REDWOOD CITY -- The first animal case of West Nile Virus in San Mateo County this year has been found in a squirrel collected last week in the city of San Mateo, a San Mateo County Health Department spokeswoman confirmed Wednesday.

An eastern grey tree squirrel was found July 9 by the San Mateo County Mosquito Abatement District, and subsequent testing at the University of California, Davis concluded it tested positive for the virus, according to spokeswoman Beverly Thames.

No human cases have been reported in San Mateo County, but health officials are warning residents to avoid mosquito bites.

"The presence of infected squirrels means that there likely are infected mosquitoes nearby, which increases the chances that human cases may appear in San Mateo County," county Deputy Health Officer Alvaro Garza said in a prepared statement. "We urge residents to take steps now to protect themselves from mosquito bites."

Garza said that after birds, tree squirrels run the highest risk of West Nile Virus infection, but there is no evidence it can be transmitted to humans through proximity to squirrels, he said.

Residents can reduce the risk of contracting mosquito-borne diseases by draining standing water around the house each week, including in tires, cans, flowerpots, clogged rain gutters, rain barrels, toys and puddles, the health department reported.

Also, residents should take precautions against bites especially at dusk and dawn, when mosquitoes that carry the virus are thought to be most active. Insect repellants containing DEET are considered effective and residents should follow instructions carefully.

Residents are also advised to dress in long sleeves and pants during the dusk and dawn hours and in areas where mosquitoes are active, and to replace damaged or missing screens on windows and doors.

Areas with significant mosquito problems can be reported to the San Mateo County Mosquito Abatement District at (650) 344-8592. To report recently dead birds and squirrels, call (877) WNV-BIRD or visit http://www.westnile.ca.gov.

Though the virus is rare, symptoms such as a high fever, severe headache and stiff neck should be reported to a doctor immediately, the health department advised.

West Nile Virus activity has been reported in 32 counties throughout California this year, with human cases in four counties and one death in Kern County, the health department reported.