Occurrence of O157:H7 in Pest Flies Captured in Leafy Greens Plots Grown Near a Beef Cattle Feedlot.

Leafy greens are leading vehicles for O157:H7 foodborne illness. Pest flies can harbor this pathogen and may disseminate it to produce. We determined the occurrence of O157:H7-positive flies in leafy greens planted up to 180 m from a cattle feedlot and assessed their relative risk to transmit this pathogen to leafy greens. The primary fly groups captured on sticky traps at the feedlot and leafy greens plots included house flies ( L.), face flies ( L.), stable flies ( L.), flesh flies (family Sarcophagidae), and blow flies (family Calliphoridae). O157:H7 carriage rates of house, face, flesh, and blow flies were similar ( > 0.05), ranging from 22.3 to 29.0 flies per 1,000 flies. In contrast, the carriage rate of stable flies was lower at 1.1 flies per 1,000 flies ( < 0.05). Differences in carriage rates are likely due to the uses of fresh bovine feces and manure by these different pest fly groups. O157:H7 carriage rates of total flies did not differ ( > 0.05) by distance (ranging from 0 to 180 m) from the feedlot. Most fly isolates were the same predominant pulsed-field gel electrophoresis types found in feedlot surface manure and leafy greens, suggesting a possible role for flies in transmitting O157:H7 to the leafy greens. However, further research is needed to clarify this role and to determine set-back distances between cattle production facilities and produce crops that will reduce the risk for pathogen contamination by challenging mechanisms like flies.