Berry, Elaine D., James E. Wells, Lisa M. Durso, Kristina M. Friesen, James L. Bono, and Trevor V. Suslow (2019) Occurrence of Escherichia coli O157:H7 in Pest Flies Captured in Leafy Greens Plots Grown Near a Beef Cattle Feedlot. Journal of Food Protection: August 2019, Vol. 82, No. 8, pp. 1300-1307.

Leafy greens are leading vehicles for Escherichia coli O157:H7 foodborne illness. Pest flies can harbor this pathogen and may disseminate it to produce. We determined the occurrence of E. coli 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 (Musca domestica L.), face flies (Musca autumnalis L.), stable flies (Stomoxys calcitrans L.), flesh flies (family Sarcophagidae), and blow flies (family Calliphoridae). E. coli O157:H7 carriage rates of house, face, flesh, and blow flies were similar (P > 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 (P < 0.05). Differences in carriage rates are likely due to the uses of fresh bovine feces and manure by these different pest fly groups. E. coli O157:H7 carriage rates of total flies did not differ (P > 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 E. coli 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.

In conclusion, E. coli O157:H7–positive pest flies of several species were common in leafy greens planted within 180 m of a beef cattle feedlot. The isolation of the same PFGE types from the FSM, flies, and leafy greens suggests that flies can disseminate this pathogen from  cattle production to nearby produce crop fields. However, a definitive role for pest flies to transmit pathogens to preharvest leafy greens has not yet been demonstrated, so additional research is needed to confirm these occurrences and to guide the development of management strategies to protect fresh produce from contamination in the preharvest production environment, including the determination of appropriate setback distances from cattle production that will reduce risk from hard-to-exclude pathogen sources, such as airborne dust or flies.