Leafy greens are a common culprit of foodborne illnesses, with the produce linked to 40 outbreaks of a serious strain of E. coli from 2009 to 2018, a report published Wednesday in the journal Emerging Infectious Diseases finds.
Of outbreaks linked to a specific leafy green — rather than a mix — 54% were linked to romaine. Spinach and iceberg lettuce were each linked to 17% of the outbreaks, and cabbage, green leaf and kale were each linked to 4%.
It's not entirely clear why romaine was the most common culprit in the outbreaks. The researchers noted, for example, that more iceberg lettuce was harvested and sold each year from 2009 to 2017 than romaine.
Romaine did grow in popularity during the decade analyzed, the researchers wrote: By the end of the study period, more money was spent on romaine lettuce than on iceberg. But this alone doesn't explain why romaine was responsible for so many outbreaks.
Another possible explanation, the researchers posited, is the lettuce's shape, which could provide an entry point for pathogens: "Romaine is tall with loosely clumped leaves, open at the top; iceberg is smaller with compact leaves."
Other outbreaks were linked to mixed greens, including three romaine and iceberg mixes, a butter lettuce and radicchio mix, 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 can include diarrhea and vomiting, according to the Centers for Disease Control and Prevention. Most people recover on their own within 5 to 7 days, though some may need medical care.
The decade of outbreaks, which occurred in the United States, Canada or both countries, accounted for 1,212 illnesses, 420 hospitalizations and eight deaths, according to the report. A total of 77 cases of hemolytic uremic syndrome, a kidney problem that requires hospitalization, were also attributed to the outbreaks.
Shiga toxin-producing E. coli are linked to about 265,000 illnesses each year in the U.S., according to the report. One type of STEC in particular, STEC O157, tends to cause more severe illness. The most common source of this infection is ground beef, followed by leafy greens. Indeed, STEC O157 was responsible for 32 of the outbreaks described in the report.
Though leafy greens are grown year-round in the U.S., more outbreaks began in October and April than any other month of the year. It's unclear why this seasonality in outbreaks occurred, the study authors wrote.
There are several reasons why leafy greens are particularly susceptible to E. coli contamination, starting with how the crops are cultivated, Benjamin Chapman, a professor and food safety specialist at North Carolina State University, said.
"The vast majority of lettuce production is outside and requires a lot of water," Chapman, who wasn't involved with the report, said in an email. And in the U.S., it's mostly grown in areas where animals — a source of E. coli — are also raised. E. coli contamination can come from sources such as irrigation water, animals and handling.
"We know from earlier outbreaks that a little bit of contamination in the field can lead to cross contamination," he said.
Once that contamination has made it onto the plant, it's "very difficult to remove," Chapman said. Produce is triple-washed in processing plants, and in the home, a person may be able to rinse off "90 to 99% of what's there, but that may not be enough depending on how much" contamination there is, he added.
And because lettuce is almost always eaten raw or undercooked, "any contamination that makes it to the plate ends up in the gut," he said.
Avoiding outbreaks entirely is, unfortunately, a supply chain issue, he said.
This story was originally published on NBC News.