It’s Always Lettuce

The article discusses a significant outbreak of cyclosporiasis in the U.S., suspected to be linked to iceberg lettuce supplied by Taylor Farms, infecting over 7,100 people in Michigan alone. Historically, leafy greens like lettuce have been implicated in multiple foodborne illness outbreaks, including instances of E. coli and norovirus. The Centers for Disease Control and Prevention identifies produce as a leading source of foodborne illnesses, with leafy greens at the forefront due to their high consumption in the U.S. One of the main risks is the washing and handling processes, which may not effectively eliminate pathogens. Contamination can happen at several points in the supply chain, particularly from irrigation practices and the texture of lettuce, which can harbor pathogens. Experts suggest thorough washing, especially of salad greens, is essential to reduce infection risk.

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Salad is usually synonymous with healthy eating. Except now, as the US grapples with an expanding outbreak of cyclosporiasis that’s sickened more than 7,100 people in Michigan alone, likely caused by iceberg lettuce supplied by Taylor Farms.

In fact, leafy greens are frequently implicated in instances of foodborne illness. There was a 2022 multistate E. coli outbreak tied to romaine served on Wendy’s sandwiches and 173 cases of norovirus linked to Illinois eatery D.C. Cobb’s. In 2021, BrightFarms recalled packaged salad greens sold in several states after 31 people got salmonella. And in 2019, a total of 167 people were infected with E. coli after eating romaine harvested from the Salinas Valley region in California. This isn't even the first cyclospora outbreak linked to lettuce: In 2020, Fresh Express salad kits were linked to more than 700 cases of people afflicted with the parasite.

The Centers for Disease Control and Prevention estimates that produce is responsible for almost half of the reported foodborne illness cases, with leafy vegetables topping the list. So what is it that makes lettuce a major source of America’s foodborne tummy troubles?

One big reason is simply because lots of people eat it. Lettuce is the second-most-consumed vegetable in the US behind potatoes. (The latest outbreak ensures potatoes will remain the king of US veggies. Plus, french fries.) Wraps, subs, tacos, you name a fast food sandwich and it’s almost certainly got lettuce on it. Salad-slinging chains are fixtures across the US. Grocery store coolers are stocked with salad kits, on top of the unadorned heads of iceberg lettuce and containers of spinach.

Those ready-to-eat salads and blends of spring mix pose a particularly acute risk because they contain multiple types of leafy greens, which can come from different farms and locations. If one type of lettuce in the mix is contaminated, it can contaminate the whole bag. And because that contaminated lettuce gets packaged into lots and lots of bagged salad, it has the potential to spread germs far and wide.

How lettuce is grown can also expose it to a wide array of pathogens. A 2025 study by researchers at Cornell University who studied E. coli outbreaks in romaine lettuce found that a lot of contamination originates from irrigation with untreated surface water applied through overhead spray irrigation systems. There are ways to lower that risk, including switching to irrigation systems that do not directly touch the leaves, but that would require upgrades and downtime.

Lettuce’s satisfying crunch is also what makes it prone to pesky pathogens. Since it’s eaten raw—really, who wants wilted lettuce?—there’s no opportunity to cook off any lingering contaminants. Like other raw fruit and produce, leafy greens don’t go through what’s known as a “kill step”—heating, pasteurization, or freezing—which is designed to eliminate pathogens during the food manufacturing process.

The texture of lettuce is an additional factor in its ability to spread disease.“Many lettuce types have a lot of crevices and folds,” says Jeanne Marrazzo, CEO of the Infectious Diseases Society of America. “It’s very easy for these tiny pathogens to get in there.”

Even prewashing procedures at manufacturing facilities don’t always remove microbes, which can cling tightly to leaves. In the case of cyclospora, the parasite has thick outer walls, making it highly resistant to most food sanitizers. Experts say vigorous washing that involves scrubbing is the best way to get rid of cyclospora.

“If you don't wash these products well, you're going to end up ingesting whatever is on the lettuce,” says Amira Roess, an epidemiologist at George Mason University.

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Emily Mullin is a staff writer at WIRED, covering biotechnology. Previously, she was an MIT Knight Science Journalism project fellow and a staff writer covering biotechnology at Medium's OneZero. Before that, she served as an associate editor at MIT Technology Review, where she wrote about biomedicine. Her stories have also ... Read More

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It’s Always Lettuce

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