Researchers have detailed the largest Salmonella outbreak in Norway since the 1980s and highlighted the need for improved detection methods.
The outbreak from October to December 2024 involved four serovars: Salmonella Newport, Typhimurium, Kisarawe, and Kinondoni and was traced to alfalfa sprouts. The sprouts were produced by a Norwegian facility using seeds from an Italian supplier.
A total of 230 patients were identified, with 76 people requiring hospitalization. Symptom onset dates ranged from Oct. 11 to Dec. 18, 2024. The median age of cases was 48 but ranged from 10 months to 90 years old, and 159 were female.
Salmonella Typhimurium was most common with 120 infections followed by Salmonella Newport with 105 cases. Salmonella Kisarawe and Salmonella Kinondoni were rare, with three and two cases, respectively.
Detection challenges
The study, published in the journal Infection, reported that sprout consumption was mentioned by 69 percent of patients interviewed through trawling or targeted questionnaires. Grocery receipts were collected from 17 patients, and half of these had purchased sprouts. A matched case–control study found cases to be associated with consumption of sprouts.
Whole Genome Sequencing established genetic links between clinical, food and environmental isolates, identifying alfalfa sprouts as the outbreak source. Traceback investigations pointed to 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.
The Norwegian producer also conducted routine sampling and microbiological testing on the two received seed batches but did not find Salmonella.
Scientists said these results may reflect testing limitations rather than an actual absence of contamination.
“The challenge of obtaining representative samples from an entire seed lot — sometimes weighing several tons — makes it difficult to detect contamination, especially as it may be unevenly distributed,” they added.
Earlier outbreak and Swedish report
Sealed packages of sprouts tested positive for Salmonella Kotte, Salmonella Newport, and Salmonella Kisarawe. Open packs were positive for Salmonella Kinondoni, Salmonella Kisarawe, and Salmonella Newport. One environmental sample was positive for Salmonella Kisarawe.
“The diversity of serovars suggests that the contamination of the alfalfa sprouts was not due to a single contamination event but likely resulted from multiple contamination points or sources along the production and supply chain,” said scientists.
In late November 2024, products containing the suspected alfalfa sprouts were withdrawn from the market, and the remaining seeds were placed under quarantine. Decontamination of equipment and at the production facility was conducted and sampling was intensified.
Sweden had four cases of Salmonella Kinondoni clustering with Norwegian cases, however, their investigation found no epidemiological evidence of sprout consumption. Sweden also reported an outbreak of Salmonella Typhimurium involving 100 patients linked to alfalfa sprouts. Traceback work in both countries identified the Italian supplier as the common source. This company sourced seeds from three growers in the same region of Italy.
Earlier in 2024, two Salmonella outbreaks in Norway were linked to sprout consumption. The first batch from the Italian supplier, used between February and September 2024, was suspected to be associated with these events — one from May to August with Salmonella Typhimurium and another from August to October with Salmonella Hvittingfoss.
“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 outbreak,” said researchers.
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