By Mahita Gajanan
November 21, 2018

All the romaine lettuce in the U.S. is currently unsafe to eat, the Centers for Disease Control and Prevention has said, warning consumers that an E. coli outbreak is linked to the salad staple.

The warning comes after 32 people in 11 states reported E. coli infections. The number of ill people is expected to increase, says Laura Gieraltowski, team lead of the CDC’s foodborne outbreak response team.

Gieraltowski says the CDC took the somewhat extreme measure of warning people to avoid all romaine lettuce because the agency has not yet identified a common grower or supplier that could be the source of the outbreak. Having that information could help the CDC issue a more specific and limited warning.

California, which grows the majority of romaine lettuce shipped around the country this time of year, has reported the highest number of E. coli cases in the current outbreak, with 10 people infected so far. Investigators will likely look into those cases to determine where the lettuce came from and where it was sent. “Where the cases are can often give us information on how the food was distributed,” Gieraltowski says.

Romaine lettuce itself is not inherently more likely to cause E. coli outbreaks than similar produce. But vegetable row crops, which include leafy greens like romaine, are a major source of E. coli, according to Gieraltowski. Such crops are responsible for about 40% of E. coli outbreaks.

“It’s a common vehicle for E. coli O157,” she says, adding that leafy greens pose a further risk of infection because they are often not served cooked, a process that kills germs.

The strain of E. coli causing the current outbreak matches that of the DNA in an E. coli outbreak in late 2017 that was linked to leafy greens. While investigators were not able to pinpoint the specific source of the culpable greens in that outbreak, Gieraltowski says they may be able to use the information collected last year to find a common grower and work to reduce contamination in the area, if the cause is environmental.

The DNA match marks a notable factor in the whole case, says Bill Marler, a food safety lawyer with Marler Clark whose work involves representing victims of foodborne illnesses.

“What that tells you is those two cases came from the same source, and likely came from the same environmental area, like a farm in a valley,” he says. “E. coli isn’t everywhere and where it is, it tends to populate and and share the same genetics. Hopefully they’re able to find out what caused it. That’s how people learn the hard way to prevent the next outbreak.”

Marler says that in his experience, E. coli outbreaks are typically tracked back to dairy farms and cow feedlots located near the growing areas of contaminated vegetables. Water sources that run through areas populated by cows can lead to contaminated crops.

“There’s always a cow somewhere lurking in the background,” he says. “When you grow stuff outside you’ve got to be acutely aware of the environmental pathogens. Do things to minimize risk. Like, don’t grow romaine lettuce across the street from a dairy farm.”

Write to Mahita Gajanan at mahita.gajanan@time.com.

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