Dr. Eilish Cleary, New Brunswick's chief medical officer of health, said a child from the Fredericton area under the age of two became ill about a week ago but has fully recovered.
She said lab results that came back Thursday show the case involves E. coli 0157, the same strain that was traced to shredded lettuce distributed by FreshPoint Inc. to KFC and Taco Bell restaurants late last month.
The latest case is likely linked to the lettuce sold at the fast-food restaurants, Cleary said.
The Canadian Food Inspection Agency says the lettuce is the most probable cause of the outbreak.
It was not distributed to grocery stores.
"Lettuce has a short shelf life, therefore contaminated products are unlikely to still be available," the Public Health Agency of Canada says in a statement.
The previous six cases in New Brunswick happened around Christmas.
Cleary said she doesn't know why the child got sick this month.
"Was there a food source that was left that caused the infection?" she asked. "Could it have come in contact with something else, or was there somebody else that got infected and maybe didn't report symptoms that this child contracted the disease from?"
Cleary said officials are interviewing family members to try to determine how the child became ill, but they may never know for sure.
She said hospitals and doctors are being asked to report anyone showing E. coli symptoms.
"This outbreak, by and large, seems to have settled down in New Brunswick," Cleary said. "This is one case but we have not yet had any other reports."
The Canadian Food Inspection Agency issued a recall of any remaining lettuce distributed by FreshPoint.
On Tuesday, the agency reported that product and water samples taken from FreshPoint for laboratory analyses all came back negative for the presence of E. coli 0157.
Symptoms of the gastro-intestinal illness include severe cramps, bloody diarrhea and vomiting.
To prevent the infections from spreading, Cleary said people should wash their hands thoroughly after using the washroom or changing diapers.
Other precautions include washing and peeling raw vegetables and fruits, thoroughly cooking all meats and preventing contact between cooked foods, raw poultry and other meats.