After six hours of fogging by the San Mateo County Mosquito and Vector Control District, infected mosquitoes were found in San Mateo but not in Ladera, district officials said. Another round of fogging is set for San Mateo.
Infected mosquitoes had been found in Ladera on July 18 and in the San Mateo area, officials said.
While birds are the primary hosts, humans, horses and other animals can be infected. Among human victims, 80 percent show no symptoms at all, officials say.
Infection requires the bite of an infected mosquito; the virus cannot be spread person-to-person.
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