Residents are being urged to eliminate any standing water on their property so the mosquitoes don't have a place to breed.
A bite from a mosquito can spread the potentially deadly disease to humans and pets.
A dead crow found in Sierra Madre last week also tested positive for the virus.
Statewide, 91 positive mosquito samples have been identified this year in six counties, compared with five counties at this point in 2011.
City News Service contributed to this report.