(CNN) Why is lettuce so often the culprit in illness outbreaks linked to the bacteria E. coli?
A total of 121 people from 25 US 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.
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."
She added that, although commercial producers do some testing for E. coli on wash water and irrigation water, not every single product that makes it into the hands of a customer is tested.
The E. coli testing is based on the Food Safety Modernization Act, a set of regulations enacted in the US in August 2015 that requires growers with a certain size farm to sample water associated with produce, Noble said.
"The goal was to set up a monitoring scheme to protect the public," she said. The regulations are still being phased in, so some growers have begun monitoring programs but others have not.
Though these monitoring programs measure the total volume of E. coli in the water, it might not take a high number of bacteria to make someone sick, since the Shiga toxin-producing strains can be potent, Noble said.
All told, Farber believes, "both the US and Canada have a very safe food supply."
Still, consumers have "a role to play," he said, by paying attention to food recalls and asking questions when they are unsure of quality or safety of a food product. They also need to know "that 'best before' or 'use by' dates are only based on quality and not safety."
With the growing season in the Yuma region at an end, Harrison Farms and others in the region are not growing any lettuce now, but the CDC still warns consumers against eating romaine lettuce at this time unless it isn't from the region.
Generally, Farber recommends washing your hands thoroughly with warm water and soap for at least 20 seconds before and after handling lettuce and then washing lettuce thoroughly under fresh, cool running water. Wilted or brown leaves should be discarded along with the outer lettuce layer, he said.
The CDC also offers recommendations for consumers to avoid becoming infected with a harmful strain of E. coli. Generally, the agency advises using proper handwashing and kitchen sanitation when preparing food; cooking meat at proper temperatures; avoiding raw milk, unpasteurized dairy products and juices; and not swallowing water when swimming.
"There is no need to use anything other than water to wash lettuce," Farber said. "Washing it gently with water is as effective as using produce cleansers."