SAN DIEGO COUNTY, CA – County vector control crews will begin treating San Diego area waterways Wednesday to try to eliminate kill mosquito larvae in an effort to reduce the risk of potential transmission of the West Nile or Zika viruses.

The aerial larvicide drops will target nearly 50 rivers, streams, ponds and other waterways in the region, The larvicide is harmless to people and pets but kills mosquito larvae before they can turn into biting adults, according to the county. The exact number of waterways that receive drops varies, depending on whether or not they've dried up, but vector control flies over 48 areas. The drops will continue monthly through October.

Another 1,400 potential mosquito-breeding areas each year are treated by hand. The vector control program also gives out free mosquito-eating fish to the public, tracks down and treats neglected swimming pools and tests dead birds for West Nile. Mosquitoes breed in standing water. County officials have issued several recent calls for the public to help by dumping out water that may have gathered from the rains over the past few months, such as in clogged rain gutters, overturned toys or inside old tires.

WATCH: Top 10 Worst Mosquito Cities In The United States Native mosquitoes spread the West Nile virus, while an invasive form of the species is capable of carrying Zika. However, no mosquitoes in San Diego County have tested positive for Zika, and the only locally acquired cases of the illness have been sexually transmitted, according to county health officials.

While Zika and other tropical diseases, including chikungunya and dengue, are not naturally found in San Diego County, county residents who visit countries and places where these diseases are established can return home infected, where invasive Aedes mosquitoes could bite them and spread disease to others. In 2016, 82 county residents tested positive for Zika after traveling abroad.

In 10 instances, County Vector Control found invasive Aedes mosquitoes living near people who contracted Zika. To make sure those mosquitoes could not spread Zika to others, Vector Control hand-sprayed people's yards in 10 neighborhoods to kill mosquitoes and protect the public.