Leafy greens contaminated with Shiga toxin-producing Escherichia coli have continued to cause foodborne illness outbreaks in recent years and present a threat to public health. An important component of foodborne illness outbreak investigations is determining the source of the outbreak vehicle through traceback investigations. The U.S. Food and Drug Administration is home to traceback investigation experts who use a standardized process to initiate, execute, and interpret the results of traceback investigations in collaboration with the Centers for Disease Control and Prevention and state and local partners. Traceback investigations of three outbreaks of Shiga toxin-producing E. coli infections linked to romaine lettuce in 2018 and 2019 were examined to demonstrate challenges, limitations, and opportunities for improvement. The three outbreaks resulted in a total of 474 illnesses, 215 hospitalizations, and 5 deaths. These illnesses were linked to the consumption of romaine lettuce from three distinct growing regions in Arizona and California. Some of the challenges encountered included the time it took to initiate a traceback, limited product-identifying information throughout the supply chain, lack of interoperability in record-keeping systems, and comingling of product from multiple suppliers. These challenges led to time delays in the identification of the farm source of the leafy greens and the inability to identify the root cause of contamination. Implementation of technology-enabled traceability systems, testing of these systems, and future regulations to incentivize adoption of traceability systems are some of the initiatives that will help address these challenges by improving traceback investigations and ultimately preventing foodborne illnesses and future outbreaks from occurring.

On February 20, 2019, PulseNet, the molecular subtyping network for foodborne disease surveillance, identified six Shiga toxin-producing Escherichia coli (STEC) O26:H11 infections with the same pulsed-field gel electrophoresis (PFGE) pattern combination.This PFGE pattern combination matched that of infections from a July 2018 outbreak that was associated with ground beef.In response, CDC initiated an investigation with federal, state, and local partners to identify the outbreak source and implement prevention measures.CDC defined a case as STEC O26 infection with an isolate matching the outbreak strain by PFGE or related by core genome multilocus sequence typing scheme (cgMLST), with dates of illness onset during December 11, 2018-May 21, 2019.Investigators initially hypothesized that ground beef was the outbreak cause because of the PFGE match to the July 2018 outbreak and because in early interviews, patients commonly reported eating ground beef and leafy greens.Investigators used cgMLST to compare the genetic sequences of isolates from both outbreaks and determined that they fell into separate genetic clades (differing by 6-11 alleles), suggesting that something other than ground beef caused the illness in 2019.CDC noted that one patient consumed raw cookie dough and that most patients were young adult females, similar to demographic distributions of past flour-associated STEC outbreaks (1-3).Investigators developed a supplemental questionnaire focusing on beef, leafy greens, and flour exposures.Twenty-one cases were reported from nine states (Figure).The median age of patients was 24 years (range = 7-86 years); 71% were female.Three patients were hospitalized, and none died.Among 13 patients asked about flour exposures, six reported eating, licking, or tasting raw homemade dough or batter during the week before illness onset.Three patients reported eating raw dough or batter made with the same grocery store brand of all-purpose flour, including a patient who reported eating raw dough at a bakery in Rhode Island.Overall, of 18 patients with store information, 11 reported shopping at this same grocery store chain.The Rhode Island Department of Health visited the bakery reported by the patient and collected flour for testing.On