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Hexemer said that’s because of Listeria’s incubation period, which can last up to 70 days, and the reporting delay that accompanies new cases, "just to be sure that there isn't an illness that's making its way through the system."

Hexemer said the last tally of cases reported on Aug. 12 remains unchanged, with 20 confirmed infections in Ontario, Quebec, Nova Scotia and Alberta, and three deaths. The illnesses prompted a national recall of several Silk and Great Value plant-based milk products on July 8.

The first three cases connected to the outbreak were reported in August, September and December of 2023, according to a timeline on the federal health agency's website.

Hexemer said it would have been an “outbreak miracle� to find a common source of infection in those cases at the time.

But after nine cases were reported in June, investigators examined bacteria samples and compared them not only to each other, but also to the three dispersed cases from 2023. They discovered the same genetic strain.

The federal agency also interviewed infected individuals and asked them to recall what they ate and drank in search of commonalities that could point investigators to the source. In this case, Hexemer said it was helpful that the product was consumed on a daily basis.

Some of the people who got sick in June still had the plant-based milk in their fridge, a finding that Hexemer considers “very, very lucky."

The beverages all contained the same product code – 7825 – and pointed to a packaging facility in Pickering, Ont.

On Aug. 7, the Canadian Food Inspection Agency announced that the source of the illness was traced to a specific production line at the third-party facility used by plant milk manufacturer Danone Canada.

Hexemer said the way Listeria behaves in production environments played a role in the gaps between confirmed cases.

“It kind of lays dormant, probably has a little bit of a protective shell, and a little bit got out, and then something happened, and a lot got out. And that's not uncommon for Listeria,� she said.

Given such challenges, Hexemer said, “I actually see this as quite a successful investigation.�

This report by °µÍø½ûÇø was first published Aug. 23, 2024.

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