A significant proportion of listeriosis cases in Germany are caused by eating smoked or cured salmon products, according to the Robert Koch-Institut (RKI).

A total of 22 cross-federal state outbreaks of listeriosis have evidence pointing to smoked or cured salmon products as the cause of infections. This includes 15 illnesses from 2010 to 2015 and 41 in both 2019 and 2020.

These outbreaks include 236 clinical Listeria monocytogenes isolates and 208 cases of listeriosis could be assigned to these isolates.

Patients were diagnosed between 2010 and 2020. This past year 41 people got sick in 14 of the 22 outbreaks which suggests that the sources of infection persist and outbreaks are ongoing.

Matching product and patient samples
The number affected in the outbreaks ranges from two to 35 people. Patients were 0 to 99 years old and 110 males and 98 females were affected. Four pregnancy-associated illnesses were reported. Forty-four people were reported to the RKI as deceased, of which 17 died directly or indirectly from listeriosis.

Analysis of Listeria monocytogenes isolates from smoked or cured salmon products has found they are a close match to those from outbreak patients.

The genome sequences of clinical Listeria monocytogenes isolates are closely related within the individual outbreaks, so it can be assumed that patients were infected by a common source.

However, clinical isolates of the 22 outbreaks are not very closely related, which indicates they are probably caused by smoked or cured salmon products from different origins, according to the report.

In 2018, there were 701 invasive listeriosis infectionsreported in Germany compared to 591 for 2019.

Eleven of the outbreaks also involved illness in 17 other European countries including Great Britain, according to data from the Epidemic Intelligence Information System of the European Centre for Disease Prevention and Control (ECDC). This is not unexpected, as smoked or cured salmon products are produced, processed and sold internationally, according to the report.

A total of 21 patients or their relatives from 12 outbreaks have been questioned about food consumption and shopping behavior. Of these, 19 had eaten smoked salmon or smoked fish in the two weeks before the onset of disease. This is significantly more than would be expected in the general population.

In a survey used for outbreak investigations by the RKI, 24 percent of healthy respondents stated they had consumed smoked salmon two weeks prior to being asked.

Challenging situation
Individual outbreaks mostly involve infections during a long period of time. Under-reporting is to be expected for all listeriosis outbreaks, because not all patients are diagnosed and isolates from diagnosed cases are not always sent to the laboratory at RKI.

It can be assumed that the number of illnesses in the 22 outbreaks in Germany is significantly higher, said RKI.

Persistent occurrence of cases in the outbreaks described is an indication that there is contamination in production facilities and further illnesses are to be expected. It is not yet clear at which plants and processing steps the contamination occurs that causes the illnesses in Germany, according to RKI and BfR.

In September 2020, the Federal Institute for Risk Assessment (BfR) highlighted the risks of consuming smoked fish because of Listeria.

Raw, smoked or cured fish products and seafood such as sushi, sashimi, oysters, cold or hot smoked fish and cured fish such as graved salmon are frequently contaminated with Listeria. In total, 7 percent to 18 percent of the samples of cold-smoked or cured fish products examined by food monitoring authorities in Germany between 2007 and 2017, and 3 percent to 9 percent of hot-smoked fish products contained Listeria monocytogenes.

The BfR recommends that people who have an increased risk of developing listeriosis should only eat fish or seafood that has been thoroughly heated.