Outbreak investigators say a cattle feedlot near a canal providing water to growing regions in Arizona is a key element in their hypothesis about the source of E. coli that contaminated romaine lettuce earlier this year.

The Food and Drug Administration has been investigating the outbreak, linked to romaine grown in the Yuma, AZ, area since the first week of April. Monday the agency reported on the hypothesis about a concentrated animal feeding operation near the canal. Dust from such feedlots is a known vector for the spread of E. coli bacteria and other pathogens to growing fields and surface water.

“FDA notes that the canal is close to a Concentrated Animal Feeding Operation (CAFO), a facility with a large number of cattle on the premises,” the agency reported Monday.

“The CAFO can hold in excess of 100,000 head of cattle at any one time and the FDA traceback information showed a clustering of romaine lettuce farms nearby. Our experts continue to work on examining potential links between the CAFO, adjacent water, and geologic and other factors that may explain the contamination and its relationship to the outbreak.”

Five people died and more than 200 others were confirmed with infections from a particularly virulent strain of E. coli O157:H7 that investigators found in the canal water in June. Sick people were spread across 36 states. Almost half of them were so sick they had to be admitted to hospitals. Federal officials declared the outbreak over on June 28.

Producer growers in the area, including those who produce romaine and other leafy greens, have been meeting on a regular basis about the outbreak for months. The industry set up a Leafy Greens Food Safety Task Force to review growing practices. The task force has suggested tripling an industry-imposed 400-foot buffer zone to separate leafy greens growing fields from animal feedlots. 

Members of the California and Arizona Leafy Greens Marketing Agreement were scheduled to review the 1,200-foot setback suggestion on Friday. The industry groups are accepting comments for 15 days before making a final decision. Produce industry leaders have said they want to have preventive measures in place before next season’s leafy greens are planted in the Yuma area.

The task force suggesting the increased buffer zone distance and other potential food safety measures includes not only growers, packers, shippers and distributors of leafy greens, but also buyers such as Taco Bell, Chick-fil-A, Blue Apron, and Sysco, which is the largest foodservice supplier in the country. Staff from the Centers for Disease Control and Prevention as well as the FDA assisted the task force.

In its updat Monday, the FDA doubled down on its position that the canal water was the likely source of E. coli. The cattle feedlot, however, could have easily contaminated the canal.

“As FDA has previously stated, samples of canal water have tested positive for the outbreak strain of E. coli. FDA continues to consider that contaminated water coming into contact with produce, either through direct irrigation or other means, is a viable explanation for the pattern of contamination,” according to the agency’s update.

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