The Public Health Agency of Canada (PHAC) said there are 41 cases under investigation.
Romaine lettuce has been identified as the source but the cause of contamination is not known.
As romaine lettuce can have a shelf life of up to five weeks the agency is advising people in Ontario (eight cases), Quebec (14), New Brunswick (five), Nova Scotia (one) and Newfoundland and Labrador (13) to consume other types of lettuce.
17 hospitalised and one dead
Individuals aged between three and 85 became sick in November and early December.
Seventeen people have been hospitalized and one has died.
Canada has almost 500 Shiga toxin producing E. coli (STEC) O157:H7 infections annually.
PHAC said ‘most’ of those sickened ate romaine lettuce before illnesses either at home or in prepared salads purchased at grocery stores, restaurants and fast food chains.
“The outbreak appears to be ongoing, as illnesses linked to romaine lettuce continue to be reported to the Public Health Agency of Canada.
“These illnesses indicate that contaminated romaine lettuce may still be on the market (including in restaurants, grocery stores and any establishments that serve food).”
The Canadian Food Inspection Agency is testing romaine lettuce for E. coli.
Time from when a person is exposed to E. coli O157 to beginning of symptoms is typically one to three days but may be as long as 10 days.
The Centers for Disease Control and Prevention (CDC) said 17 STEC O157:H7 infections have been reported in 13 states.
California (three cases), Connecticut and New Hampshire (two), Illinois, Indiana, Michigan, Nebraska, New York, Ohio, Pennsylvania, Virginia, Vermont and Washington (all one case) are affected.
Illness dates range from 15 November to 8 December.
Whole genome sequencing on samples of bacteria making people sick in the US show the type of E. coli is closely related genetically to that in Canadian patients, meaning the ill people are likely to share a common source of infection.
Interviews in the US are ongoing with sick people to determine what they ate in the week before illness to see if there is a common food item.
An estimated 265,000 STEC infections occur each year in the US with STEC O157 causing about 36% of these.
Leafy greens, such as lettuce, can become affected in the field by soil, contaminated water, animals or improperly composted manure.
Lettuce can pick up bacteria during and after harvest from handling, storing and transporting produce.
Contamination is also possible at the grocery store, in the refrigerator or from counters and cutting boards through cross-contamination with bacteria from raw meat, poultry or seafood.