Listeria is found in all food groups, but the probability that pregnant women and other vulnerable groups get so much of the bacteria from food that they can become ill vary for different foods. The effect of measures that reduce probability also varies.
This is the main message in an assessment of health advice on Listeria monocytogenes for pregnant women and other vulnerable groups, ie children, the elderly, and those with a reduced immune system, conducted by Norwegian Scientific Committee for Food and Environment (VKM) based on the request from Norwegian Food Safety Authority.
The background is that the Norwegian Food Safety Authority want to update their health advice on Listeria monocytogenes.
Listeria can cause the disease listeriosis. Listeriosis can be transmitted from mother to fetus during pregnancy and lead to life-threatening disease in the fetus.
VKM has evaluated a wide range of products within four food groups: fish and seafood products, meat products, dairy products and vegetables. Vegetables include fruits, berries, vegetables, potatoes, nuts, seeds and cereal products.
In the assessment, VKM highlights some food products that, under certain conditions, may increase the likelihood for vulnerable groups to develop listeriosis.
"Byn certain conditions we mean packing methods, storage time and temperature from the food is produced until it is eaten," says Taran Skjerdal, who has been the lead expert in the project.
Cold-smoked and hot-smoked fish, dilled fish, boiled shrimps and fish food packed after being chilled are fish products that, according to VKM, may increase the probability of acquiring listeriosis.
Heat-treated meat products intended to be reheated before being eaten, but which are not heated by the consumer, may also increase the likelihood of listeriosis. Such products are, for example, meat patties, meatballs and sausages.
Dilled raw meat, smoked raw meat without curing or use of preservatives or heat treatment and chicken salad can also increase the likelihood of listeriosis.
All types of unpasteurized dairy products can increase the likelihood of listeriosis, VKM's assessment shows.
That also applies to pasteurised products like sliced or grated cheese, mozzarella used in salad, all spreadable cheeses and soft cheeses.
Some cut, fresh vegetables and fruit, especially melons, can increase the likelihood of listeriosis.
This applies also to sprouts and freshly cut ready-to-eat products with several ingredients as well as to inappropriately preserved home-canned and fermented food.
VKM has also evaluated the effect of various measures that can reduce the likelihood of listeriosis.
Heat treating the food before eating is killing the bacteria if the treatment is sufficient. The food should be heated until the coldest point reaches 75 degrees. Then the food should rest so that the heat is distributed evenly throughout so that no parts of the food remain cold, says Skjerdal.
Listeria grows slower at low temperatures. Therefore, it is important that the temperature in the refrigerator is not higher than 4 degrees. The effect is even better if low temperature is combined with short storage time in the refrigerator.
"One measure may be to avoid leftovers stored in the refrigerator for several days. Listeria bacteria often grow faster in opened than in unopened packaging, Skjerdal states.
VKM's Scientific Panel on Biological Hazards is responsible for the assessment.