An Overview of Traceback Investigations and Three Case Studies of Recent Outbreaks of Escherichia coli O157:H7 Infections Linked to Romaine Lettuce

Abstract

Abstract: 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.

Keywords: Foodborne illness outbreaks; Leafy greens; Shiga toxin–producing Escherichia coli; Traceback investigations.

FIGURE 1.

An example of a traceback timeline for a multi-state foodborne illness outbreak linked to the consumption of romaine lettuce. The source of origin and the destination of each shipment is displayed vertically on the left. The dates of shipments are displayed horizontally on the top. Dates of shipments, product amounts, and lot code information of purchases of romaine lettuce products are traced and displayed in each box from the point of service, through the distribution chain, to processors, distributors, and farms. Implicated shipments that match the timeframe of illness and purchase date (May 18) for the selected timeframe of interest in the boxes are highlighted and arrows display the direction of the shipments from farms, to distributors, to processors, and to the points of service.

FIGURE 2.

An example of a traceback diagram for a multi-state foodborne illness outbreak linked to the consumption of romaine lettuce. Purchases of romaine lettuce products are traced from the point of service, through the distribution chain, to processors, distributors, and farms. Product originates from individual ranches, a subdivision of farms, denoted on the right side of the diagram.

FIGURE 3.

Level and strength of convergence identified by traceback investigations in scenario A, where three POS trace back to a single distributor and three different farms and scenario B, where three POS trace back to three distributors and a common farm.

FIGURE 4.

Traceback diagram for multi-state foodborne illness outbreak of STEC infections linked to the consumption of romaine lettuce from the Yuma growing region in spring, 2018. Purchases of romaine lettuce products are traced from the point of service, through the distribution chain, to processors, distributors, and farms. Product originates from individual ranches, a subdivision of farms, denoted on the right side of the diagram*. *Updated as of October 22, 2020

FIGURE 5.

Traceback diagram for multi-state foodborne illness outbreak of STEC infections linked to the consumption of romaine lettuce from the Santa Maria growing region in fall, 2018. Purchases of romaine lettuce products are traced from the point of service, through the distribution chain, to processors, distributors, and farms. Product originates from individual ranches, a subdivision of farms, denoted on the right side of the diagram.

FIGURE 6.

Traceback diagram for multi-state foodborne illness outbreak of STEC infections linked to the consumption of romaine lettuce from the Salinas growing region in spring, 2019. Purchases of romaine lettuce products are traced from the point of service, through the distribution chain, to processors, distributors, and farms. Product originates from individual ranches, a subdivision of farms, denoted on the right side of the diagram.