Four product tests at a Maple Leaf plant in Toronto have come up positive for listeria, the deli meat producer at the heart of a deadly nationwide listeriosis outbreak said Wednesday.

More than 5,000 product tests have been conducted since the Toronto plant re-opened in September and results have come back for 3,850 of them, Maple Leaf said. Four tested positive for Listeria monocytogenes, the bacteria responsible for a disease linked to the deaths of 20 people across the country.

The Canadian Food Inspection Agency's website confirmed the new listeria findings Wednesday night. It said all Maple Leaf products manufactured since the plant reopened Sept. 17 were being held by the inspection agency, and that none had been allowed to enter the marketplace.

"The CFIA and Health Canada will do a full and comprehensive assessment and further scientific evaluation of the new findings over the coming days," a notice on the website said.

Federal officials from the Prime Minister's Office, the Privy Council Office and the Canadian Food Inspection Agency met late Wednesday to discuss the findings, according to a report by the Globe and Mail.

"We've just received results related to these four positives," Paul Mayers, associate vice-president of the CFIA, told the Globe. "The plant is operating. The product isn't reaching the marketplace.

"We will, with Health Canada, conduct an assessment to assess what these findings might mean in terms of the ongoing activities in the plant and whether any additional measures would be necessary in relation to product from that plant. Of course, that assessment has not yet been conducted."

Immediate product quarantine

Michael McCain, president of Maple Leaf Foods, said in a statement released late Wednesday that "listeria exists in all food plants, all supermarkets and presumably in all kitchens. Our testing protocols are designed to find positive results so we can remediate them immediately."

McCain said the company is working with the CFIA to withhold distribution of any products from the Toronto facility. He said the products do not pose a risk to the public, and that standard protocol for positive tests is immediate product quarantine.

"We are being ultra-cautious in this facility," said McCain, "more cautious than any other plant in North America. Under the circumstances, we consider this to be an appropriate action plan."

The number of confirmed deaths linked to the listeriosis outbreak has risen to 20 since federal officials announced the first death on Aug 20. Fifteen of the deaths have occurred in Ontario, two have been in B.C. and one each in Alberta, Quebec and New Brunswick, according to federal figures.

Six deaths across the country are still under investigation.

Listeria monocytogenes can be found in unpasteurized dairy products, raw vegetables and meats, and processed foods, including deli meats and hot dogs. Ingesting the bacteria can cause serious illness, including brain and blood infections that can lead to death. The elderly, infants and people with compromised immune systems are the most vulnerable.

191 products recalled since August

Some of the flu-like symptoms of listeriosis are nausea, vomiting, diarrhea, severe headaches, stiff neck and fever. It can take up to 70 days for symptoms to appear after the contaminated food is consumed.

A total of 191 items produced at Maple Leaf's Toronto plant were recalled during the crisis that began in August, costing the company an estimated $20 million. The plant was reopened Sept. 17.

Since then, 841 environmental samples have been taken and results came back for 671, with one turning up positive for listeria.

Citing the Canadian Food Directorate, Maple Leaf said in its release Wednesday that one to 10 per cent of all ready-to-eat food contains listeria, "and it is benign to the vast majority of Canadians."

When the plant closed in September, McCain said slicing equipment at the plant would be subjected to stricter cleaning and testing protocols, including disassembling the machines every day to check for gatherings of listeria. Tests and samples would also be done more frequently.

McCain said at the time that the company had cleaned its machines on a daily, weekly and monthly basis as recommended by the manufacturer, but it wasn't until the machines were disassembled that Maple Leaf found an area where listeria bacterium likely embedded itself.

With files from the Canadian Press