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 WGS (cluster cases) with salmonellosis cases notified in 2023 (historical cases) and calculated odds ratios for food exposures in surveillance data using logistic regression. We performed WGS on 58 isolates from the outbreak period (11/03/2024-18/06/2024), and all belonged to a single cgMLST cluster (HierCC HC5₄10410). Compared with the 552 historical cases, cluster cases more frequently reported fresh cheese consumption (OR 18;. 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 using surveillance data. Using WGS cluster cases as the case definition, rather than all S. Typhimurium cases, 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 and similar contexts.
Schreeb et al. (2026) studied this question.