West Nile virus back in Bay Area 

Fears about West Nile-infected mosquitoes in San Mateo County are in the air since a squirrel, which is unlikely to have traveled very far, was confirmed to be carrying the virus.

The case is this year’s first confirmed in the county. The dead squirrel was found July 28.

While the case is not likely to trigger an epidemic, officials at the county’s Mosquito and Vector Control District are concerned that the squirrel may have contracted the virus locally.

“Squirrels don’t travel very far, so when we find one that’s infected, it means the virus was contracted in the area,” vector ecologist Angie Nakano said. “Infected squirrels indicate that there are infected mosquitoes nearby.”

The West Nile virus is transmitted through bites by infected mosquitoes. Finding an infected squirrel is greater cause for concern than an infected bird, which could have flown in from an outside area where the virus is present, Nakano said.

Last year, the county only had one case of West Nile, which was confirmed in a bird found in Redwood City.

When the virus first arrived in California in 2004, San Mateo County saw around 10 cases, but the numbers have dropped over the past few years and Nakano said the county generally only sees one or two cases per year.

“It’s possible that cooler weather has slowed the transmission cycle or a natural immunity has been building up in the wild population of birds,” said Nakano.

All confirmed cases in San Mateo County have been in animals, with no human cases to date. Santa Clara and Contra Costa counties have had human cases in the past, but the infections were not contracted locally.

Now that a local squirrel has tested positive for West Nile virus, Nakano said people need to be aware that the possibility of infection is still a source of concern.

“Public interest isn’t there anymore because the virus isn’t in the news,” Nakano said. “It’s important that people report sick or dead animals so we can detect the virus before it has the ability to spread to humans.”

The Mosquito and Vector Control District recommends that people take precautionary measures to prevent exposure to infected mosquitoes, including draining standing-water sources, avoiding outdoor activities at dusk or dawn, using insect repellents with DEET and dressing in long sleeves and pants if in an area with mosquitoes.



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A daily newspaper covering San Francisco, San Mateo County and serving Alameda, Marin and Santa Clara counties.
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