Outbreak of Salmonella Typhimurium linked to Swedish rocket salad, Sweden, September-November 2022

In September 2022, the Public Health Agency of Sweden detected an increase in the incidence of Salmonella Typhimurium cases through the Swedish electronic notification system, SmiNet, and an outbreak strain was identified through whole genome sequencing. 109 cases with symptom onset between 17 September and 24 November, 2022 were reported from 20 out of 21 Swedish regions. The median age of cases was 52 years (range 4 - 87 years) and 62 % were female. A case–control study identified rocket salad (adjusted odds ratio (aOR): 4.9, 95% confidence interval (CI): 2.4-10, p-value <0.001) and mixed salad bags (aOR: 4.0 95% CI: 1.9-8.1, p-value <0.001) as source of the outbreak. Trace-back, supported by investigations of an outbreak cluster with the same WGS-strain in Finland, identified rocket salad cultivated, washed and packed in Sweden as the likely source of the outbreak. No food samples were available for microbiological analysis. Our investigation emphasises the need for primary producers of leafy greens to identify possible sources of Salmonella and other pathogens to prevent contamination of the products. However, bagged rocket salad and other leafy greens can contain pathogens and pose a health risk regardless of pre-washing procedures.