The otherwise unmarked squirrel is a sign of West Nile's increasing pervasiveness all over California in 2010, Chindi Peavey, laboratory director for the San Mateo County Mosquito and Vector Control District said.
"West Nile virus is much more prevalent this year throughout the state," she said. "It's not something people should panic about, but yes, we have it."
Birds are the most common carriers of the virus, Peavey said. Human infection typically occurs from a bite from a mosquito that has previously bitten a bird.
The squirrel was found on July 28 by a lab technician on a road near a creek and tested positive for West Nile virus this week.
Mosquitoes and a bird in Oakley were found to be carriers of West Nile virus on Wednesday, officials said. Teams in Santa Clara, Sacramento, Yolo and Placer counties have all fogged their areas with mosquito deterrent to rid any threat of West Nile virus.
There is no planned fogging in San Mateo County because scientists haven't uncovered enough of the West Nile virus in the area, Peavey said.
Residents who discover nearby dead birds, squirrels or other animals should notify scientists at the San Mateo County Mosquito and Vector Control District.
People should also call the office if they own or are near standing water. The water can be treated with bacteria that will prevent mosquitoes from transmitting the virus.
If dead birds or animals are found, people can call (877) 968-2743. For other questions or concerns, call the vector control office at (650) 344-8592.