A recent study investigated listeriosis outbreaks in Germany using whole genome sequencing (WGS) and patient interviews. The researchers were able to link a significant number of listeriosis cases to smoked and graved salmon products that were contaminated with Listeria monocytogenes. The findings highlight the need to improve hygiene practices in facilities that process uncooked salmon products, as well as to better inform the public and vulnerable populations of the risk of contracting listeriosis from smoked and graved salmon.

Listeriosis by the Numbers in Germany

The study focused on 22 outbreaks encompassing a total of 228 cases of listeriosis that occurred from 2010–2021 in Germany. The cases were identified by a prior German national surveillance program using WGS. Eight of the 22 outbreaks include cases from 2021 and are considered to be ongoing. A probability was previously established that uncooked salmon products caused the 22 outbreaks studied by the researchers, who focused on salmon-associated outbreaks to illustrate the “significant and preventable” public health risk of smoked and graved salmon contaminated by L. monocytogenes.

Of the 228 cases studied, listeriosis was the confirmed cause for 17 of 50 reported deaths. Additionally, the average incident rate of listeriosis was 0.69 per 100,000 people, 91 percent of listeriosis cases identified were non-pregnancy associated, and 60 percent of listeriosis cases occurred in men. A steady increase of listeriosis cases was observed from 2011–2017, followed by a slight decrease in 2019 and 2020.

Review of Past Outbreaks

In the study, researchers included 166 L. monocytogenes isolates from fish and fish processing environments, as well as 259 isolates from patients. At the time of their diagnosis, the patients were interviewed about their food consumption history, food purchasing behaviors, and medical history. Patient interviews focused on individuals whose cases were verified by medical professionals and belonged to specific clusters. Over 80 percent of interviewed patients recalled consuming smoked salmon or smoked fish in the two weeks before the onset of their illness, which is more than would be expected in the general population.

The researchers’ surveillance of genomic data reveals a close genetic relationship between the clinical isolates from the case patients and the non-clinical isolates from smoked or graved salmon products. However, the L. monocytogenes clones in the 22 outbreaks were not closely related at the molecular level. The findings show that 81 percent of the outbreaks were caused by the same molecular serogroup, despite the fact that only 40 percent of previous human listeriosis cases in Germany were caused by the same strain.

Furthermore, the 166 non-clinical isolates (153 from fish products and 13 from fish processing plants) were closely related to the 22 outbreak clusters assessed by the 259 clinical isolates. Of the 22 outbreaks, 20 were closely related to food isolates, one was closely related to isolates from the fish processing environment, and one was not linked to any closely related isolates in Germany. At least one of the closely related food isolates in each outbreak was linked to smoked or graved salmon. Various other fish products were linked to another 15 non-clinical isolates that were closely related to the outbreak clusters, such as smoked trout, pickled herring salad, and smoked halibut.

A “Significant and Preventable” Problem

Smoked and graved salmon had previously been identified as possible listeriosis hazards. The study’s findings confirm this assessment, as well as highlight the magnitude of the issue of listeriosis contracted from uncooked salmon products. In 2018–2020, 27 percent of German listeriosis cases caused by contaminated foods were linked to uncooked salmon products; it is probable that a number of the additional 1,407 cases caused by unknown food products can also be attributed to smoked or graved salmon. The researchers also assume that the number of listeriosis cases caused by uncooked salmon is much higher than the data suggests, due to under-recording.

The researchers assert that the problem of listeriosis caused by contaminated, uncooked salmon products is severe and preventable. From the data surveilled in the present study and other findings from studies across Europe, the researchers believe that fish processing plants are in need of enhanced guidance for controlling L. monocytogenes from regulatory bodies, that continuous control measures and testing for L. monocytogenes should be implemented in fish processing, and that international risk management strategies must be implemented to stop the distribution of contaminated products across borders. The researchers also stress the importance of developing a real-time system for sharing WGS data internationally to minimize the scope and harm of foodborne illness outbreaks.