This Type of Lettuce Has Been Linked to More Outbreaks of a Deadly Bacteria Than Any Other Leafy Green

Romaine lettuce has been linked to more outbreaks of a potentially deadly bacteria than any other leafy green in North America, according to a study. But experts told Newsweek consumers shouldn't regard such foods as unsafe.

Between 2009 to 2018, there were 40 outbreaks of Shiga toxin–producing Escherichia coli (STAC) associated with leafy greens in the U.S. and Canada, causing eight deaths.

Of those outbreaks, 31 were in the U.S., 22 at a multi-state level; four in Canada, all affecting multiple provinces; and five in both countries.

A total of 1,212 people fell ill, 420 were hospitalized, and 77 had the potentially life-threatening kidney problem hemolytic uremic syndrome (HUS).

STEC bacteria are estimated to cause 265,000 illnesses per year in the U.S., according to the paper published in the journal Emerging Infectious Diseases. The bacteria fit into one of two categories: STEC O157 and non-O157 STECs.

STEC O157 is less common but can cause severe disease, and is more likely to cause a person to become hospitalized and develop HUS than non-O157 types, the authors wrote.

Some 98 percent of leafy greens produced in the U.S. are grown in California and Arizona. Mexico also imports leafy greens to the U.S. Most of those eaten in Canada are imported from the U.S., according to the paper.

What are known as ruminant animals, such as cows, are thought to be the most common host. Animals can contaminate leafy greens directly or indirectly, through irrigation water, or dust containing feces.

The researchers drew data from the U.S. Centers for Disease Control and Prevention's (CDC) Foodborne Disease Outbreak Surveillance System, the Public Health Agency of Canada, and a national network for foodborne disease surveillance called Pulse Net.

This included information gathered by health officials who interviewed the sick and asked them about what they ate and where the food originated, for instance a restaurant or grocery store. The authorities also sampled foods and the environments like restaurants or food production facilities thought to be the source of outbreaks.

In the study, an outbreak was defined as where at least two people had had a similar illness with a common exposure.

Of the total 40 outbreaks, there was information on the specific leafy green in 29 cases, and 24 linked to a single type.

Most of the outbreaks were linked to romaine lettuce, making up 54 percent, followed by spinach (17 percent), iceberg (17 percent), and 4 percent each for cabbage, green leaf, and kale. Five outbreaks were linked to multiple leaf types: three romaine and iceberg, 1 butter and radicchio, and 1 spinach and spring mix.

The study revealed most outbreaks occurred in the fall and spring, at 45 percent and 28 percent, respectively. It was unclear what was behind the seasonality, the team said.

The researchers found STEC O157 was the most common cause of leafy green STEC outbreaks, making up 32 (80 percent) of those studied.

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A stock image shows a man looking at a packet of salad. Scientists have investigated outbreaks of STEC in the U.S. Getty

Of the total outbreaks, five resulted in a food recall, involving shredded romaine, bagged spinach and spring mix, shredded iceberg and romaine, and ready-to-eat salads and sandwich wraps containing romaine.

The team said it may be the case that the shape of romaine lettuce, which has loosely clumped leaves which are open at the top, may make it more prone to contamination, but more research is needed to prove this.

"Over the past decade, multiple STEC outbreaks linked to leafy greens occurred in the United States and Canada, causing illness that was widespread and often severe," the authors wrote.

Katherine Marshall, senior epidemiologist in the CDC's Outbreak Response and Prevention Branch, told Newsweek her team's study showed "we have more work to do to make leafy greens safer."

Marshall said it wasn't clear why romaine lettuce was linked to more outbreaks than other leafy greens. It may be because it is more popular, or its shape.

"Even though we identified 40 outbreaks over a ten year period in the US and Canada, these are relatively rare events. The U.S. food supply is one of the safest in the world, and millions of servings of leafy greens are eaten safely every day," she said.

Martin Wiedmann, a professor in food safety at Cornell CALS who did not work on the study, told Newsweek the study was limited because the root causes of contamination could typically not be determined, which meant the article provided limited information on how to reduce the risk of E. coli contamination.

'Small risks'

Wiedman said readers should not regard leafy greens and romaine lettuce as unsafe, as about 8 billion pounds of leafy green are consumed annually in the U.S. alone. "This clearly indicates that the risk of getting sick from leafy greens is very low."

No food is zero risk, "there really is no activity in life that carries zero risk," said Wiedman, and the benefits of eating leafy greens "by far outweigh the small risks."

Wiedman said consumers should rinse produce under cold running water, and not treat produce with sanitizers and disinfectants as there is a risk of intoxication and negative health effects.

Barbara Kowalcyk, a director of the Center for Foodborne Illness Research and Prevention at The Ohio State University who did not work on the study, told Newsweek it is important to note that outbreaks represent a small percentage of all foodborne illness cases.

She said: "Children, pregnant and postpartum women, senior citizens and individuals with compromised immune systems should be aware that they are at high risk of serious complications from foodborne illness and may want to consider that when making decisions about consuming foods—such as leafy greens—that have been associated outbreaks."