At the end of September 2022, the Public Health Agency of Sweden (PHAS) observed an increase of domestic Salmonella cases reported by treating physicians and clinical microbiological laboratories through the Swedish national electronic notification system for notifiable diseases (SmiNet). Data obtained from regional clinical microbiological laboratories performing subtyping of Salmonella isolates on serogroup level suggested that Salmonella serogroup B was causing the national increase of cases.
On 4 October 2022, PHAS identified a cluster of eight cases of S. Typhimurium belonging to the 7-gene multilocus sequence type (ST) 19 based on whole genome sequencing (WGS), suggesting a common source. The cluster was identified as part of the PHAS microbial surveillance programme, which is based on voluntary submission of metadata and isolates of Salmonella from domestic cases, by the regional clinical microbiological laboratories. At PHAS, typing is performed using WGS and conventional slide agglutination can be done if needed.
On 7 October 2022, a national outbreak was declared, following the laboratory confirmation of another 11 cases belonging to the cluster distributed in 11 of 21 Swedish regions. The outbreak control team was coordinated by PHAS and included the Swedish Food Agency (SFA) and regional departments of communicable disease control and prevention (CDC).
We describe an outbreak investigation of S. Typhimurium in Sweden linked to cases in Finland, in September–November 2022. We collaborated with Finnish authorities and aimed to assess the extent of the outbreak, identify its source and initiate appropriate control measures to prevent further cases.
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