Green leafy plants are a common culprit of foodborne illness, with the products linked to 40 outbreaks of a serious strain of E. coli from 2009 to 2018, according to a report published Wednesday in the journal Emerging Infectious Diseases.
Among these outbreaks, one salad in particular was to blame: Romaine. (Remember 2018, a year shaken by two massive romain-linked E. coli outbreaks?)
Of outbreaks associated with a particular leaf green – rather than a mixture – 54 percent were associated with romaine. Spinach and iceberg lettuce were each linked with 1
It is not entirely clear why romaine was the most common sinner in the outbreak. The researchers noted, for example, that more iceberg lettuce was harvested and sold each year from 2009 to 2017 than romaine.
Romaine grew in popularity during the decade analyzed, the researchers wrote: At the end of the study period, more money was spent on romaine lettuce than on the iceberg. But this alone does not explain why romaine was responsible for so many outbreaks.
Another possible explanation, as the researchers claimed, is the lettuce form, which can provide an entry point for pathogens: “Romaine is tall with loosely clumped leaves, open at the top; the iceberg is smaller with compact leaves.”
Other outbursts were linked to mixed greens, including three blends of romaine and icebergs, a butter salad and a radicchio blend, and a spinach and spring mix.
The report focused on outbreaks of a type of E. coli called Shiga toxin-producing E. coli or STEC. The name refers to a toxin produced by the bacteria that makes people sick. Symptoms may include diarrhea and vomiting, according to the Centers for Disease Control and Prevention. Most people recover on their own within 5 to 7 days, although some may need medical attention.
The decade of outbreaks that occurred in the United States, Canada, or both countries accounted for 1,212 diseases, 420 admissions, and eight deaths, according to the report. A total of 77 cases of hemolytic uraemic syndrome, a kidney problem requiring hospitalization, were also attributed to the outbreaks.
Shiga toxin-producing E. coli is associated with approx. 265,000 diseases each year in the United States, according to the report. In particular, one type of STEC, STEC O157, tends to cause more serious illness. The most common source of this infection is minced beef followed by leafy vegetables. In fact, STEC O157 was responsible for 32 of the outbreaks described in the report.
Although leafy vegetables are grown year-round in the United States, more outbreaks began in October and April than any other month of the year. It is unclear why this seasonality in outbreaks occurred, the study authors wrote.
There are several reasons why leafy vegetables are particularly susceptible to E. coli contamination, starting with how crops are grown, said Benjamin Chapman, professor and food safety specialist at North Carolina State University.
“The vast majority of lettuce production is outside and requires a lot of water,” Chapman, who was not involved in the report, said in an email. And in the United States, it is mostly grown in areas where animals – a source of E. coli – are also bred. E. coli contamination can come from sources such as irrigation water, animals and handling.
“We know from previous outbreaks that a bit of contamination in the field can lead to cross-contamination,” he said.
Once this contamination has entered the plant, it is “very difficult to remove,” Chapman said. The product triples in processing plants, and in the home, a person may be able to flush “90 to 99 percent of what is there, but that may not be enough, depending on how much” pollution there is, he added.
And because lettuce is almost always eaten raw or undercooked, “any contaminant that makes it to the plate ends up in the gut,” he said.
Avoiding outbreaks is unfortunately a problem with the supply chain, he said.
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