All suspect lettuce has been recalled, they said Friday afternoon.
"The evidence from our collaborative investigation leads us to believe that the common food source was distributed to this fast-food restaurant chain," said Dr. Frank Atherton, Nova Scotia's deputy chief medical officer of health.
"Lettuce has a limited shelf life, and we have not seen a new case in more than a week. This tells us it is highly unlikely the food item remains in the food chain. As an added precaution, the Canadian Food Inspection Agency is issuing a recall of the lettuce products."
He added that the fault did not lie with the restaurants, but with FreshPoint, the company that supplied the lettuce.
No new cases of E. coli O157 are expected, officials said.
Nova Scotia has had 10 confirmed cases of E. coli O157 in the past couple of weeks. At least five are linked to the outbreak.
New Brunswick has also had six confirmed cases, while Ontario has had five. All of the patients have been treated and are recovering, Atherton said.
Dr. Eilish Cleary, New Brunswick's chief medical officer, said it was hard to pin down the particular outlet. The lettuce was distributed to other fast-food chains, but the cases were all linked to Taco Bell/KFC.
"When we looked at the food history of our patients, they had eaten at several locations, so we were unable to pinpoint exactly which ones they were exposed at," she said.
Sabir Sami, president of KFC/Taco Bell parent company Yum Restaurants, said his company takes the developments seriously.
"We’re obviously concerned, as this lettuce provided to us by FreshPoint has been distributed to many area restaurants in Canada, including ours," he said in a news release.
"We have removed all the affected lettuce from our restaurants in Canada and want to reassure our customers that our food is perfectly safe to eat. The health and safety of our customers is our top priority."
E. coli O157 is the same strain that killed seven people in Walkerton, Ont., in 2000. It secretes a powerful toxin that can cause severe illness, high blood pressure and kidney damage.
Romaine lettuce was determined to be the likely source of an E. coli outbreak in Miramichi., N.B., in April. At least 13 people in the northern New Brunswick city were infected with that strain of E. coli O157, while another 11 people may have also been infected with that strain, officials said at the time.Suggest a correction