TORONTO -- When news broke Tuesday that consumers should avoid eating romaine lettuce because of an E. coli outbreak, the U.S. Centers for Disease Control and Prevention swiftly demanded that retailers and restaurants remove it from store shelves and stop including it in meals.

But in Canada, the country's public health and food inspection agencies stopped short of insisting on its removal, despite it being linked to the illnesses of 19 people in Ontario, Quebec and New Brunswick -- of whom six required hospitalization.

Experts said the difference in approach likely stems from how many cases linked to the bacteria crop up in a country, how cautious nations want to be about protecting industries and how comfortable a country is with their hunches about the outbreak's origins.

E. coli is a bacteria that lives in the intestines of cattle, poultry and other animals. According to Health Canada, most strains are harmless to humans but some varieties are capable of causing illness. Typical symptoms include stomach cramps, diarrhea and vomiting.

Norman Neumann, the vice-dean of the University of Alberta's School of Public Health, said during outbreaks impacting Canada and the U.S., health bodies from both countries will likely consult each other on investigating the source, but don't always co-ordinate their responses.

He suspects the U.S. has gone a step further than Canada in part because U.S. authorities reported 32 cases of E. coli, 13 of which involved a person who was hospitalized.

"The caseloads are higher in the U.S. so it might suggest a little bit more of a severe response in the U.S.," he said.

Pinpointing the exact cause of the outbreak can be difficult because public health officials often have to search for similarities in places those who fall ill have visited or what they've eaten.

It can take a week for symptoms to appear in some cases and by then, asking someone to recall everything they ate the week before might be difficult and thus, impact a health agency's comfort in taking action against a particular source of the outbreak, Neumann said.

"When there are outbreaks, certain things have been implicated only to find out years later the epidemiology evidence wasn't sound or secure," he said. "You can pinpoint a potential source only to find out a few weeks, months or years later it was maybe not the source and we ruined an industry in response."

The Canadian Food Inspection Agency did not say why it had stopped short of instructing grocery stores to stop selling romaine lettuce and restaurants to cease serving it.

However, the CFIA has said if the contaminated food products are identified in Canada, they will take the necessary steps to protect the public, including recalling the product.

"As partners in ensuring the health and safety of Canadians, retailers and restaurants should take appropriate action as per the advice provided to Canadians in the public health notice," agency spokeswoman Maryse Durette said in an email statement Wednesday evening.

"The Government of Canada advises residents in provinces affected by the outbreak to avoid eating romaine lettuce -- including salad mixes containing romaine lettuce -- until more is known about the outbreak and the cause of contamination."

However, the CFIA has said if the contaminated food products are identified in Canada, they will take the necessary steps to protect the public, including recalling the product.

Grocery giants Empire Company Ltd., Loblaw Companies Ltd. and Metro Inc. haven't waited for an official request though. On Wednesday, they said they were temporarily taking hundreds of products containing the vegetable off shelves at thousands of grocery stores that they own.

Jim Chan, a former health inspector and manager at Toronto Public Health, said he believes it is within the provincial public health body's abilities to issue a warning to all food premises, including restaurants, quick-dining options and supermarkets, to stop serving romaine until the CFIA confirms the product is safe.

"I strongly believe that Health Canada/CFIA should call for retailers and wholesalers to remove all off shelves, as well as a recall to consumers," he told The Canadian Press. "I think food safety should take priority."

Chan added that most of Canada's romaine lettuce is imported from the U.S. because Canada's growing season ended in August.

The Public Health Agency of Canada said the 18 people who fell ill in connection to the outbreak in Canada reported their cases between mid-October and early November, and one complained of suffering a severe complication related to it.

Those affected were between the ages of five and 93.

The agency said before their illnesses occurred, the people affected reported eating romaine lettuce at home, in prepared salads purchased at grocery stores and from menu items ordered at restaurants and fast food chains.