Smoked and graved salmon continues to pose a Listeria infection risk with evidence showing a large proportion of listeriosis cases are caused by these products, according to scientists in Germany.

In 2021 and 2022, there were 66 cases of listeriosis in 15 outbreaks, reported the Robert Koch Institute (RKI).

A total of 24 outbreaks of listeriosis across federal states with links to smoked or graved salmon products as the cause have been found since 2010.

This is an update from a previous study that identified 22 outbreaks between 2010 and 2021 that were associated with eating smoked and graved salmon products. Many also affected people in other countries. 

Severe underreporting is to be expected for listeriosis outbreaks, so it can be assumed that the number of cases in Germany is significantly higher, said scientists.

Updated figures
In 2021 and 2022, closely related isolates continued to be identified in smoked or graved salmon products for 11 of 22 outbreaks from 2010. For one of the two new outbreaks, related food isolates were reported in Denmark.

In 2021, 29 people were sick in 12 outbreaks and in 2022 there were 37 cases in 11 outbreaks. Another 19 cases fell ill before 2021 and were identified retrospectively.

Overall, the data includes almost 300 patients from 2010. The largest outbreak involves 41 cases.

Listeriosis cases in 2021 and 2022 were 17 to 95 years old with a median age of 78. A total of 39 males and 27 females were affected. Nine deaths were reported to the RKI and four died directly or indirectly from listeriosis. This takes the total to 57 deaths from 2010, with 21 linked to Listeria infections.

One illness associated with pregnancy was reported, bringing the total to five from 2010 to 2022.

Multi-year and international incidents
Despite the risk of Listeria from smoked or graved salmon being known for some time, 2022 data shows contamination still exists in production facilities and outbreaks are ongoing with further cases expected, said scientists.

RKI and the Federal Institute for Risk Assessment (BfR) have not yet been able to find in what companies and at which processing steps contamination occurs. The agencies said any measures already taken don’t seem to be enough to protect consumers.

The fact that cases continue to occur for many outbreaks, although possible fish products and manufacturers have been identified, casts doubt on the efficiency of measures taken and strategies pursued to minimize Listeria contamination, said RKI and BfR.

Communication by RKI via the EpiPulse system of the European Center for Disease Prevention and Control (ECDC) found cases in other EU countries or the United Kingdom for two more outbreaks, taking the total to 13 international events. The similarity of isolates makes it likely that sources of infection are the same, said scientists.

A total of 259 non-clinical isolates from fish products and fish-associated production environments in Germany showed a close relationship to 23 listeriosis clusters. Most isolates came from smoked or graved salmon products.

In 2021 and 2022, 25 cases of listeriosis or their relatives were asked about food consumption and shopping behavior. Of these, 16 said they had consumed smoked salmon or smoked fish in the two weeks before the onset of the illness.

Food safety and infection control measures are increasingly necessary to minimize the risk of listeriosis from smoked or graved salmon. Risk communication and risk management are needed in Germany and at the international level to stop contamination of these products with Listeria and also the outbreaks said scientists.

People at an increased risk of listeriosis should only eat fish and seafood that have been well-cooked. Smoked and graved salmon products should not be offered to vulnerable groups, such as immunocompromised people and the elderly in healthcare facilities, said the BfR.