A University of British Columbia study found minute traces of Listeria in one fifth of ready-to-eat fish products purchased from Vancouver stores, but no Listeria in ready-to-eat meat products from the same retailers.
UBC food microbiologist Kevin Allen tested 40 ready-to-eat fish samples, all purchased prior to their best-before date from seven large chain stores and 10 small retailers in Metro Vancouver. The products included lox, smoked tuna, candied salmon and fish jerky.
The findings – published in a recent issue of the journal Food Microbiology – showed that Listeria was present in 20 percent of the ready-to-eat fish products. Of those, five percent had the more virulent strain of Listeria monocytogenes.
The researchers also tested ready-to-eat meat products from the same Metro Vancouver retailers where they bought the fish. They did not find Listeria in any of the 40 meat samples, which included bologna, corned beef, cooked ham and pepperoni.
In 2008, an outbreak of listeriosis in Canada traced to ready-to-eat meats killed 23. Since then, Canada has had an increased focus on Listeria in ready-to-eat meats with more inspection, environmental sampling and end-product testing.
Although the Listeria monocytogenes levels in the ready-to-eat fish products met federal guidelines in Canada — under 100 listeria cells per gram — Allen noted that the bacteria can multiply during handling and storage – particularly toward the end of shelf life.
“Additional handling of ready-to-eat foods in stores, such as slicing, weighing, and packaging, may increase the potential for cross-contamination,” said Allen in a UBC news release. “While Listeria bacteria can be killed by high heat, most people eat these fish products without further cooking.”
Unlike most bacterial causes of foodborne illness, Listeria monocytogenes continue to grow under refrigeration. Storage times are therefore an important consideration for prevention of listeriosis, particularly for high risk foods such as soft cheeses, unpasteurized dairy products (including raw milk); deli and lunch meats, cold cuts, hot dogs, pâté and meat spreads.
Listeriosis can cause high fever, severe headache, neck stiffness and nausea. Pregnant women, the elderly and people with weakened immune systems are particularly at risk. While infected pregnant women may experience only a mild, flu-like illness, infections during pregnancy can lead to premature delivery, infection of the newborn, or even stillbirth.
Listeriosis, in severe cases, can result in brain and blood infections and is fatal in an estimated 18-40 percent of cases.
Allen, an assistant professor at the Faculty of Land and Food Systems (LFS), led the Vancouver, BC study with co-authors Lili Mesak, a UBC research assistant, and Jovana Kovačevic, an LFS food science graduate student.