20c - Listeria monocytogenes outbreak linked to artisanal blackened olives in Baden-Wuerttemberg, Germany 2023
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Background : In December 2020 whole genome sequencing of the National Reference Laboratory detected a cluster of previously unrelated Listeriosis cases of cgMLST-type 5498 (“pi8”). First cases were notified in December 2020 and clustered in southern Germany. We investigated the outbreak to identify the source and stop it.
Methods : We defined cases as culture or PCR positive and having cgMLST5498 with disease onset from December 2020. Cases were interviewed with the structured Listeriosis-questionnaire of the Federal Health Agency about food consumption during the exposure period. Answers were shared with the State laboratory for food control (CVUAS-S) to inform enhanced environmental sampling.
Results : Of the 20 pi8-cases, five were pregnancy related, nine were from Baden-Wuerttemberg and eight from neighbouring States. Median age was 58 years (range: 0-93), 13 were female and seven male. Two cases died, however of reasons other than Listeriosis. Ten of 13 cases interviewed mentioned having eaten blackened pickled olives during their incubation periods. In September 2023, CVUA-S sequenced a pi8-Listeria submitted by a voluntary self-monitoring artisanal pickle manufacturer. The spike of an industrial tin opener for blackened olives was identified as reservoir. The German food monitoring program reported in 2022 that among olives tested, only blackened had Listeria.
Conclusions Voluntary self-monitoring and sequencing of all bacterial food samples led to the rapid attribution of blackened olives as outbreak vehicle for the Germany wide pi8-Listeriosis outbreak. Regular sampling and daily disinfection of the tin opener were ordered. After October 2023, no new cases were notified. Non-pasteurised, blackened olives should be considered as risk factors for Listeriosis and not eaten during pregnancy. Treating green olives with ferrous gluconate for blackening might enhance growth or invasive capabilities of Listeria.