Scientists have shared details about a major Salmonella outbreak investigation in Finland, including cucumbers and iceberg lettuce served at educational establishments.

Over 700 people fell ill during the 2021 outbreak from Salmonella Typhimurium associated with a daycare lunch. Similar Salmonella Typhimurium was found in patients and in a vegetable mix containing iceberg lettuce, cucumber, and peas served for lunch.

It was the largest Salmonella Typhimurium outbreak in Europe since 2004 and the biggest Salmonella outbreak ever reported in Finland, according to the Zoonoses and Public Health study.

There was a sudden increase in gastroenteritis patients in central Finland in June 2021. The outbreak was reported to the Finnish Institute for Health and Welfare (THL). 

The majority of patients had fever, nausea, abdominal pain and diarrhea, with many of them having bloody stools. Between June 19 and July 5, 239 patients with gastroenteritis sought treatment. Their median age was 4 , ranging from 1 to 61 years old. 

Role of the central kitchen
Healthcare services encountered many challenges during the outbreak. This included patients with gastrointestinal symptoms in an area not designed for infectious disease patients, especially not children. Cleaning facilities were limited, and infection control measures due to the COVID-19 pandemic slowed the process. Also, the holiday season resulted in a medical and nursing staff shortage.

The central kitchen delivered food to 61 daycare centers. based on the interviews of sick people and daycare managers, patients attended 59 daycare centers that received food from the same central kitchen. Investigations suggested that lunch or snacks served on June 18 were suspected meals.

620 children and 108 staff reported gastroenteritis to the daycare managers. All 15 staff members at the central kitchen were asymptomatic and gave Salmonella-negative stool samples.

THL found a similar Salmonella Typhimurium strain in 35 patients from Central Finland and in three patients’ samples from other parts of the country between July and November 2021. Since Salmonella Typhimurium strains similar to those in outbreak cases were also found elsewher in Finland, the source was likely in wider distribution, said scientists.

Salmonella was detected in the vegetable mix. Patient isolates and three isolates from the vegetable mix were closely related.

Lettuce quality complaints
An inspection of the central kitchen found no abnormalities. The cucumber was of domestic origin, while iceberg lettuce was imported from another European country.

Traceback’s work revealed that the information on the batch of vegetables from the wholesaler was incomplete. The wholesaler had received quality complaints about the iceberg lettuce from the central kitchen. The manufacturer did not test the suspected batch for Salmonella since the production plant had given a certificate declaring it was negative.

Pre-chopped iceberg lettuce use was suspended in the central kitchen. The kitchen staff was advised to freeze vegetable samples separately in the future. No measures were carried out against the wholesaler and manufacturer of iceberg lettuce as the product was no longer on the market or in stock.

Because of quality complaints, the main suspect ingredient was one batch of iceberg lettuce. However, cucumbers could not be excluded. A small batch of cucumbers might have been contaminated and spread contamination to other cucumbers during the washing procedure. 

“We recommend that to enable thorough microbiological investigation. Institutional kitchens store the food samples separately for internal quality control. Food items should always be tested when Salmonella contamination is suspected in an outbreak. Improving traceability throughout the supply chain is important to enable traceback investigations of fresh vegetables,” said researchers.