Why E. coli romaine lettuce outbreak mystifies scientists

May 11, 2018

The struggle to identify the origins of the E. coli outbreak, which has sickened people from Washington state to New Jersey, highlights the difficulty of tracking a pathogen through a complex food supply chain, said a produce safety expert at Washington State University.

Conducting a traceback investigation from farm to plate is far from a direct line, but more of a massive web that requires tedious detective work, said Faith Critzer, a produce safety specialist and associate professor of food sciences at WSU extension in Prosser.

“The exact source of contamination could remain a mystery for some time. There are many dots to connect – on the human side and the product side,” she said.

Bare shelves in a Richland, Wash., grocery store after packages of romaine lettuce were voluntarily pulled. Stores and restaurants are restocking shelves with romaine grown in another region. Photo: Faith Critzer/WSU

Twenty-eight more illnesses have been attributed to the E. coli outbreak linked to romaine from Yuma, bringing the total to 149 cases.

As of May 9, cases had been reported from 29 states. Sixty-four people have been hospitalized, including 17 people who have developed kidney failure. One person has died.

The most recent illness started April 25, according to the Centers for Disease Control and Prevention, which issued an update May 9.

The Minnesota Department of Health listed May 2 as the onset date of the most recent illness in the state in a news release May 8.

California remains the area most affected by the outbreak, with 30 illnesses reported. Twenty cases have been reported by Pennsylvania.

Interviews with ill people continue to indicate romaine, with 102 of the 112 people interviewed telling the CDC they ate it in the week before their illness began.

Six people have been infected in Washington state.

Tracing product, patients

To unravel the mystery, investigators with the Centers for Disease Control and Prevention – working with state and local health officials – must cover a vast geographic area to interview infected people about everything they ate. The Food and Drug Administration then traces the many channels the lettuce took from the field to point of sale, Critzer said.

“Several weeks can pass from the time a person develops symptoms, goes to a doctor, gets the lab test results and is confirmed as part of the outbreak.  By then, the lettuce that person ate at a restaurant or home may be long gone and can’t be traced or tested,” she explained.

Adding to the challenge is the product supply chain. Leafy greens such as romaine are grown, packed and shipped by different companies before reaching retails stores and restaurants, making it more difficult to pinpoint where the food became contaminated.

“Investigators must examine all the routes between where the lettuce was grown and where it was sold,” said Critzer, requiring them to collect and analyze hundreds of records to narrow the investigation’s focus, she said.

Though a farm in Yuma, Arizona, was identified as the general source of whole-head romaine that sickened eight inmates in Alaska, health officials are not certain whether the contamination occurred in the field or along the packaging and distribution chain, according to the FDA.

“Most of the illnesses in this outbreak are not linked to romaine lettuce from this farm and are associated with chopped romaine lettuce (not whole head lettuce),” the agency said in a statement, adding that other farms are being investigated as well.

Use bacteria’s DNA

In the meantime, the Yuma growing season has ended, and California is the growing region throughout summer. However, the CDC’s warning not to eat romaine unless it’s known not to be from Yuma remains in effect.

Ideally, investigators will be able to isolate the E. coli strain from the source and use DNA sequencing to match it with the people who got sick, said Critzer.

Even then, the sleuth work will continue. Among other things, scientists will try to determine the reason for the outbreak’s high number of severe illnesses. It may be that a more virulent strain of E. coli is to blame, she said, or that the lettuce was contaminated with many bacteria.

– Linda Weiford, WSU News







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