The test results show a dramatically high percentage of bacteria-laced ham, corned beef, turkey and roast beef was being served to hundreds of vulnerable hospital patients and seniors.

Public health units from across Ontario were also ordered to collect samples.

A federal government source confirmed yesterday that half of the provincial samples tested positive for listeria.

Experts say it's more contamination than they have seen and further evidence of a health risk that should have reached the public's attention sooner.

"There shouldn't be any positives," said Rick Holley, a food safety expert 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 that are positive ... And it is a particularly virulent strain of listeria. It's one of the bad ones."

The lab tests, conducted by the Canadian Food Inspection Agency and compiled in a document labelled "confidential," reveal 17 of 26 samples collected in Toronto on Aug. 14 and 15 tested positive for the pathogen.

Some samples showed contamination of more than 20,000 Listeria monocytogenes bacteria per gram. The tests prompted the national recall of various Maple Leaf meat products.

The listeria outbreak has killed 20 people, 15 of them in Ontario.

The Toronto samples were collected by city public health inspectors from 13 nursing homes, hospitals and an HIV/AIDS hospice on orders from Ontario's Ministry of Health and the CFIA as part of their investigation into the outbreak.

"I'd never seen anything like this," said Dr. Vinita Dubey, Toronto's associate medical officer of health. "The fact that so many came back positive shows how contaminated the source was."

She said the samples of meat were unopened, ruling out any contamination from food handlers in the institutions.

Positive tests showed up on meat with "best before" dates ranging from early August to Oct. 1, which suggests the meat from the North York plant was being contaminated over nearly two months, said Holley, of the University of Manitoba.

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

The strain listed in the test results -- 1/2a -- is considered one of the most dangerous because of its ability to survive in meat vacuum packages and to resist digestion in white blood cells. This strain and strains 1/2b and 4b are responsible for about 98 per cent of human listeriosis cases involving food.

The test results from mid-August were actually from the second round of meat samples Toronto Public Health collected during the probe. In late July, inspectors collected samples from a North York nursing home where two people were reported ill with listeriosis -- a highly unusual event.

Samples from that home were collected July 21, Dubey said. But the testing process -- which included sending the meat to a lab in Ottawa and a federal lab in Winnipeg -- took more than two weeks.

Results didn't come back until Aug. 5. Those results showed five of the 11 samples were positive for listeria. Experts say, on average, no more than 3 to 5 per cent of samples should test positive for listeria.

Yet it would be another 12 days and another round of meat tests before a public recall was announced by the CFIA on Aug. 17, then vastly expanded a few days later.

Linda Smith, a spokesperson for Maple Leaf, said the company was informed of the second batch of test results on the evening of Aug. 16.

"On the basis of (the test results), we did a recall. When we looked at that, we felt it was important to do more and that's why we closed the plant and recalled 191 products."

Dubey and many of her colleagues in the public health community say it took too long to test meat samples -- and get word to the public about the threat.

CFIA officials have vigorously defended their response to the outbreak, saying recalls must be based on science and it takes time to identify the source of such an outbreak. Asked this week for comment on the results, the agency did not respond.

Maple Leaf officials have previously pinpointed the outbreak to meat slicers in its North York plant. The test results show the slicers must have been "heavily contaminated to make the results come back this positive," Dubey said.

The test results show contaminated samples were sitting in kitchens across Toronto where meals for elderly, highly vulnerable residents were prepared.

"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 a significant challenge for those people," Holley said.

CFIA inspectors who spoke to the Star and the CBC say the agency's move toward self-regulation of meat safety has compromised public safety. They say policy changes imposed earlier this year shifted the responsibility for inspection to profit-driven companies.

"We shouldn't be called inspectors any more," said one CFIA inspector. "We should be called auditors."