The Greater Los Angeles County Vector Control District said signs of the virus have now been found in Carson, Northridge, Reseda and Signal Hill.
Residents are urged to use EPA-registered repellents when spending time outdoors to prevent mosquito bites.
The summer heat can increase virus activity and mosquito populations, according to a district statement.
A Tustin woman in her 50s has also been confirmed as the first human case of West Nile Virus in Orange County this year.
According to the Orange County Health Care Agency, there were 12 human cases of the disease in the county last year and one death.