Infected bird carcasses found within county boundaries are likely to have picked up the disease in another county, and that county may no longer harbor the disease as it once did. "It varies tremendously," Ms. Nakano said.
A statement released by the vector control agency called infected dead birds "an early indication that the virus is circulating in the environment." The agency encourages residents to report fresh carcasses of birds or tree squirrels to the West Nile virus hotline at westnile.ca.gov or by calling (877) 968-2473.
If the carcass is more than 24 hours old, infectious agents will likely have degraded to the point at which they may not be detectable, but cooler temperatures can lengthen that testing window, Ms. Nakano said.
Between 2004 and 2011, San Mateo County officials detected the virus in 38 dead birds and 12 squirrels, the statement said. With this June 11 discovery, San Mateo County now joins 15 other California counties in having reported the presence of West Nile in 2012.
Humans are susceptible. About 20 percent of people bitten by an infected mosquito come down with the symptoms of West Nile fever, which include days and sometimes weeks of fever, headache, body aches, rashes and swollen lymph glands, according to the Centers for Disease Control.
In one of 150 cases, the disease can take a serious turn toward West Nile encephalitis or meningitis or West Nile poliomyelitis, with symptoms that include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. People 50 or older are at the highest risk of severe illness, the CDC says.
"West Nile virus activity may increase as we head into the summer months, and it is important for residents to protect themselves from mosquito bites," Ms. Nakano said.
Protection on your property begins with three steps, all of which begin with the letter D.
• Drain standing water wherever you find it, including flower pots, old car tires, rain gutters and pet bowls.
• Dawn and dusk are when mosquitoes bite. Use repellant and wear protective clothing such as long pants and long-sleeved shirts. Keep mosquitoes out of the house with tight-fitting screens.
• DEET, or N,N-Diethyl-meta-toluamide, is a common insect repellant recommended for clothing and skin to protect against mosquito bites. Other recommended repellants include picaradin, lemon-eucalyptus oil and IR3535.
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