Introduction
In Belgium, the annual incidence of Listeria monocytogenes infections reported at the National Reference Centre (NRC) ranges between 0.65 and 0.75 cases/100 000 inhabitants over the last ten years. Although fluctuations in total numbers were usually related to fluctuations in serotype 1/2a prevalence, since 2019 this trend has changed to fluctuations in serotype 4b. The main cause of these fluctuations are small and large outbreaks. Therefore, it is key to closely monitor the genetic relatedness of clinical strains to enable real-time tracking of potential outbreaks and initiate further investigations promptly.
Materials and Methods
Since 2020, each clinical isolate sent to the NRC is sequenced on a weekly basis by Next-Generation sequencing. The obtained data are analysed using an in-house Listeria pipeline (v1.2) in Galaxy, which amongst other analyses performs cgMLST (core genome MultiLocus Sequence Typing) based on the scheme of Moura et al 2016. Cluster analyses are performed after uploading the profiles in BioNumerics v8. A genetic cluster is defined from the moment that at least two isolates have a maximum of 7 allelic differences (AD) within a 12-month timeframe. All genetically related strains that fall outside this timeframe, are subsequently added to the cluster.
Discussion
From January 2020 until June 2024, a total of 26 ‘active’ genetic clusters of Belgian clinical isolates were identified; involving 168 cases, 10 reported deaths and 8 reported fatal pregnancy outcomes. Thirteen of the clusters are of serotype 4b (92 cases), while 11 clusters are of serotype 1/2a (65 cases). Twenty clusters contain a smaller number of cases (2-6), whereas 6 clusters consist of a larger number of cases (8 to 35). Additionally, 12 clusters span a period of more than 2 years (range 2-13 years), indicating a persistent source. Seven of the clusters (of which 6 persistent, and 2 notified by our NRC) are part of a cross-border event/investigation, with smoked salmon, soft cheese, and pig meat as potential sources. For 2 of the 4 clusters reported internationally by our NRC, no closely related international isolates were detected, suggesting a national food source, although it remains unidentified.
The four outbreaks with the highest number of cases were (1) cluster 2020_Beta: 35 cases of CC4 (2020, 2021, 2022), no internationally related cases, potential link to meat salad but the direct source was not identified; (2) cluster 2023_Alpha: 22 cases of CC1 (2019, 2020, 2023, 2024), 6 international cases involved, source identified as pig meat of Belgian origin; (3) cluster 2017_Alpha: 17 cases of CC155 (2016-2023), 10 countries involved, related to smoked fish products of Lithuanian origin; (4) cluster 2014_Alpha: 14 cases of CC14 (2014, 2018, 2019, 2020, 2021), 9 Swedish cases involved, related to smoked Salmon of Norwegian origin.
