Ongoing High Incidence and Case-Fatality Rates for Invasive Listeriosis, Germany, 2010–2019

We used 10 years of surveillance data to describe listeriosis frequency in Germany. Altogether, 5,576 cases were reported, 91% not pregnancy associated; case counts increased over time. Case-fatality rate was 13% in non–pregnancy-associated cases, most in adults ≥65 years of age. Detecting, investigating, and ending outbreaks might have the greatest effect on incidence

Unless listeriosis was mentioned as part of the causal chain leading to death, "other cause" would be selected as cause of death.
The consultant laboratory for Listeria at RKI collects and performs whole genome sequencing (WGS) on clinical L. monocytogenes isolates from human infections in Germany.
Unlike in the reportable disease notification system, submission of clinical L. monocytogenes isolates from primary laboratories to the RKI consultant laboratory is voluntary but is encouraged by public health authorities. Genome sequence information from isolates in individual cases in L. monocytogenes clusters or outbreaks can thus be linked with notified listeriosis cases throughout Germany. In case of outbreak detection by WGS, mandatory notification data from listeriosis cases and typing data for the L. monocytogenes isolates that were sent to the RKI can be merged for investigation.
Subtyping of L. monocytogenes isolates from food samples was performed at the National Reference Laboratory for Listeria monocytogenes at the German Federal Institute for Risk Assessment (BfR) in Berlin. The consultant laboratory and the BfR regularly compare L. monocytogenes strain identities from clinical and food isolates.
The information on outbreaks is submitted to the federal state and local public health departments. After detection, cross-regional listeriosis outbreaks are jointly investigated by the public health services as well as food safety agencies of the districts, cities, federal states, and at the national level. Listeriosis surveillance in Germany is limited in the sense that food consumption histories are only collected after detection of outbreaks. Patients or their relatives who consent are interviewed using a standard comprehensive exploratory questionnaire. If a certain food is already indicated as the probable vehicle, a shorter, more focused questionnaire is used for the interview. Furthermore, several listeriosis outbreak investigations require international collaboration, including with the European Center for Disease Prevention and Control (ECDC) and its Epidemic Intelligence Systems. In parallel, food safety agencies (e.g., European Food Safety Authority) play important roles on all levels of the federal system to safeguard necessary investigations and interventions. The OneHealth approach has been incorporated by stakeholders for foodborne disease surveillance and control in Germany. Data on the occurrence of zoonotic pathogens and related antimicrobial resistance in food has been collected by food safety authorities of the federal states since 2009 as part of official food and veterinary surveillance guidelines. Information is evaluated and published annually (3).

Description of listeriosis outbreaks relevant for Germany
In recent years, several protracted and geographically widespread listeriosis outbreaks have been identified in Germany through the systematic application of bacterial typing identified by WGS in the context of disease surveillance. An outbreak with 79 cases occurring during 2012-2016 (4), predominantly in south Germany, could be successfully traced back to meat products from a specific company by systematic typing of L. monocytogenes isolates (5).
Another listeriosis outbreak with 83 cases during 2013-2017 was associated with meatballs (6), a source pinpointed by patient interviews and confirmed by WGS. In the course of this outbreak investigation, 2 distinct genotypes turned out to be involved. One of the largest European outbreaks included 112 notified listeriosis cases throughout Germany and happened during 2018-2019. A case-control study followed by targeted household food sampling of listeriosis patients identified blood sausage from 1 supermarket chain as a possible vehicle.
The association between the outbreak cases and the blood sausage was confirmed by WGS (7). In 2019, there was a nationwide outbreak with 39 cases primarily in healthcare facilities.
Epidemiologic and food tracing investigations in healthcare facilities enabled the detection of contaminated meat products from a manufacturer that provided catering for hospitals (8). The detection of this company as the source led to a large-scale product recall and was accompanied by extremely broad media coverage.
In 2021, BfR and RKI published a report on 22 listeriosis outbreaks in Germany in recent years that focused on the role of salmon or salmon products (9). There were a total of 218 cases reported. Forty-four case-patients died, 17 from listeriosis; 4 cases were pregnancyassociated. The clinical isolates from the 22 outbreaks identified by WGS are not genetically related to each other but each outbreak closely related to isolates from different salmon products (9).
Federal states in eastern Germany show higher incidence compared with those in western Germany (Appendix Figure). The lowest average annual incidence was seen in Saarland (0.48/100,000) and the highest in Saxony (1.28/100,000). The geographical pattern of listeriosis distribution in Germany remains largely unexplained. Recently identified listeriosis outbreaks had regional patterns most likely determined by retail supply organization but outbreak areas were not concordant with high-incidence areas.
Even though these prominent outbreaks highlight meat and salmon products as vehicles for listeriosis outbreaks, it should be kept in mind that other food items, particularly cheeses, sliced fruits, and prepared salads, might cause outbreaks or sporadic cases.