A report on the largest multistate produce-linked Shiga toxin-producing Escherichia coli (STEC) outbreak in several decades in the United States highlighted challenges in determining the route of contaminated lettuce and in providing rapid, actionable guidance during an outbreak. The report was published in Clinical Infectious Diseases.

In April 2018, a cluster of infections with STEC strain O157 associated with multiple locations of a restaurant chain were reported in New Jersey and Pennsylvania. The Centers for Disease Control and Prevention queried the national laboratory network for foodborne disease surveillance for additional cases and began a national investigation.

Cases were defined as infections occurring between March 13, 2018 and August 22, 2018 with 1 of 22 identified outbreak-associated E coli O157:H7 or E coli O61 pulsed-field gel electrophoresis pattern combinations or by being a strain of STEC O157 that was closely related to the main outbreak strain, as demonstrated by whole genome sequencing. Further, the CDC developed a focused questionnaire to assess leafy-green consumption and grocery stores and restaurants from which individuals had obtained food.

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In total, 240 case-patients were identified from 37 states, of whom 104 were hospitalized, 28 experienced hemolytic uremic syndrome, and 5 individuals died. Interviews conducted with 179 patients found that 152 (85%) reported consuming romaine lettuce in the week before illness onset. Among these cases, 22 subclusters were identified. Product traceback from subcluster restaurants identified numerous romaine lettuce distributors and growers. However, all of the lettuce originated from the Yuma County growing region of Arizona. Further water sampling from an irrigation canal in the region yielded the outbreak strain of STEC O157.

This outbreak was larger than any of the 28-state outbreaks related to leafy green produce reported from 1998 to 2016. Although it was unclear why this outbreak was so large, investigators pointed to the volume of contaminated produce as playing a role. A conclusion supported by the findings that the outbreak strain was identified along multiple points of a 3.5-mile stretch of an irrigation canal with traceback that led to 36 romaine fields.

Along with the unusually large size of the outbreak, the case-patient clinical course was also unusually severe. Investigators believed this could be explained by the STEC strain’s toxin subtype, which produces more virulent toxins than other strains. The study authors stressed that, “communicating unusually severe outcomes associated with STEC infections during an outbreak to healthcare providers is important.” They recommended that clinicians be made aware of ongoing outbreaks and test patients exhibiting diarrhea, especially bloody, diarrhea with low or absent fever. Antibiotics should not be administered to these patients because doing so increases the risk of severe outcomes, even if the bacteria are susceptible to antibiotics.

During this outbreak,  a challenge persisted to provide clear and actionable messaging to consumers and retailers because a single source of contamination had not yet been identified. Although the growing region was identified, packaging rarely contains this information. Furthermore, several factors complicated the traceback investigation during this outbreak.

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“This outbreak highlighted challenges in quickly determining the source and route of outbreak pathogens contaminating leafy greens, and difficulties in rapidly providing actionable guidance to consumers and retailers during leafy greens-associated outbreaks,” reported investigators. They highlighted the need for more research into STEC’s ecology in regions growing ready-to-eat foods, evaluations of efforts to assure agricultural water monitoring, and improved labelling and record keeping. Overall, the investigators stressed enhanced collaboration between health and agriculture officials, food industries, and academia.


Bottichio L, Keaton A, Thomas D, et al. Shiga toxin-producing E. coli infections associated with romaine lettuce – United States, 2018 [published online December 9 2019]. Clin Infect Dis. doi:10.1093/cid/ciz1182

This article originally appeared on Infectious Disease Advisor