An E. coli Outbreak Traced to a Fast-Food Chain — but Never to One Ingredient

In August 2022, Michigan noticed about five times the usual number of Escherichia coli O157:H7 infections. Most of the sick had eaten at the same national fast-food chain. Genetic testing, interviews and supply-chain tracing followed — and yet investigators never pinned the outbreak on a single ingredient. Why is as instructive as the outbreak itself.

The outbreak

Cases were defined by whole-genome sequencing: infections within 0–2 alleles of the outbreak strain, with illness beginning July 26–August 24, 2022. CDC's PulseNet network found 109.

Patients ranged from 1 to 94 years old (median 22), and 45 percent were female. Fifty-two (48 percent) were hospitalized and 13 (12 percent) developed hemolytic-uremic syndrome, a serious complication; no one died.

Following the food

• Of 84 patients interviewed, 70 (83 percent) had eaten at the chain in the week before falling ill, and 11 restaurant clusters were identified.
• Among 68 who gave details, the most common exposures were beef patties (78 percent) and romaine lettuce on sandwiches (68 percent). Early in the investigation, lettuce exposure topped 90 percent, and the chain pulled lettuce in the affected states.
• The Food and Drug Administration traced the romaine and the U.S. Department of Agriculture traced the beef. Neither found a single lot that could explain all the illnesses. Tests of restaurant food and supply-chain samples never found the outbreak strain.
• Some infected food handlers were identified and may have amplified the outbreak locally, but many restaurant clusters had none, so they were unlikely to be the source.

Why the source stayed hidden

• Ingredient collinearity. The same ingredients appear together in many menu items, so no one item stood out.
• Cross-contamination among ingredients or from sick workers blurred the picture.
• No outside cluster. Investigators usually triangulate a source by finding the same food at an unrelated restaurant with a different supply chain. There was no such cluster here.

What worked anyway

Clear communication among the states, FDA, USDA and the chain led to quick removal of the suspected romaine lettuce. No outbreak-associated illnesses were reported after it was removed.

Sources

Based on "Notes from the Field: Multistate Outbreak of Escherichia coli O157:H7 Infections Linked to a National Fast-Food Chain — United States, 2022," by Christan Stager and colleagues (CDC, Michigan health and agriculture departments, FDA, USDA Food Safety and Inspection Service), Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.