Interregional outbreak of Salmonella Typhimurium linked to fresh cheese: A case-case study guided by whole-genome sequencing (WGS), Portugal, March-June 2024.

In 2024, an outbreak of Salmonella typhimurium affected two regions in Portugal. To identify the vehicle, we conducted a case-case study using a 'same disease, different time period' design. We compared S. typhimurium cases linked by whole-genome sequencing (WGS) (cluster cases) with salmonellosis cases notified in 2023 (historical cases) and calculated odds ratios (OR) for food exposures in surveillance data using logistic regression. We performed WGS on 58 isolates from the outbreak period (11/03/2024-2118/06/2024), and all belonged to a single cgMLST cluster (HierCC HC5_410,410). Compared with the 552 historical cases, cluster cases more frequently reported fresh cheese consumption (OR 18; 95% CI: 8.5-38). We visited the implicated cheese production site, identified food safety non-conformities, and enforced hygiene measures. Environmental and product specimens collected at the visit tested negative for Salmonella spp. Taken together, the most plausible vehicle in this outbreak was fresh cheese. The case-case design enabled a rapid, low-cost analysis to support targeted investigation using surveillance data. Using WGS cluster cases as the case definition, rather than all S. typhimurium cases during the outbreak period, yielded a higher OR in the case-case study, increasing confidence in the findings. We recommend this combined approach as part of the toolkit for foodborne outbreak investigations in Portugal in similar contexts.