Dr. David Acheson, partner and managing director of food safety at Leavitt Partners, healthcare and food safety consulting firm, said it is likely that the onions came into contact with the listeria at some point in the processing facility, perhaps from condensation on the conveyer belt, refrigeration units or on the chopping equipment.
Nearly 600 cases of whole cantaloupes from Burch Farms in Faison, N.C., are also currently under recall due to possible Listeria contamination. The cantaloupes were distributed to retail stores in New York, Maine and potentially other states.
Earlier this year, River Ranch Fresh Foods issued a voluntary recall for its bagged salad from stores across the nation after concerns over lsteria contamination and in 2011, lettuce from Oregon distributor True Leaf Farms that possibly contained Listeria was recalled from stores in Washington and Idaho.
However, none of those incidents compared to a 28-state outbreak in 2011, which resulted in 33 deaths, 147 infections and one miscarriage. The listeria contamination, linked to Holly, Colo.-based Jensen Farms, was the declared the largest outbreak of food-borne disease to have been identified in decades.
Despite the increase in cases, Acheson said he doesn't believe the number of cases is increasing, but more cases are being found because testing is now more frequent.
"The Jensen Farms episode was triggered because of illness, this one was triggered through testing," Acheson said. "We're getting better at recognizing it. There is more testing going on so it's being found."
Serena Marshall contributed to this story.