Fly Reservoirs of E. Coli O157:H7 and Their Role in Contamination of Leafy Greens

Filth flies (house flies, blow flies) have long been associated with the transmission of human disease pathogens, but they have never been implicated to transmit these pathogens to pre-harvest plants. <P>
After the 2006 E. coli O157:H7 outbreak in spinach, our group was funded by Fresh Express to examine movement of this pathogen to leafy greens by insects. To our surprise, we found large numbers of filth flies in some commercial lettuce fields and a small number of them were tested and found to harbor E. coli O157:H7. <P>This begs the question: what proportion of flies found in or near leafy greens production areas are carrying E. coli O157:H7 and are they capable of contaminating pre-harvest leafy greens? <P>To answer this, we propose to test a large sample of flies collected from E. coli O157:H7 risk areas (cattle feedlots and rangeland) and also from leafy green production areas to determine if any proportion of the insects are carrying the pathogen. <P>We will do the feedlot sampling in two areas, one in California and the other in Oklahoma. Leafy greens in Monterey and San Benito counties will be sampled with the collaboration of three industry leaders, Fresh Express, Inc., Dole Fresh Vegetables and Pinnacle Organics. <P>Finally, we will also determine if the E. coli bacterium can be transmitted to plants through fly feces. Together, the objectives outlined in our project will help to determine if flies are a route for E. coli O157:H7 (and possibly other human pathogens) contamination of pre-harvest greens and will generate information that will be useful to growers and distributers in making risk-assessment and food safety decisions.