Purpose: The objective was to determine the occurrence of E. coli O157:H7-positive pest flies in leafy greens planted at different distances from a cattle feedlot.
Methods: Spinach was planted to nine plots sited at 200, 400, and 600 feet from a cattle feedlot (3 plots each distance). Sticky traps were used to capture flies at the spinach plots and the feedlot in June through September. Flies were identified, counted, and pooled by species (up to ten flies per pool). E. coli O157:H7 was determined by immunomagnetic separation and plating in up to ten fly pools per site.
Results: Prevalence of E. coli O157:H7 in house flies (Musca domestica) and face flies (Musca autumnalis) was 17.1 and 15.8%, respectively, and did not differ (P > 0.05). Prevalence in house flies tended to be higher (P = 0.09) than in flesh flies (Sarcophagidae; 10.7%) and was higher (P < 0.05) than in blow flies (Calliphoridae; 7.6%) and stable flies (Stomoxys calcitrans; 0.63%). Overall, the percentage of O157:H7-positive fly pools was highest (P < 0.05) at the edge of the feedlot (18.5%), although the pathogen was found in 10.4, 8.5, and 9.5% of fly pools at 200, 400, and 600 feet from the feedlot, respectively.
Significance: Current leafy green field distance guidelines may not be adequate to limit the occurrence of pest flies in crops planted near cattle feedlots. Further work is needed to clarify the risk for transmission of E. coli O157:H7 to leafy greens by flies.