Why is lettuce so often the culprit in illness outbreaks linked to the bacteria E. coli?
A total of 121 people from 25 states have become ill from E. coli contamination linked to romaine lettuce between March 13 and April 21, the US Centers for Disease Control and Prevention reported Wednesday. There has been one death in California resulting from an E. coli infection.
E. coli can be found living in the intestines of both people and animals, as well as in food and in the environment. Almost all strains of E. coli are harmless, but some can cause stomach cramps, diarrhea and vomiting.
Healthy adults usually recover from an infection of E. coli within a week, but some strains can cause more severe illness, especially in young children and older adults, who are at greater risk of developing kidney failure.
"Leafy greens, such as lettuce, can become contaminated in the field by soil, contaminated water, animals or improperly composted manure," said Jeff Farber, director of the Canadian Research Institute for Food Safety and a professor at University of Guelph in Ontario. "Lettuce can also be contaminated by bacteria during and after harvest from handling, storing and transporting the produce."
Usually, people eat romaine lettuce without cooking it, which could kill the germs. "Other raw fruits and vegetables that have come into contact with feces from infected animals are another common source of infection," Farber said.
Popularity also plays a role in why lettuce is a frequent bad actor: "Lettuce is also eaten the most out of all the produce items," he said.
From 2010 through the current outbreak, nine outbreaks have been caused by green lettuce or sprouts, compared with 12 from all other food groups, including meat, flour and prepared products, the CDC reports.
Many modes of contamination
In the current outbreak, 52 of the 102 patients who have been interviewed by public health officials have been hospitalized, including 14 who developed a type of kidney failure called hemolytic uremic syndrome. This 51% hospitalization rate is higher than the 30% typically seen in E. coli outbreaks.
The strain of bacteria involved in the outbreak is Shiga toxin-producing Escherichia coli O157:H7. This "tends to cause more severe illness, which may explain why there is a high hospitalization rate," the CDC said in its outbreak investigation update.
Between 1998 and 2016, there were 45 outbreaks associated with Shiga toxin-producing E. coli in leafy vegetables reported in the United States, CDC spokeswoman Brittany Behm said. The new one is the largest outbreak of its kind since a deadly E. coli outbreak in 2006 that was linked to spinach.
In the new outbreak, the investigation revealed that several people in an Alaska correctional facility who became sick had consumed romaine lettuce sourced from Harrison Farms of Yuma, Arizona. The agency has not determined where in the supply chain contamination occurred.
"Lettuce can be contaminated in many different ways from the farm through the distribution chain," Behm said. "It could be from manure in the fields to contaminated water to contamination within a processing facility."
Rachel Noble, a biologist and professor in the Department of Environmental Sciences and Engineering at the University of North Carolina at Chapel Hill, explained that because "lettuce is grown very close to the ground," rain and the process of irrigation allow dirt and silt to "jump up onto the lettuce," leading to contamination.
"Any commercially grown lettuce product will be put through some basic wash step before it's sold," Noble explained. The series of baths and tumblers is not a thorough cleaning, however; it's just enough that the end product is "appealing to the customer."
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