Health officials reportedly collected the samples in Encino, Northridge, Panorama City, Porter Ranch, Sherman Oaks and Van Nuys.
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"We are beginning to see the anticipated increase in West Nile virus activity in the region," the district's director of community affairs Kelly Middleton said.
The Los Angeles County Department of Public Health recently reported six human infections in the county so far this year.
Residents, business owners and property managers are encouraged to ensure no standing water is permitted on their property and to remove or seal items where mosquitoes gather.
These areas include yard and parking lot drains, air conditioner drip pans and buckets, rain barrels, unused swimming pools, plant saucers and old tires.
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West Nile virus is transmitted to people and animals through the bite of an infected mosquito and is a leading cause of severe infections of the nervous system among adults over 50.
There is no cure for West Nile virus.
The City News Service contributed to this report.