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Last week the Canadian Food Inspection Agency (CFIA) issued a notice about a recall of bagged kale because of a risk of Listeria, a nasty foodborne bacteria that is able to thrive in the cool temperatures of a household fridge.

The product — Eat Smart Sweet Kale Vegetable Salad Bag Kit — was sold in Atlantic Canada, Ontario and Quebec.

A spokesperson for the company told CBC News in an email that the Listeria contamination was discovered after routine sampling by the CFIA and "involved a single lot of production run in one of the Curation Foods facilities in Ohio."

So far no illnesses have been reported.

"No other products are affected," the Curation Foods spokesperson said. "As a precaution, we have intensified our cleaning and testing efforts and will continue to evaluate our processes as part of our program of continuous improvement."

It's the third time in eight months that an Eat Smart product has been recalled in Canada because of Listeria, one of the most dangerous foodborne pathogens.

It highlights one of the challenges with this particular bug — that it can lurk on leafy vegetables and other fresh produce after they've been packaged and processed.

Unlike other foodborne bacteria, the Listeria pathogen can multiply in the cold, and it can persist even after the food is washed. If the product is eaten without cooking, there is no chance to kill the bacteria through heat.

The bacterium Listeria monocytogenes is a Gram-positive species that causes Listeriosis. (Kateryna Kon/Shutterstock )

The good news about Listeria is that infections are rare even though the bacteria is common in the environment. The bad news is that it can cause serious illness, especially in people with weakened immune systems. It's also a particular hazard for pregnant women.

"It can actually cross three different barriers in the human body. It's quite a clever bacteria," said Jeffrey Farber, director of the Canadian Research Institute for Food Safety at the University of Guelph. "It can cross the epithelial cells in the intestines, it can cross the blood-brain barrier and it can also cross the placental barrier."



The food risk from Listeria was first identified in Canada in 1981 when people in the Maritimes began getting sick and dying after eating coleslaw.

"At that time nobody really thought about Listeria as a foodborne pathogen," said Farber.

Then in Southern California in 1985, 62 people died after eating cheese contaminated with Listeria.

"That really put Listeria on the map," said Farber.

At first experts assumed the Listeria risk was limited to deli meats and raw cheese. Canadians might recall the major Listeria outbreak in 2008 associated with ready-to-eat meat from Maple Leaf Foods that killed 22 people.

Listeria is 'quite a clever bacteria,' says Jeffrey Farber of the University of Guelph. (Submitted by Jeffrey Farber)

Then it started showing up in unexpected places — on caramel apples, celery and cantaloupe.

And in 2015 another Canadian outbreak alerted officials that packaged leafy greens could harbour Listeria.

"This is the first multi-state outbreak associated with prepackaged salads," said Dr. Colin Basler, an epidemiologist with the U.S. Centers for Disease Control in Atlanta who led the investigation.

Basler's story of that discovery, published last week, reveals the intricacies of a cross-border investigation where inspectors relied on new DNA fingerprinting technology and old-fashioned luck.

Basler started connecting the dots in the fall of 2015 when people in several U.S. states were infected by Listeria bacteria with an identical genetic fingerprint. It was a red flag suggesting they'd all been exposed to the same source of contamination.

But where was the Listeria lurking? It can take up to 10 weeks for symptoms to start, including fever, vomiting, cramps and sometimes headache, stiff neck, and a loss of balance. By then people can't remember precisely what they ate. In this case the only common food item people could remember eating seemed to be salad.

Smoking lettuce leaf

The break in the case came from Canada when Basler learned that Canadians in several provinces were also getting sick from the same DNA-fingerprinted bug.

Then Canadian inspectors found the smoking lettuce leaf — Listeria discovered in prepackaged leafy salad collected from stores with an exact genetic match to the bacteria that had infected patients.

The investigators concluded the leafy salads had been contaminated during processing. The company halted production for four months and recalled 22 varieties of packaged salads sold under a variety of brand names in the U.S. and Canada.

The seven-month outbreak took a heavy toll on victims. In Canada 14 people went to hospital and three people died. In the U.S. 19 people got sick and one of them later died. A child developed meningitis and a pregnant woman gave birth prematurely.

Chocolate milk cases

The same year, at almost the same time, four Canadians died and 34 others got sick from exposure to Listeria in chocolate milk.

A recent review of that outbreak, which was limited to Ontario, reflects a challenge of this pathogen. Even though the milk was pasteurized, the contamination happened later in the packaging process.

"If you look at a lot of the outbreaks, a lot of them can actually be traced back to the lack of basic what we call 'good manufacturing practices.'" Farber said. "Within the environment of a food processing establishment it's known to survive for a very long period of time. We're talking months or years."

"If plants do a rigorous job of cleaning and sanitizing the environment, that's half the battle."

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