Along with feedlot dust blowing in the wind and surface irrigation water flowing adjacent to feedlots, flies captured in leafy greens plots near feedlots are capable of transferring E. coli from animal operations to produce fields.

Set for publication in August in the “Journal of Food Protection,” new research from a team of experts links contamination of leafy greens with E. coli from concentrated animal feeding operations (CAFOs), also referred to as feedlots, via “pest flies.”  

“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,” according to the research abstract.

The report, “Occurrence of Escherichia coli O157:H7 in Pest Flies Captured in Leafy Greens Plots Grown Near a Beef Cattle Feedlot,” comes a little more than a year after a 2018 E. coli outbreak linked to romaine lettuce was declared over.

More than 200 people from 36 states were sickened in that outbreak, which was the first of two in 2018 that implicated romaine. Five of the patients in the first outbreak died, according to the Centers for Disease Control and Prevention. The second outbreak, declared over in January this year, sickened 62 people in 16 states. No confirmed deaths were reported.

The romaine implicated in the first 2018 outbreak was from the Yuma, AZ, area. The majority of those romaine fields are near or adjacent to a cattle feedlot that can handle more than 100,000 animals at a time. Many growers use water from open canals that are next to the feedlot to irrigate their fresh produce.

The second outbreak was linked to a grower in California who had an “agricultural water reservoir” wher E. coli was found.

In the first outbreak, no single grower, shipper or brand of romaine was specifically identified as the source of the implicated leafy greens. The relatively short shelf life of lettuce and a tangle of incomplete shipping and receiving records, some hand written, made it virtually impossible for outbreak investigators to pinpoint the origin of the romaine.

There was lively debate among growers, government agencies, academics and consumer groups regarding how the romaine in the Yuma area might have become contaminated with the E. coli O157:H7. Winds, however, were known from the beginning to blow through the massive feedlot and toward the open irrigation canals and lettuce fields.

In their study about flies, feedlots and leafy greens, the research team determined the occurrence of E. coli O157:H7 in flies collected in leafy greens fields that were up to 180 meters from a cattle feedlot. The scientists then assessed the flies’ relative risk to transmit the pathogen to the leafy greens. 

The risk is present at roughly the same level for four of the five captured fly species: house, face, flesh, and blow flies. The fifth species, stable flies, had lower levels of E. coli. The scientists also discovered the carriage rates for the pathogen were as strong at 180 meters from the feedlot as they were at zero meters.

“Leafy greens are leading vehicles for Escherichia coli O157:H7 foodborne illness. Pest flies can harbor this pathogen and may disseminate it to produce,” the researchers reported.

“However, further research is needed to clarify this role and to determine set-back distances between cattle production facilities and produce crops that will reduce the risk for pathogen contamination by challenging mechanisms like flies.”

The research team included:

Elaine D. Berry of the U.S. Department of Agriculture, Agricultural Research Service, U.S. Meat Animal Research Center;

James E. Wells of the U.S. Department of Agriculture, Agricultural Research Service, U.S. Meat Animal Research Center;

Lisa M. Durso of the U.S. Department of Agriculture, Agricultural Research Service, Agroecosystem Management Unit;

Kristina M. Friesen of the U.S. Department of Agriculture, Agricultural Research Service, Agroecosystem Management Unit;

James L. Bono of the U.S. Department of Agriculture, Agricultural Research Service, U.S. Meat Animal Research Center; and

Trevor V. Suslow previously of the Department of Plant Sciences, University of California-Davisand currently with the Produce Marketing Association.