Dissemination of Escherichia coli O157:H7 from a cattle feedlot: Effect of proximity on contamination of leafy greens, bioaerosols, and pest flies

Escherichia coli O157:H7 outbreaks linked to the consumption of leafy green vegetables have focused attention on cattle as potential sources of contamination, and fueled the need for information regarding the dissemination of this pathogen from cattle production facilities.  Questions include: can E. coli O157:H7 be transported in bioaerosols or by insects?  How far can it be disseminated, and is this transport significant to preharvest produce contamination?  What are appropriate set-back distances between cattle production and crop fields that will reduce contamination risk?  The impact of proximity on E. coli O157:H7 contamination of leafy greens planted in plots that were 60, 120, and 180 meters from the edge of a beef cattle feedlot was examined in a two-year study.  E. coli O157:H7 was recovered from 3.5% of leafy green samples per plot at 60 meters, which was higher (P < 0.05) than the 1.8% of positive samples per plot at 180 meters, indicating a decrease in contamination as distance from the feedlot was increased.  Although E. coli O157:H7 was not recovered from air samples at any distance, total E. coli was recovered from air samples at the feedlot edge and all plot distances, indicating that airborne transport of the pathogen can occur.  Results suggest that risk for airborne transport of E. coli O157:H7 is increased when cattle pen surfaces are very dry and dusty. House flies, face flies, blow flies, flesh flies, and stable flies were found to carry E. coli O157:H7, and the pathogen was found at high percentages on some fly species, with little to no effect of distance from the feedlot.  Additional research is needed to determine safe set-back distances between cattle feedlots and crop production that will reduce fresh produce contamination, and to clarify the roles for bioaerosols and flies in E. coli O157:H7 dissemination.