Norwegian authorities are investigating a potential outbreak of Listeria monocytogenes that has affected six people.

The Norwegian Institute of Public Health (Folkehelseinstituttet) reported an increase in cases of people infected with Listeria in December. Of the six cases, four are from Oppland and Hedmark. Usually, one to two patients with listeriosis are reported monthly.

The agency did not disclose any demographic details about the patients or if there had been any hospitalizations or deaths.

The Norwegian Food Safety Authority (Mattilsynet) has repeated advice that those with compromised immune systems, pregnant women and elderly people with weakened health, should avoid foods that may contain Listeria.

Listeria is often found in foods that are eaten without heating and so called ready-to-eat foods stored in the refrigerator such as sliced meat, fish, raw milk and unpasteurized milk cheeses. Listeria bacteria can survive freezing. Increasingly, Listeria monocytogenes is being found in fresh produce, ice cream and processed foods. Four previous outbreaks of listeriosis in Norway were linked to fish, organic camembert cheese and cold meats.

The agency is working with the Norwegian Institute of Public Health and municipal health services to determine whether patients have a common source of infection. This takes time and it is not always possible to find the source of infection.

Time from infection to the start of symptoms, referred to as the incubation period, is usually from a few days to three weeks, but in some patients it can be up to three months.

The fish outbreak in 2013 affected three people who all had another underlying disease. The outbreak linked to cheese in 2007 affected 21 people with five deaths. Eight cases were reported in 1992 linked to a meat producer and in 2005 three cases were reported at a hospital and the strain was found on equipment in the hospital’s kitchen but a food source was not determined.

Number of cases of Campylobacter infection per week, infected in Sweden or unknown country, 2017 and 2018 (including week 49). Click to enlarge

Meanwhile, an increase in Campylobacter cases in Sweden linked to chicken appears to have leveled off.

The number of domestic cases of infection was higher in November than expected for the season but fewer people were reported to have been infected with Campylobacter in recent weeks. Authorities warned it was still too early to say that the outbreak is over.

During the first three weeks in November last year, Sweden had an average of domestic cases, including those with unknown country of infection, just below 70 per week. During the same period this year, the average number of cases was more than 90 per week.

Finally, in Denmark a case-control study has pointed to a type of spiced sausage called medisterpølse as the source of a Salmonella outbreak with 33 cases.

Example of medisterpølse

Statens Serum Institut (SSI) carried out the study to find the source of infection with 21 patients and 67 healthy controls. However, not all patients reported eating this product so it is still possible there may be other pork contaminated with Salmonella.

It has not yet been possible to find out where the type of sausage was produced and sold or to pinpoint a specific batch from a food company as the outbreak source.

Almost 20 people were hospitalized with Salmonella monophasic Typhimurium (O: 4.5,12; H: i, -) but no deaths have been reported.

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