Blow Flies Pose a Public Health Hazard near Urban Combined Sewer Overflows

Over 700 municipalities in the United States have combined sewer systems. Following rainfall events, flows to wastewater treatment plants (WWTPs) may exceed capacity, resulting in billions of gallons of wastewater being discharged annually. In Indianapolis, Indiana, precipitation events of ≥6 mm result in up to 90 sewage overflow events annually. As blow flies visit organic wastes that include fecal material, we hypothesized that flies act as vectors of enteric pathogens present in wastewater from overflow events. Blow flies were collected over a 2 year period at six urban parks in Indianapolis, located varying distances from the nearest overflow location. Fly gut DNA was extracted from 997 flies, followed by human fecal source tracking via mitochondrial DNA sequencing. Sixty-eight blow flies collected near an overflow point underwent qPCR screening for 23 enteric pathogen gene markers. Compared to flies caught within 1 km of the nearest combined sewer overflow, flies caught further away were associated with a 68% reduction (RR = 0.32) in the risk of detecting human mitochondrial DNA. A majority of flies (53%, 36/68) tested positive for ≥1 enteric pathogen-associated gene. These results indicate that blow flies near combined sewer overflows can carry enteric pathogens associated with sewage and pose a public health hazard.