Summary of Awards to Date

Fly reservoirs of E. Coli O157:H7 and their role in contamination of leafy greens.


Apr. 1, 2009 - Mar. 31, 2010

Award Number


Amount Awarded



Astri Wayadande, Ph.D
Oklahoma State University


Justin Talley, Ph.D.


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. 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. 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? 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. 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. 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.

Technical Abstract

Filth flies are well-documented mechanical vectors of human pathogens in hospitals, food areas, and confined animal production areas. Filth flies develop in animal waste, decomposing plant matter, and feed containers, all of which can be contaminated with multiples strains of bacteria, including Escherichia coli O157:H7 and other enteric species. Fly transmission of human pathogens to unprocessed, fresh produce is less certain. Recent evidence documented the presence of E. coli O157:H7 in feral flies captured in leafy green production areas and transmission of E. coli O157:H7 to spinach under experimental conditions, but the prevalence of this pathogen in the feral fly population and propensity of flies to transmit to leafy greens is unknown. We propose to examine flies captured in several locations on or near E. coli O157:H7 reservoirs and leafy green production areas in California and Oklahoma to determine if a significant number of flies harbor the O157:H7 pathogen. Furthermore, we will test whether fly excrement can serve as a source of E. coli O157:H7 colonization of leafy greens phyllosphere. The objectives of this proposal are: 1. Capture emerging adult filth flies from manure pats deposited by cattle maintained in feedlots in California and Oklahoma and test them for E. coli O157:H7, 2. Test flies captured in Salinas Valley leafy green production areas located within a 10 mile radius of a confined cattle operation or rangeland for presence of E. coli O157:H7, and 3. Determine if E. coli O157:H7 colonization of the leaf surface occurs after fly defecation. This project addresses Research Priority # 4: Human pathogen reservoirs and vectors (other animal risk factors). We anticipate that the data collected will further define whether or not flies are a significant risk to pre-harvest contamination of leafy greens. This project will support our ongoing research collaboration with our colleagues at the University of California, Riverside.