With raw milk, chances of Listeria infection grows

Soft-ripened cheese made from raw milk is 50 to 160 times more likely to cause a Listeria infection than cheese made with pasteurized milk, according to a joint study by Health Canada and the U.S. Food and Drug Administration.

The findings are worth noting since Listeria is one of the most deadly foodborne illnesses in North America, with the elderly, pregnant women and people with pre-existing medical conditions especially susceptible.

In a summary report, Health Canada and the FDA noted that some outbreaks of Listeriosis have been “strongly” associated with cheese, especially soft and soft-ripened cheese. The two agencies said they wanted to assess the risk posed to consumers by these products.

Using camembert cheese and mathematical models, they predicted the chances of one Listeria infection per number of servings.

Health Canada predicted one outbreak of Listeriosis per 7.3 billion servings of pasteurized cheese and one outbreak per 105 million servings of raw-milk cheese in the general population.

Among the elderly, Health Canada predicted one case of Listeriosis per 138 million servings of pasteurized cheese and one outbreak per 2.6 million servings of raw-milk cheese. The chances of a Listeria infection were also higher in pregnant women and people with pre-existing medical conditions.

Although outbreaks are more likely to occur with raw-milk cheese, Health Canada and the FDA did note that there are risks associated with pasteurized cheese, especially in the home.

“The main factor that influences risk per serving of pasteurized cheese is the amount of monocytogens growth in cheese, particularly while the consumer stores the cheese at home. monocytogens is a bacterium that can grow at refrigerator temperatures, given the right conditions,” the authors of the report wrote.

The report, “Joint FDA/Health Canada Quantitative Assessment of the Risk of Listeriosis from Soft-Ripened Cheese Consumption in the United States and Canada” is still in the draft stage.

In addition to outlining the risks of soft-ripened cheese, the report examined ways to reduce Listeria monocytogens in raw-milk cheese. Those include removing the regulation that requires a minimum of 60 days aging at 2 degrees C; testing of bulk milk used to make raw-milk cheese; and a treatment (which stops short of full pasteurization) that would kill 99.9 per cent of the bacteria in bulk raw milk.