Nationwide outbreak of invasive listeriosis associated with consumption of meat products in health care facilities, Germany, 2014-2019

Abstract Objectives Invasive listeriosis is a severe foodborne infection caused by Listeria (L.) monocytogenes. The aim of this investigation was to verify and describe a molecular cluster of listeriosis patients and identify factors leading to this outbreak. Methods Whole genome sequencing (WGS) and core genome multi locus sequence typing were used for subtyping L. monocytogenes isolates from listeriosis cases and food samples in Germany. Patient interviews and investigational tracing of foodstuffs offered in health care facilities (HCF), where some of the cases occurred, were conducted. Results We identified a German-wide listeriosis outbreak with 39 genetically related cases occurring between 2014 and 2019. Three patients died because of listeriosis. After identification of HCF in different regions of Germany for at least 13 cases as places of exposure, investigational tracing of food supplies in 6 prioritized HCF revealed meat products from one company (X) as commonality and subsequently the outbreak strain was analysed in 6 isolates from ready-to-eat meat products and one isolate from the production environment from company X. No further Sigma1 cases were detected afterrecall of the meat products from the market and closure of company X (as of August 2020). Conclusions Interdisciplinary efforts including WGS, epidemiological investigations in patients and investigational tracing of foods were essential to identify the source of infections, and thereby preventing further illnesses and deaths. This outbreak underlines the vulnerability of hospitalised patients for foodborne diseases, such as listeriosis. Food producers and HCF should minimise the risk of microbiological hazards when producing, selecting and preparing food for patients.