Cause of 'unusual' salmonella outbreak linked to onions still unknown

The cause of a widespread salmonella outbreak tied to onions, which has made at least 149 people in Alberta sick, remains a mystery, says Michael Gänzle, professor of food microbiology at the University of Alberta.

The Canadian Food Inspection Agency issued a recall of some red onions, imported from California, on July 30. Since then, the recall has been expanded six times, to include yellow, white and sweet onions, as well as salsas, salads and sandwiches made with the onions. The onions have been linked to 239 confirmed cases of salmonella across Canada, with the greatest number of cases in Alberta.

"It's very unusual," Gänzle told CBC's Edmonton AM. "This is the first case linked to onions, that I am aware of."

Onions are a rare source of salmonella outbreaks, Gänzle said, since they grow underground, are peeled before eating and are often cooked, which can kill the bacteria that makes people sick.

The investigation into what caused the contamination is ongoing, but Gänzle has some theories.

"I would guess that contaminated irrigation water would be the source," he told CBC. "Growing onions in California means you are irrigating a lot."

Contaminated irrigation water has been the source of other salmonella outbreaks in California produce, Gänzle said. However, crops like spinach are more commonly contaminated, often as a result of cattle being too close to the fields and contaminating irrigation water with feces.

Anyone who thinks they might still have the recalled onions lurking in their cold storage, or at the back of their fridge, should toss them immediately. There is no way to tell a good onion from a bad one, just by looking. Washing the onions won't work, said Gänzle, because salmonella could have gotten on the onion early in its growth and it could have been growing there for some time.

When it comes to symptoms, "most of us will eat it and not get ill," said Gänzle. However, people who are young, old, or have weak immune systems are at higher risk for serious illness or even death.

Salmonella can cause diarrhea, fever and cramps. In rare cases, the bacteria can get into the blood, where it can be fatal.

The recall includes onions sold under the brand names Krown, Onions 52, Tender Loving Care, Thomson International and Thomson International Premium.

A full list of the recalled products is available on the Canadian Food Inspection Agency website.