The infected raven and squirrel found in August raised concern about the possibility of West Nile Virus circulation in the county. The disease is usually spread through bites from infected mosquitoes, the statement said.
The squirrel, which was found in Redwood City on Aug. 17, is the second dead squirrel to test positive for the virus in San Mateo County this year, the statement said.
The raven was picked up in Portola Valley on Aug. 24, but the low level of West Nile Virus in that bird indicates that it may have been infected as long ago as last year, the statement said.
Ravens do not travel long distances, the Control District said, so it is likely that the bird became infected somewhere in or near Portola Valley. Ravens, crows, and jays are all particularly susceptible to West Nile Virus.
The organization is asking county residents to help them figure out whether the virus is circulating in the environment or being carried by infected mosquitoes by reporting dead birds and tree squirrels either online at www.westnile.ca.gov or by phone at (877) 968-2473.
The district suggests that people can reduce the risk of being bitten by contaminated mosquitoes by eliminating standing water, wearing mosquito repellent when an abundance of the bugs is present, and staying covered or indoors during dawn and dusk.
People over the age of 55 are most at risk for contracting the virus, the Control District said.
Most people who get the disease do not show any symptoms, but about 20 percent of those infected may experience flu-like symptoms, the statement said. One in 150 people may develop a serious illness that can leave permanent neurological damage.
To report a major mosquito problem in San Mateo County, contact the Control District at (650) 344-8592. More information about West Nile Virus can be found online at www.smcmad.org.