The Food and Drug Administration has identified one farm in California as a likely source of E. coli-tainted romaine lettuce that sickened 62 people in 16 states and the District of Columbia last fall, but can't rule out if others were involved.

The agency said in its investigation, released today, that there was “insufficient evidence” to show the Adams Bros. Farming Inc. location in Santa Barbara County, California was the sole source of the outbreak, leaving questions about the definitive origin and scope of the outbreak.

The final report did not specify a singular point of origin for the outbreak, only noting it was "most likely" way the lettuce became contaminated was due to the "use of water from this reservoir as agricultural water."

Investigators collected and tested more than 150 samples but could only find the specific strain of E. coli in a reservoir used by the farm.

“It is believed that this water came into contact with the harvested portion of the romaine lettuce, since the outbreak strain of E. coli O157: H7 was found in sediment from the reservoir and in no other sampled locations,” FDA Commissioner Scott Gottlieb and Deputy Commissioner Frank Yiannas said in a statement. “The water from the reservoir doesn’t explain how lettuce grown on other ranches or farms identified by traceback may have been contaminated. So, this one farm cannot explain the entire outbreak.”

On Dec. 17, that farm “issued a recall of romaine lettuce and other products — including red leaf lettuce, green leaf lettuce and cauliflower — that may have also been contaminated due to use of agricultural water from the on-farm reservoir,” they said.

The officials said the investigation concluded the farm had a procedure “to collect and test reservoir agricultural water for generic E. coli and to treat the agricultural water with a sanitizer before use. However, the investigation team noted the verification procedure records did not document that sufficient sanitizer was present to adequately reduce any pathogens present in the water when this water was used for direct contact with romaine lettuce at harvest, during postharvest handling, and to wash/rinse harvest equipment food contact surfaces.”

FDA's investigation report noted traceback procedures indicated "the contaminated lettuce may have been grown on multiple farms as not all traceback legs led to the farm on which sediment from a water reservoir tested positive for the E. coli O157: H7 outbreak strain.

"An alternative explanation for this lack of traceback convergence to a single farm may be due to ill consumers having multiple romaine lettuce exposures and limited recollection of exposures and brands they ate," the report added.

Food safety problems related to raw whole and bagged salad leafy greens “are a longstanding issue,” Gottlieb and Yiannas said. “The FDA and CDC identified 29 foodborne illness outbreaks of Shiga-toxin producing E. coli with a confirmed or suspected link to leafy greens in the United States between 2009 and 2017.”

The investigation was conducted by the FDA’s Produce Safety Network, the Centers for Disease Control and Prevention, the California Department of Food and Agriculture, and the California Department of Public Health.

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