The number of people affected by Listeria in Norway in the past few months has grown to 13. Two-thirds ate a traditional dish made with fermented, uncooked fish before becoming sick.

The Norwegian Institute of Public Health (Folkehelseinstituttet) said most cases are more than 70 years old and have compromised immune systems. The agency is investigating the increase as an outbreak but not all cases are part of it.

Eight cases ate rakfisk before they fell ill. This is a traditional seasonal Norwegian fish dish made from trout or char, salted and then fermented for up to a year before being eaten without cooking. The Norwegian Institute of Public Health said Listeria has been detected in rakfisk produced by Slidre Ørretsenter, which is the suspected source of the outbreak.

Slidre Ørretsenter has withdrawn all varieties of rakfisk from the market. They were distributed and sold all over the country.

The company asked people who bought the products to throw them away or take them back to the store wher they were purchased. Rakfisk is a seasonal item and production is not going on now, according to the manufacturer.

Officials at the Norwegian Institute of Public Health said there was a “strong suspicion” that rakfisk caused the outbreak and emphasized that people in risk groups such as pregnant women, those with impaired immune systems, and the elderly should avoid eating rakfisk and other products at risk of Listeria contamination including items made from unpasteurized milk, soft and semi-soft cheeses, smoked salmon and sliced cooked meats.

The agency received the reports of people with listeriosis from mid-November 2018 through the second week of January this year. Most of them live in Hedmark, Oppland and Buskerud. Usually, the country only has one to two patients are reported monthly.

Last month, The Norwegian Institute of Public Health reported an increase in Listeria monocytogenes cases with six people affected in December. The agency started the outbreak investigations with the municipal health services, the Norwegian Food Safety Authority (Mattilsynet) and Veterinary Institute.

Laboratory testing is continuing and bacteria with similar DNA profiles have been detected in eight of the patients, meaning they were likely infected by a common source. Bacterial isolates from three of the 13 patients have other DNA profiles and do not belong to the outbreak. Typing of the remaining bacterial strains is ongoing.

The municipal health service and the Norwegian Food Safety Authority has interviewed patients and collected food samples for analysis. Food tests are analyzed at the Veterinary Institute while information from interviews is compiled at the Norwegian Institute of Public Health.

Mattilsynet is carrying out further investigations with the manufacturer. Samples of rakfisk were taken from one of the infected people and analyzed by the Veterinary Institute.

Food contaminated with Listeria monocytogenes may not look or smell spoiled but can still cause serious and sometimes life-threatening infections. Anyone who has eaten any of the recalled product and developed symptoms of Listeria infection should seek medical treatment and tell their doctors about the possible Listeria exposure.

Also, anyone who has eaten any of the recalled product should monitor themselves for symptoms during the coming weeks because it can take up to 70 days after exposure to Listeria for symptoms of listeriosis to develop. Although infected pregnant women may experience only mild, flu-like symptoms, their infections can lead to premature delivery, infection of the newborn or even stillbirth. 

Symptoms of Listeria infection can include vomiting, nausea, persistent fever, muscle aches, severe headache and neck stiffness. Specific laboratory tests are required to diagnose Listeria infections, which can mimic other illnesses.