In 2019, a large outbreak caused 167 cases and hospitalized 85 people from 27 states. Image Credit: Shutterstock

The Centers for Disease Control and Prevention said Wednesday that a relatively new strain of E. coli is responsible for multiple outbreaks of foodborne illness in recent years, including those related to romaine lettuce and other leafy greens.

The strain, which the CDC refers to as REPEXH02, is believed to have emerged in late 2015 and is responsible for dozens of hospitalizations and many cases of hemolytic uremic syndrome, which can impede blood clotting in infected people and cause kidney failure, the agency said in a report. More study is needed to understand factors that contribute to the bacteria's emergence and persistence in specific environments, the authors wrote.

Although cattle have long served as the main vector for E. coli illnesses in humans, recent outbreaks have been associated with consuming leafy greens. The CDC has classified this strain as reoccurring, emerging and persistent.

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"We typically think of foodborne illness as either part of an outbreak or not," said Jessica Chen, lead author of the study. "In 2019 we switched to using whole genome sequencing to look at the DNA of a strain and track the bacteria which cause foodborne illness. With this tool we can see if multiple outbreaks are caused by the same strain, and link related illnesses over months and years. We call strains that recur, are emerging or persisting over time REP strains of bacteria."

Chen said this newly identified strain has a toxin type associated with more severe disease in those infected.

E. coli bacteria live in the intestines of healthy people and animals, and most types are harmless or can cause brief bouts of diarrhea. But one particular strain, E. coli O157:H7, is estimated to cause 63,000 foodborne illnesses and 20 deaths in the United States each year. REPEXH02 is a sub-strain of this.

The pace and magnitude of E. coli outbreaks have increased in recent years, according to CDC data.

According to the new study, 58 percent of recent E. coli illnesses were attributed to vegetable row crops, largely leafy greens. In 2019, a large outbreak caused 167 cases and hospitalized 85 people from 27 states. It was associated with the consumption of romaine lettuce from California's Salinas Valley. In another outbreak in late 2020, 40 infections occurred in 19 states, 20 people were hospitalized and four developed hemolytic uremic syndrome.

There are several reasons for an uptick in outbreaks related to leafy greens, said Bill Marler, a foodborne illness lawyer and star of Netflix's new documentary "Poisoned," which examines the U.S. food industry and its history of foodborne illness outbreaks.

"There are more and more people wanting products like triple-washed bagged lettuce, but bagged salad is a great vector for E.coli growth," he said. "And farms have expanded closer and closer to animal feedlots and dairies, and these are now more prone to flooding."

With that flooding, he says, E. coli from cattle fecal matter is making it into water used to irrigate leafy greens - greens that often don't have a "kill step" such as cooking to render the bacteria harmless.

And with the current regulatory system, federal agencies in charge of food safety "aren't allowed to go onto these feedlots and do any kind of testing to help them get a sense for the source of these outbreaks. They can only take educated guesses, which limits their ability to find the source and to recall products," said Brian Ronholm, director of food policy for Consumer Reports.

"In the current system, there is not much water testing," and much of the testing that does occur is on a voluntary basis, Ronholm said. "And the agencies have a limited ability to test."

This week, the Environmental Protection Agency denied a 2017 petition submitted by Food & Water Watch and dozens of co-petitioners urging the agency to tighten regulations on factory farm water pollution under the Clean Water Act. The EPA said it plans to study the issue.

Ronholm describes "a frustrating conundrum for consumers": Leafy greens are integral for a healthy diet. And yet they tend to be the highest-risk products you can purchase from a food safety standpoint. And that risk grows in the fall.

E. coli outbreaks in bagged romaine are more common at the end of the growing season, according to U.S. Department of Agriculture data. Most recorded outbreaks have involved romaine lettuce harvested in the fall on the California Central Coast such as in Salinas, and in late winter in Southern California and Arizona. Those two states are the major lettuce-growing areas in the United States, with farm production valued at nearly $3 billion.

One of the most significant findings of the USDA's study is that the E. coli pathogen survived on average 5.6 times better in cold-stored packaged romaine harvested in the fall than on the same varieties harvested in late spring.

"Our results strongly indicate that fall-harvested romaine and the microbe communities it harbors have intrinsic characteristics that make them a better place for E. coli to survive in fresh-cut product," said microbiologist Maria Brandl, leader of the study.

It is unknown whether these hardier fall-harvested pathogens are in fact the REPEXH02 strain.

Chen says that investigating REP strains will hopefully provide the CDC opportunities to partner with regulatory agencies, academia and industry to better understand how these strains cause illnesses over long periods and how to prevent them in the future.