created considerable discomfort for residents and businesses in nearby areas.

Of course, news about hungry mosquitoes prompts worries about West Nile virus, especially in light of media coverage such as today's New York Times piece about the West Nile outbreaks in Texas.

To clarify whether local salt marsh mosquitoes pose a health risk, I called Russ Parman, a spokesman for the county Vector Control District. West Nile isn't a big issue with the salt marsh mosquitoes, a species called Aedes dorsalis, Parman said, because mammals are their target meal. In contrast, the Culex mosquitoes that transmit West Nile bite both birds and humans, carrying the virus from its natural reservoir in birds to human hosts. (There are many other mosquito species, too – California has about 50 different types of mosquito.)

Instead, the problem with Aedes is that they're big and vicious. "They'll bite you right through your blue jeans," Parman said.

The methods used to control Aedes mosquitoes also differ from those used on Culex. To go after the West Nile carriers, the pest control experts use "fogging," which disperses pesticide over the landscape in tiny droplets of water that cling to and kill adult mosquitoes. In contrast, today's spraying delivered two pest control agents in a large volume of water to the salt marsh, where the aim is to kill mosquito larvae and pupae before they hatch, Parman said. The agents used, says the county's press release:

are mosquito-specific and short-lived in the environment: they effectively control the immature (aquatic stage) mosquitoes, but are not harmful to birds, fish, other insects, wildlife, or humans.

All this doesn't mean, however, that Santa Clara residents can be completely complacent about West Nile.

"So far, we are having a relatively quiet [West Nile] season," Parman told me. The county performed one fogging operation a few weeks ago in Los Altos to eradicate Culex mosquitoes there that tested positive for West Nile, but Parman points to surprises in the Texas outbreaks – including a relatively young man who became very ill – to say that there are no guarantees with the viral disease, for which no vaccines or drugs exist. "This underscores the fact that these viruses are variable," he said. "You just never know from one year to the next."

Parman encouraged everyone, especially those over 50 and people who have chronic diseases such as diabetes or reduced immune function, to take precautions to avoid mosquito bites: Limiting outdoor activities at dawn and dusk, when mosquitoes are most active, and using insect repellents or wearing long sleeves and pants if you're outside at those times of day.

Previously: Image of the Week: West Nile Virus and Researchers identify dominant chemical that attracts mosquitoes to humans
Photo by dr_relling

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