SAN DIEGO -- County environmental health workers began dropping bacterium-based larvicide into ponds, rivers and wetland areas Wednesday in an effort to kill the larvae of mosquitoes that can transmit West Nile virus.

The solid cereal-sized larvicide pellets are released into 48 or so local waterways about once a month during mosquito season. County officials said the bacterium used to kill mosquito larvae will not affect people and pets.

So far this year, five San Diego County residents have tested positive for West Nile. County environmental health officials said 44 of the 45 people who contracted West Nile in 2015 did so after mid-September.

The county is also reminding residents to help control mosquitoes themselves by dumping out standing water around their homes, using mosquito fish to stem breeding in horse troughs, fountains and other water sources, and using insect repellent when outdoors.

The routine larvicide applications are unrelated to recent hand-spraying of a pesticide derived from chrysanthemums to kill adult, invasive Aedes mosquitoes in Grant Hill, South Park, Mount Hope and Normal Heights. All four areas where Pyrenone 25-5 was sprayed over the last couple months were near the home of someone who contracted the Zika virus while traveling.