By Ethan Covey
A large, multistate outbreak of Escherichia coli O157:H7 infections has been traced to food eaten at a national fast-food chain.
However, an investigation into the outbreak has demonstrated the challenges faced when trying to appropriately identify the source of outbreaks linked to single restaurant chains, according to the epidemiologists, who did not name the restaurant (MMWR Morb Mortal Wkly Rep 2023;72[26]:732-733).
The outbreak resulted 109 identified cases from six U.S. states: Michigan (67; 61%), Ohio (24; 22%), Indiana (11; 10%), Pennsylvania (four; 4%), Kentucky (two; 2%) and New York (one; 1%).
“With outbreaks at restaurants, identification of the contaminated ingredient, even with early identification and good traceback records, can be hampered because most people eating at those establishments ate the same few ingredients (i.e., at a fast-food burger restaurant, most people are eating beef, lettuce, onions, pickles, tomatoes and buns because burgers are the main menu item and consist of those ingredients),” said Christan Stager, DVM, a CDC Epidemic Intelligence Service officer class of 2021, and the lead author of the report.
“This is called ‘ingredient collinearity,’ and this can make it difficult to identify which ingredient could be causing the outbreak,” he continued.
Outbreaks involving multiple restaurants or grocery stores allow cross-checking of items served or bought at those locations to narrow down a common item with a common source. However, outbreaks with only one restaurant or restaurant chain don’t allow for such triangulation, complicating identification of the contaminated ingredient.
In this outbreak, illness onset occurred between July 26 through Aug. 24, 2022, and resulted in a fivefold increase in regional cases of E. coli O157:H7 infection.
Interviews conducted with 77% of the patients found that 83% of the group reported having eaten at the same fast-food chain within the week preceding illness onset. The most commonly reported food exposures were beef patties (78%) and romaine lettuce (68%).
Despite epidemiological, traceback and microbiological investigations, the contaminated ingredient was not confirmed.
“Many times, in outbreaks associated with restaurants, there is high turnover of ingredients, meaning the contaminated ingredient may no longer be in the restaurant by the time the outbreak is identified,” Dr. Stager said. “Depending on adherence to food safety protocols, a single contaminated ingredient can cross-contaminate other ingredients, making it difficult to identify the original ingredient. Food handlers who eat food from the establishment and become infected also have the potential to contaminate other ingredients.”
While confirming the source of outbreaks can be difficult, healthcare professionals and the general public can assist by following the CDC’s recommendations.
Additionally, CDC and public health partners are developing new ways to investigate and solve outbreaks more efficiently. Some of these initiatives include Shopper History Outbreak Partnership, using technology to improve traceback and advancing laboratory and data analysis tools for detecting and solving outbreaks).
“Communication and cooperation with state, local and federal partners, as well as the restaurant industry, are especially important in these types of foodborne outbreaks,” Dr. Stager said.
