The number of suspected and confirmed cases of listeriosis is expected to increase in the coming weeks because of the bacterium's long incubation period, Canada's health minister said Sunday evening.

So far, there are 21 confirmed cases of listeriosis from food contaminated with Listeria monocytogenes bacteria, and 30 cases remain under investigation, Health Minister Tony Clement said at a news conference in Ottawa.

Symptoms of listeriosis, which include vomiting, nausea and diarrhea, occur up to 70 days after contaminated food is consumed, with the average incubation period of 30 days, public health officials said.

"So we expect both the number of suspected cases and the number of confirmed cases will increase as the investigation continues and samples continue to be received from provincial, territorial and federal partners," Clement said.

The outbreak has been linked to four deaths out of the 21 confirmed cases in British Columbia, Ontario, Quebec and Saskatchewan. Three deaths were in Ontario and one was in B.C.

Clement made his comments a day after government lab tests concluded that the bacteria strain linked to the listeriosis outbreak matched the one in some food products recalled last week by Maple Leaf Foods.

The establishment of the link prompted the company to immediately expand its voluntary recall to include all the foods produced at its Toronto plant, one of the country's largest food recalls.

Company president and chief executive Michael McCain said Sunday at a news conference in Toronto that the expanded recall at the Bartor Road facility was a precautionary measure and that no trace of the listeria strain had been found in any products beyond the ones that were recalled earlier this month.

"We had to take the most conservative approach possible … and recalled 100 per cent of the production from the entire facility," said McCain, whose company also paid to air public service announcements on television to warn Canadians.

Testing planned

He said that the recall will cost the company about $20 million, about 10 times more than originally estimated, and that efforts to sanitize the Toronto plant, which is expected to reopen Tuesday, have gone well. Public health officials said they will monitor and test any products that are produced from the reopened plant before they're allowed to go on store shelves.

On Saturday night, Canadian public health officials disclosed that lab tests conducted by Health Canada showed the link between the potentially deadly bacterial outbreak and two recalled meat products from Maple Leaf Foods.

"We had two products that tested positive for the outbreak strain," including Maple Leaf Foods roast beef deli slices and corn beef deli slices, said Dr. Jeff Farber, director of Health Canada's bureau of microbial hazards.

Test results for a third product were a close match to the outbreak strain, but showed a slight variance.

"While these results are highly significant, and indicate that the investigation is on the right path, the investigation is not complete," the Public Health Agency of Canada said in a news release. "Test results on additional food samples expected next week will advance the investigation further."

The results are "good news" because they confirm that health officials are looking in the right places for the cause of the outbreak, officials of the Canadian Food Inspection Agency said in Ottawa.

"We're very confident that we're on the right path," said Garfield Balsan, a food safety and recall specialist with CFIA.

Pinpointing source difficult

Trying to pinpoint the source of the microorganism has been difficult because it's so widespread and commonly found in the environment, such as in soil and in water, said Dr. Mark Raizenne, director general of  Public Health Agency of Canada's centre for food-borne, environmental and zoonotic infectious diseases.

That explains why public health and company officials have been saying it's unlikely the source of the contamination will ever be found.

"There are so many different sources of this particular organism," Farber said, adding it could have potentially been brought into the Toronto plant in raw beef and poultry, in water, in dirt or by employees.

Because it's so widespread, public health officials see 60 to 70 cases of listeriosis each year, Farber said.

Maple Leaf Foods said Sunday it expects the additional recall affecting 220 products to be implemented swiftly. A list of specific products and codes is available on its website and on Canadian Food Inspection Agency's website. CFIA officials said they will make sure that all the products will be removed from the marketplace through checks, Balsan said.

"Tragically, our products have been linked to illness and loss of life. To those people who are ill, and to the families who have lost loved ones, I offer my deepest and sincerest sympathies," McCain said Saturday. "Words cannot begin to express our sadness for their pain.

People most at risk include newborns, the elderly, pregnant women and people with weakened immune systems, public health officials said. The public should wash their produce and avoid unpasteurized milk and dairy products, they advised.