Occurrence of Escherichia coli O157:H7 in pest flies captured in leafy greens plots grown near a beef cattle feedlot

Authors: Berry, Elaine; Wells, James - Jim; Durso, Lisa; FRIESEN, KRISTINA - Retired ARS Employee; Bono, James - Jim; SUSLOW, TREVOR - University Of California, Davis

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2019
Publication Date: 7/16/2019
Citation: Berry, E.D., Wells, J.E., Durso, L.M., Friesen, K.M., Bono, J.L., Suslow, T.V. 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. 82(8):1300-1307. https://doi.org/10.4315/0362-028X.JFP-18-601.
DOI: https://doi.org/10.4315/0362-028X.JFP-18-601

Interpretive Summary: Delicate fresh produce items like leafy greens are normally consumed raw and may receive mild processing treatments following harvest, that are designed more for removal rather than inactivation of bacterial contamination. This makes it especially critical to protect fresh produce from pathogen contamination in the preharvest growing environment. Pest flies found near cattle commonly carry pathogens, and these insects are difficult or impossible to exclude from produce crop fields grown near cattle production facilities. We found that E. coli O157:H7-positive pest flies of several species (house, face, blow, and flesh flies) were common in leafy greens planted up to 600 feet from a beef cattle feedlot. The isolation of closely-related E. coli O157:H7 isolates from the feedlot pen surface, flies, and leafy greens suggests that flies can transport this pathogen from cattle production to nearby produce crops. This research provides a detailed study of the occurrence of E. coli O157:H7-positive flies and their potential risk to transmit this pathogen to leafy greens grown near a cattle feedlot, as well as a starting point for determining set-back distances to protect leafy greens from this possible contamination route.

Technical Abstract: 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 setback distances between cattle production facilities and produce crops that will reduce the risk for pathogen contamination by challenging mechanisms like flies.