More than 50 people have been hospitalized and two people have died from dangerous E. coli infections over the past seven weeks in the United States and Canada, according to health officials from both countries. Contaminated romaine lettuce has been identified as the source of the Canadian outbreak, which involves at least 41 illnesses and one death.

The Public Health Agency of Canada has advised people living in Ontario, Quebec, New Brunswick, Nova Scotia, and Newfoundland and Labrador to consider avoiding romaine lettuce until more is known about the outbreak and its cause.

The Centers for Disease Control and Prevention (CDC) has not yet identified the cause of the 17 illnesses and one death reported in the United States, but said last week that the strain of virus found in both countries—E. coli O157:H7—is the same. “Preliminary results show that the type of E. coli making people sick in both countries is closely related genetically,” the agency said in a statement on December 28, “meaning the ill people are more likely to share a common source of infection.”

The CDC says it is unable to recommend whether people should avoid a particular food while the source of the outbreak is still under investigation. But at least one large U.S. organization has taken a preemptive step: Yesterday, Consumer Reports advised Americans to stop eating romaine lettuce until the cause of the outbreak is identified and products are removed from store shelves.

“Even though we can’t say with 100% certainty that romaine lettuce is the cause of the E. coli outbreak in the U.S., a greater degree of caution is appropriate given that romaine lettuce is almost always consumed raw,” said James Rogers, director of food safety and research at Consumer Reports, in a statement.

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Most strains of E. coli are harmless to humans, but some can cause vomiting, diarrhea, severe stomach cramps and other symptoms. Fruits and vegetables can become contaminated with E. coli when they come into contact with feces from infected animals, or with bacteria from raw meat, poultry or seafood. Most E. coli outbreaks are linked to meat products, but leafy greens are occasionally the cause.

U.S. shoppers should assume that any romaine lettuce could be contaminated, says Consumer Reports, even when it’s sold in sealed bags or in salad mixes with other greens. Washing lettuce before eating it may help reduce the risk of an E. coli infection, experts say, but it won’t eliminate the risk entirely.

E. coli cases related to the current U.S. outbreak have been reported in California, Connecticut, Illinois, Indiana, Michigan, Nebraska, New Hampshire, New York, Ohio, Pennsylvania, Virginia, Vermont and Washington state. The first reported illness began on November 15.

State and local officials are interviewing people who became infected, the CDC says, to look for common threads in what they ate—including leafy greens and romaine lettuce—in the week before their illness started.

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