Exceptionally large and country-wide outbreak of invasive listeriosis associated with the consumption of black sausage, Germany 2018-2019

Listeriosis is a serious human infection caused by ingestion of food contaminated with the bacterium Listeria monocytogenes. With ~30% the lethality of listeriosis is among the highest of all bacterial gastrointestinal pathogens causing high disease burden in the population. Therefore, molecular surveillance programs have been implemented in various countries for early recognition and efficient management of listeriosis outbreaks. Between August 2018 and April 2019, the German listeriosis surveillance program identified an exceptionally large outbreak including 134 ST6 isolates assignable to 111 human listeriosis cases that formed a highly clonal cluster after core genome multi locus sequence typing (cgMLST) and single nucleotide polymorphism (SNP) calling. The outbreak clone revealed increased resistance against benzalkonium chloride and carried a prophage that was used for the development of a clone-specific PCR. Patient interviews with a standardized questionnaire on purchase and consumption of food and a subsequent case control study using multivariate analysis identified blood sausage produced by a large food company as the most likely source of infection. L. monocytogenes isolated from this food source were indistinguishable from the human isolates in cgMLST or SNP-based analyses leading to a withdrawal of the product from the market through a close collaboration with responsible food safety authorities. Since April 2019 no new cases belonging to this outbreak cluster were identified. With this high number of WGS-confirmed outbreak isolates, this cluster represents the largest European outbreak of invasive listeriosis since 25 years.