A Listeria outbreak that affected more than 50 people was the biggest fish-product-related incident in Germany, according to researchers.

The large listeriosis outbreak with 55 cases affected Germany, Austria, Denmark, and Switzerland during 2020 and 2021. Three people died.

Investigations of food samples identified Listeria monocytogenes from smoked rainbow trout filets from Agustson, a Danish producer, grouping with isolates from cases. Patient interviews confirmed the consumption of rainbow trout as the likely infection source. 

The outbreak was identified by molecular surveillance in Germany. Routine whole genome sequencing (WGS) of clinical Listeria monocytogenes isolates was implemented in the country in 2018. 

Details of people sick
In November 2020, a sudden increase in related isolates was detected. Information on the cluster was shared via the Epidemic Intelligence Information System (EPIS) platform of the European Centre for Disease Prevention and Control (ECDC) in the same month.

This returned two recent closely related isolates in both Austria and Denmark and one in Switzerland, indicating the possible cross-border distribution of a contaminated food item.

In total, 68 isolates were recorded between September 2020 and January 2022 in the four affected countries with 63 in Germany. The last isolate was collected on Jan. 31, 2022, according to the study published in the journal Microbiology Spectrum.

Overall, 55 notified cases could be allocated to the outbreak isolates. They were reported from October 2020 to January 2022 with the majority from mid-October to mid-November 2020. Seven cases were reported later in 2021, and one was reported in January 2022.

Of these cases, 50 were from Germany, two each from Austria and Denmark, and one from Switzerland.

Of German patients, 22 were female, and their median age was 80 with a range from less than 1 to 94 years old. Three people died but for one case, another cause of death was notified, and for two people, the cause was not reported. Two cases were pregnancy-associated.

The outbreak had a sudden and marked increase in case numbers and was of a comparably short duration. Other listeriosis outbreaks in Germany are normally active for years before they are detected and stopped.

Researchers interviewed 19 of the 55 cases about food consumption prior to disease onset, and 16 recalled having eaten smoked trout. It was the most frequently reported food item from the standardized questionnaire in Germany, followed by Gouda cheese.

Search for source
A matching food isolate was detected inside an emptied and reclosed package of smoked rainbow trout filets from the waste bin of a patient in Germany. The product was manufactured by Agustson in Denmark.

In December 2020, the Rapid alert System for Food and Feed (RASFF) warned of Listeria monocytogenes in another batch of smoked rainbow trout filets of this brand in an official sample taken in October, which exceeded the limit for Listeria ready-to-eat products. At the time of the notification, the affected batch had already expired. This isolate was also similar to samples from patients.

The same brand of smoked trout was sold in supermarkets in the four countries wher listeriosis cases were identified. Altogether, this strongly indicates the outbreak was caused by rainbow trout filets from the Danish producer, said scientists.

A recall was issued in December 2020 and investigations at the processing facility in Denmark identified Listeria monocytogenes, but the isolate did not belong to the outbreak cluster. Hygiene and disinfection measures at the plant were intensified.

“Our report demonstrates that international food trade can cause multi-country outbreaks that necessitate cross-border outbreak collaboration. It also corroborates the relevance of ready-to-eat smoked fish products as causes for listeriosis,” said scientists.