A large outbreak of multiple Salmonella serovars linked to alfalfa sprouts in Norway, October to December 2024

This study investigates a nationwide Salmonella outbreak in Norway during October-December 2024 involving four different serovars-S. Newport, S. Typhimurium, S. Kisarawe, and S. Kinondoni. The investigation aimed to assess the outbreak's scope, identify the source, and implement control measures.

Epidemiological analyses included trawling and targeted questionnaires, a matched case-control study, and grocery receipt analysis. Whole-genome sequencing (WGS) determined genetic links between Salmonella isolates from human cases, food, and environmental samples. Traceback investigations identified potential contamination sources.

A total of 230 cases (69% female, median age: 48 years) were identified, with 33% requiring hospitalization. Sprout consumption was reported by 69% of cases interviewed through trawling or targeted questionnaires. Grocery receipts were collected from some of the cases, and half of these had purchased sprouts. A matched case-control study found cases to be associated with consumption of sprouts (penalized adjusted odds ratio of 3.13). WGS established genetic links between clinical, food, and environmental isolates, identifying alfalfa sprouts as the outbreak source. Traceback investigations identified potential risk associated with seeds from an Italian supplier, previously associated with two Salmonella outbreaks in Norway in 2024 and multiple outbreaks across the European Union. The Italian supplier reported negative findings for Salmonella in their self-monitoring checks on seeds sent to Norway. Control measures included product withdrawal, seed batch quarantine, and public health advisories.

This multi-serovar outbreak highlights the public health risks associated with consumption of raw sprouts and emphasizes the need for improved detection methods and stricter regulations to prevent future outbreaks.