Two-thirds of meat samples taken from Toronto-area nursing homes and hospitals in mid-August for testing were contaminated with the bacteria listeria, according to records obtained by CBC News and the Toronto Star in a joint investigation.

Health officials warned the public in August about an outbreak of listeriosis, the illness caused by Listeria monocytogenes. The outbreak has been linked to the deaths of 20 Canadians and prompted the largest food recall in Canadian history.

Vinita Dubey, Toronto's associate medical officer of health, said the test results illustrate the extent of the contamination.

"I'd never seen anything like this," Dr. Dubey said after reviewing the test data for the first time.

'The fact that so many came back positive shows how contaminated the source was.'—Vinita Dubey, Toronto associate medical officer of health

"Usually in our food investigations, we'll send a number of samples for testing and most will be negative. The fact that so many came back positive shows how contaminated the source was," she added.

The Ontario Ministry of Health and the Canadian Food Inspection Agency (CFIA) in mid-August ordered public health units across the province to collect samples of processed meats such as turkey, ham and roast beef as part of the investigation into the outbreak. A federal government source confirmed Wednesday that half of the samples tested positive for listeria, and Toronto was a hot spot.

Toronto Public Health inspectors collected 26 samples from nursing homes, hospitals and HIV/AIDS hospices on Aug. 14 and Aug. 15. Seventeen of those samples — roughly two-thirds of the total — tested positive for a dangerous strain of listeria.

"There shouldn't be any positives," said Rick Holley, a microbiologist who teaches at the University of Manitoba. "The reality is if you did a survey in the market, you might find one or two at most out of this sample [size] that are positive.… And it is a particularly virulent strain of listeria. It's one of the bad ones."

The results are also disturbing given that the samples were taken from institutions that house the elderly and infirm, Holley said. The highest risk groups in listeriosis outbreaks include people with weaker immune systems, such as children, pregnant women and the elderly.

"In an environment where these products are going to be consumed by that minority of the population that has some predisposition to some serious infection, this would represent significant challenge for those people," he said.

Maple Leaf Foods has now confirmed that the test results, which it received on the evening of Aug. 16, prompted it to launch the massive recall of its meat products on Aug. 17.

"When we looked at [the test results], we felt it was important to do more and that's why we closed the plant and recalled 191 products," said Linda Smith, a spokeswoman for Maple Leaf.

The test results obtained by the CBC and the Toronto Star show that the meat had various levels of contamination, when the standard set by Health Canada calls for zero tolerance.

The meat samples that tested positive for listeria had best-before dates ranging from early August to early October. Holley said this suggests the meat from the Toronto Maple Leaf Foods plant was being contaminated over a period of nearly two months.

"Whatever the defect was here, it had to be a continuing source of contamination. There had to be a reservoir of the bacteria growing."

With files from David McKie (david_mckie@cbc.ca), Susanne Reber and Robert Cribb