Maple Leaf Foods workers clad in protective clothing clean equipment on one of the suspect food processing lines at the facility in Toronto on Thursday. (Frank Gunn/Canadian Press)

If you don't know where your deli meat came from, don't eat it, Ontario health officials advised Thursday as they investigated more deaths for links to an outbreak of listeriosis.

The bacterial infection can be caused by eating contaminated food. One person, a woman from Hamilton, has died so far in the outbreak, and officials are investigating whether another four deaths are linked to it.

A total of 13 cases of listeriosis have been confirmed as outbreak-related, including the one death, and another 17 are under investigation to determine whether they, too, were spawned by the outbreak, Dr. David Williams, Ontario's acting chief medical officer of health, told a news conference on Thursday.

While the elderly, pregnant women and those with weak immune systems should avoid consuming ready-to-eat meat, Williams said the "when in doubt, throw it out" messages applies to all consumers.

"I expect some more cases," Williams said.

The incubation period for listeria can be as long as three months, and unlike most microbes that can cause food sickness, listeria thrives at cool temperatures.

The Canadian Food Inspection Agency has identified listeria bacteria at a Maple Leaf Foods meat plant in Toronto, but the federal agency is waiting for definitive test results to see whether it is the same strain as the one responsible for the outbreak, Williams said.

While the Ontario health ministry was aware of a surge in the number of cases of listeriosis weeks ago, there was a delay in alerting doctors and consumers.

"You really like to, before you start causing panic among the public, to say, 'Do you have something substantial to deal with?'" Williams said.

Dr. Jim Brunton, an infectious disease specialist with Toronto's University Health Network, treated some of the patients who became sick. He credits public health officials for responding quickly to the outbreak, but acknowledged that doctors who treat ill and elderly people could have been informed sooner.

"We have a lot of interest in severe autoimmune diseases like lupus, and those are patients most likely to suffer severely from listeria," Brunton said. "So yes, it could've helped us a bit and might've helped our patients."

The Public Health Agency of Canada said there are 17 confirmed cases of listeriosis nationwide, including the 13 in Ontario, as well as two in B.C., one in Saskatchewan and one in Quebec.

Two of the Ontario cases were in Ottawa, said the city's medical officer of health, Dr. Isra Levy. One was a woman who lived in a long-term care home and was sent to hospital after she fell ill at the end of July, but she has since recovered and is ready to be released. The other was a man who never grew sick enough to require hospitalization, Levy reported.

Number of cases could grow

"The pattern was people in certain long-term care facilities" across southern Ontario, Williams said.

Maple Leaf Foods was in the process of a $2-million recall on Thursday of packaged meats that might contain the Listeria monocytogenes strain. The company's Toronto plant will be closed until Monday.

"What we're going to do is a comprehensive sanitization, and in fact dismantle the two production lines in question with external supervision to ensure that we can determine the source, do some testing, if in fact it is there," said Linda Smith, a spokesperson for Maple Leaf.

It is difficult to contain an outbreak when food is produced at such a large scale, said Ann Clark, a professor of agriculture at the University of Guelph.

"It's essentially an unavoidable risk that people are taking, and probably don't realize they're taking whenever you have that much stuff going through one facility," said Clark.

Expanded recall

On Sunday, Maple Leaf Foods announced it had discovered the bacteria that causes listeriosis in Sure Slice roast beef and corned beef produced at a Toronto plant, and it issued a recall on those products.

Then on Wednesday, Maple Leaf expanded the recall of its packaged meats and said it was temporarily closing the Toronto plant to re-evaluate its food safety procedures. As of that day, some 23 products, including a variety of turkey, smoked meat and roast beef products, were subject to a recall.

A number of the recalled products are part of the investigation into the outbreak, Canadian Food Inspection Agency official Garfield Balsom said.

The agency said some of the Maple Leaf meat products, which are distributed to nursing homes, restaurants and deli counters across the country — including McDonald's and Mr. Sub — tested positive for listeria bacteria.

Brand names for the recalled products include Schneiders, Sure Slice, Deli Gourmet and Burns Bites. A complete list of affected products is available on both the Maple Leaf Foods website and that of the Canadian Food Inspection Agency.

The 23 products involved in the expanded recall carry an "establishment number" of 97B and have best-before dates ranging from Sept. 30 to Jan. 1, 2009.

Consumers who are concerned their meat products may be part of the recall should check with their retailers, such as deli counters, the Canadian Food Inspection Agency advised.

Consumers and retailers may also call Maple Leaf at 1-800-568-5801, or the Canadian Food Inspection Agency at 1-800-442-2342 ( TTY: 1-800-465-7735) from 8 a.m. to 8 p.m. ET, Monday to Friday.

Product code	Description	Package size	Up to and including Best Before Date
26365	Sliced Cooked Turkey Breast	470 g	SE 30
02106	Schneiders Bavarian Smokies	1 kg	OC 28
02126	Schneiders Cheddar Smokies	1 kg	OC 28
21333	Sure Slice Roast Beef	1 kg	SE 30
21388	Sure Slice Combo Pack	1 kg	SE 30
60243	Deli Gourmet Roast Beef slices	1 kg	SE 30
02356	Seasoned Cooked Roast Beef	500 g	OC 07
21334	Sure Slice Turkey Breast Roast	1 kg	OC 14
21444	Sure Slice Corned Beef	1 kg	OC 14
44938	Montreal Style Corned Beef	500 g	OC 14
21440	Sure Slice Black Forest Style Ham	1 kg	OC 21
21447	Sure Slice Salami	1 kg	OC 21
21331	Sure Slice Smoked Ham	1 kg	OC 21
48019	Schneiders Deli Shaved Corned Beef	200 g	OC 21
48020	Schneiders Deli Shaved Smoked Meat	200 g	OC 21
48016	Schneiders Deli Shaved Smoked Ham	200 g	OC 21
48018	Schneiders Deli Shaved Smoked Turkey Breast	150 g	OC 21
48017	Schneiders Deli Shaved Fully Cooked Smoked Honey Ham	200 g	OC 21
21360	Burns Bites Pepperoni	500 g	09 JA 01
99158	Turkey Breast Roast	1 kg	SE 30
71330	Roast Beef Cooked, Seasoned	2.5 kg	SE 30
71331	Corned Beef, Smoked Meat	2.5 kg	SE 30
26365	McDonald's Sliced Cooked Turkey Breast	6 X 470g	SE 30

With files from the Canadian Press