Maple Leaf's CEO says the most recent findings of listeria at the company's Toronto plant are a sign its testing system is working, and the results are "completely different" from those that occurred in August.

"Experts know that if the program does not detect listeria, you actually aren't running an effective program," Michael McCain said Thursday at a Toronto news conference. "It is impossible, impossible to eliminate."

Maple Leaf confirmed Wednesday that four out of more than 3,850 product test results at the plant since Sept. 17 came back positive for listeria.

McCain stressed that four of 3,850 was a "very, very low incidence" and to be expected. He also pointed out that none of the products made at the plant since it reopened Sept. 17 have made it to the marketplace.

The Canadian Food Inspection Agency (CFIA) has been holding back all Maple Leaf products manufactured since then.

The deli meat producer is at the centre of a deadly nationwide outbreak of Listeria monocytogenes, the bacteria responsible for a disease linked to the deaths of 20 people. Six deaths across the country are still under investigation.

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.

McCain suggested Thursday that media reports about the outbreak have not been balanced.

"Canada has one of the highest standards of vigilance in food safety in the world," he said.

"To suggest shock at a positive environmental test is at best misguided and at worst fearmongering," he said, referring to the one positive result for listeria out of 671 environmental samples taken since Sept. 17.

"This kind of sensationalism may actually discourage companies from doing what is right and necessary in their environmental programs and have the reverse effect of compromising food safety."

Dr. Vanessa Allen, a microbiologist Ontario Public Health Laboratories, said she agreed with Maple Leaf's policy of making sure there is no listeria found in meat before the company starts to distribute its products again.

"Kudos to them for being vigilant insofar as they have continued to test all of their products and the environmental conditions of the plant," Allen. "However, I think it is critical that none is found before any release of products, which they have not initiated to this date."

The CFIA and Health Canada are assessing the most recent findings to determine whether additional measures need to be taken, officials said Wednesday.

Listeria 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.

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.