Multi-state E. coli Outbreak Linked to Romaine Lettuce

 By Heidi Meier and Ann Filmer

The U.S. Centers for Disease Control and Prevention (CDC) released a media statement in late December profiling a multi-state outbreak of food poisoning caused by the bacteria E. coli O157:H7 with 17 reported illnesses. Romaine and leafy greens are among the suspected sources of contamination, but no definitive source or location has been confirmed at this time, according to the CDC.

A lettuce field in California (photo by Trevor Suslow, UC Davis)

According to Trevor Suslow, produce safety scientist in the UC Davis Department of Plant Sciences, and Director of the Postharvest Center at UC Davis, “If this outbreak is confirmed to have been caused by romaine lettuce, we emphasize that this is a highly perishable product with a limited usable shelf life. It is unlikely a compromised lot is still available for sale or in a home refrigerator.”

Canada recently experienced a similar outbreak, but neither the epidemiology and trace-back of the clinical cases in Canada, nor the U.S., have identified a source of contamination. In Canada, the case for romaine lettuce as the implicated food vehicle seems strong, but this is not clear in U.S. cases, Suslow said.

“To the best of my knowledge, no public health agency has contacted any romaine lettuce grower, shipper, or processor and requested that they either stop shipping or recall product already in the marketplace,” said Suslow. “It is not definitive, at this time, that romaine lettuce is the involved food in U.S. cases, so agencies have not issued any consumer advisories. Currently, a lot of dissatisfaction with this reluctance to share what is known has emerged from legislators, industry, and consumer advocacy groups.”

Although the CDC has not identified a specific food or foods that caused this outbreak, preliminary analysis of the genetic relation between isolates found in Canada and the U.S. indicates strong similarity in some, but not all, of the cases. This suggests that some cases may share a common source, but the current public position is that the epidemiologic information fails to meet standards to link these outbreaks. Whole Genome Sequence information in the public database is a powerful tool for linking widespread and non-clustered illnesses.

A shelf life of five weeks (as announced by Canada Public Health) is not realistic or practical in cold-chain temperature regimes, foodservice, or in home consumer refrigeration conditions. If held at 4C (39F), the maximum timeframe expected shelf life to maintain acceptable visual and textural quality would not exceed 21–24 days.

“One key challenge for consumers, expressed in several broadcast media interviews, is to accept that they trusted the supply of romaine lettuce a few months ago, even a few days ago,” said Suslow. “How can an individual assess their personal risk:benefit decision? None of us can reconcile this uncertainty but, personally, while continual improvement in our food safety systems and knowledge gaps must be the attitude, as a consumer I have to trust in the processes and oversight currently in place.”

More information

Centers for Disease Control information on coli in romaine

Information on Whole Genome Sequencing (WGS)

UC Davis Postharvest Center information about shelf life and quality indices of romaine lettuce 

NBC News information about E. coli and romaine lettuce

Ann Filmer is communications director for the UC Davis Department of Plant Sciences. Heidi Meier manages communications for the UC Davis Postharvest Center

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