Multi-cluster outbreak of Salmonella Typhimurium sequence type 36 linked to alfalfa sprouts, Sweden, August-November 2024.

Abstract

Salmonella Typhimurium sequence type (ST) 36 is a rare sequence type in Sweden. During August-November 2024, 100 cases of Salmonella Typhimurium ST36 were reported nationwide. The highest proportions of cases were observed among individuals aged 0-10 years (17%) and 41-50 years (20%), with females representing 65% of the total cases. Microbiological analysis of the S. Typhimurium ST36 isolates identified nine clusters and five individual strains clustering within 53 single-nucleotide polymorphisms. A matched case-control study found cases to be associated with the consumption of alfalfa sprouts (adjusted odds ratio = 8.94, 95% CI: 2.96-27.1). Trace-back investigation identified seeds used by two alfalfa sprout producers in Sweden as the likely source of the outbreak, although microbiological analysis of sprouted alfalfa seeds from the producers did not detect Salmonella. However, continued international investigations further implicated seeds sourced from the same origin, supporting that alfalfa seeds were the ultimate source of the Swedish outbreak. Our investigation and findings indicate that alfalfa sprouts may contain Salmonella and thus pose a health risk to consumers. We emphasize the need for primary producers of alfalfa seeds and sprouts to identify and prevent possible contamination points.