Why E. coli linked to romaine lettuce remains 'an extremely difficult problem'
Leafy greens such as lettuce can become contaminated in many ways
The fourth E. coli outbreak in two years linked to romaine lettuce affecting Canadian consumers demonstrates how difficult it is to definitively identify the source of contamination, but there are ways to improve food safety, experts say.
As of Nov. 22, the Public Health Agency of Canada said there is one person sickened with an illness with a "similar genetic fingerprint" to the illnesses related to a current U.S. outbreak. The individual in Manitoba became ill in mid-October.
While there is no outbreak of E. coli in Canada, U.S. public health officials report multiple illnesses in several states linked to lettuce grown in Salinas, Calif.
Consumers in both countries are advised not to eat, and retailers not to sell or serve, any romaine lettuce harvested from Salinas. If you don't know if the lettuce is romaine or whether a salad mix contains romaine, don't eat it or serve it. Throw it away, the U.S. Centers for Disease Control advised.
One of the contributors to the problem lies in how romaine is usually grown in irrigated fields.
Timothy Lytton, a professor of law at Georgia State University and author of the new book Outbreak: Foodborne Illness and the Struggle for Food Safety, said the problem is that E. coli is endemic in irrigation sources. That is especially true where the sources are above ground, as in the California valley and in Arizona, which supply most leafy greens grown in the U.S.
Most varieties of E. coli are harmless to us, but the E. coli O157:H7 bacteria that sicken people also live in cattle without making them ill.
Leafy greens such as lettuce can become contaminated in the field by soil, water, animals or improperly composted manure or during handling, storing and transport.
"It's an extremely difficult problem due to in part the proximity of cattle-feeding operations, which are a major industry in those areas, and the growing [of] leafy greens," Lytton said.
Since the 2006 outbreak of E. coli strain O157:H7 from baby spinach, the produce industry has made strides in figuring out the pathways of contamination, he said.
Food scientists say the goal is to trace the E. coli organism using sophisticated microbiological "fingerprinting" techniques, to make a link between the ill patient, the contaminated food and the source of the bacteria: the cattle.
"But the industry has been less successful in figuring out just how clean the irrigation water needs to be to reduce the danger of microbial contamination to an acceptable level," Lytton said. Irrigating with tap water would probably reduce the danger, but it would be extraordinarily expensive and not practical.
"We're not sure what the acceptable level of risk is, and the government has basically taken its cues from industry," Lytton said.
A short-lived smoking gun
While industry often employs top talent from universities in food safety, Lytton expects that the problems with leafy greens will be with us for some time.
"I think nobody is more eager to reduce the problem … than the leafy greens industry because this is threatening to completely destroy their business model."
A lot of the outbreak tracing spans the Canada-U.S. border, Lytton said, and regulators from both countries learn from each other.
For Lawrence Goodridge, a professor of food safety at the University of Guelph, irrigation water is also suspected. But it's hard to come up with definitive proof.
The strain of E. coli O157:H7 identified as the cause of this latest outbreak has been linked using DNA methods to other outbreaks involving romaine, which suggests the source of the contamination is likely the same, Goodridge said.
The problem is, the contamination can be short-lived and the testing frequency and volumes are often small, Goodridge said.
Goodridge pointed to three measures to improve the safety of romaine:
- Treat irrigation water.
- Label where the lettuce comes from, so only lettuce from the contaminated area needs to be recalled.
- Separate livestock production from vegetable production.
"Irrigation water treatment and labelling as to the source, I'd like to see those become mandatory within the industry," Goodridge said.
Meat is usually cooked but fresh vegetables, particularly lettuce, aren't, which reduces one of the steps to kill disease-causing bacteria like E. coli O157:H7.
Epidemiologist Tim Sly, a professor emeritus at Ryerson University's School of Public Health in Toronto, said produce grown close to the soil and eaten raw is always vulnerable. Romaine lettuce is the top-selling variety of lettuce in North America, with iceberg a close second, Sly said.
"But romaine grows with open leaves — open to dust and blowing soil — while iceberg grows in a 'protective' closed-leaf formation," Sly said in an email.
More fresh produce is being grown in greenhouses, which decreases the risk, Goodridge said.
The CDC said people get sick from Shiga toxin-producing E. coli three to four days after swallowing the germ. Most people get diarrhea — often bloody — severe stomach cramps, and vomiting.
Most people recover within a week, but some illnesses can last longer and be more severe.
"It's important to understand that while an E. coli infection of this type causes gastroenteritis, much as many other food-borne diseases, it can, in 11 to 15 per cent of cases (especially in children) cause more serious complications involving the kidneys," Sly said.
There is no real treatment for E. coli infections other than monitoring the illness, providing comfort, and preventing dehydration through proper hydration and nutrition, the Public Health Agency of Canada said on its website.