Invasive listeriosis outbreaks and salmon products: a genomic, epidemiological study

Abstract

Invasive listeriosis, caused by Listeria (L.) monocytogenes, is a severe foodborne infection, especially for immunocompromised individuals. The aim of our investigation was the identification and analysis of listeriosis outbreaks in Germany with smoked and graved salmon products as the most likely source of infection using whole-genome sequencing (WGS) and patient interviews. In a national surveillance programme, WGS was used for subtyping and core genome multi locus sequence typing (cgMLST) for cluster detection of L. monocytogenes isolates from listeriosis cases as well as food and environmental samples in Germany. Patient interviews were conducted to complement the molecular typing. We identified 22 independent listeriosis outbreaks occurring between 2010 and 2021 that were most likely associated with the consumption of smoked and graved salmon products. In Germany, 228 cases were identified, of 50 deaths (22%) reported 17 were confirmed to have died from listeriosis. Many of these 22 outbreaks were cross-border outbreaks with further cases in other countries. This report shows that smoked and graved salmon products contaminated with L. monocytogenes pose a serious risk for listeriosis infection in Germany. Interdisciplinary efforts including WGS and epidemiological investigations were essential to identifying the source of infection. Uncooked salmon products are high-risk foods frequently contaminated with L. monocytogenes. In order to minimize the risk of infection for consumers, food producers need to improve hygiene measures and reduce the entry of pathogens into food processing. Furthermore, susceptible individuals should be better informed of the risk of acquiring listeriosis from consuming smoked and graved salmon products.

Keywords: Listeriosis; foodborne outbreaks; molecular surveillance; public health; salmon products.

Figure 1.

Minimum spanning tree showing 22 listeriosis clusters associated with salmon consumption. The tree was calculated using 1701 locus cgMLST data in the pairwise ignore missing values mode. Clinical isolates are shown in various colours, food isolates are shown in grey. Clusters consisting of isolates with ≤7 different alleles are highlighted by a grey background. Numbers indicate allelic distances between the single nodes of the tree.

Figure 2.

Outbreak cases by month and year of notification, Germany 2018–2021. Cases are shown since 2018 when all available clinical isolates where sequenced. Three large ongoing outbreaks (Beta2a in green, My2 in grey, and Omikron1 in brown) are presented individually, the other 19 outbreaks are shown in blue.