An outbreak of Yersinia in Sweden has ended with iceberg lettuce as the suspected source of infection.

From January until the beginning of February this year, twice as many people fell ill with Yersinia infection as did during the same period in a normal year.

From 53 cases of Yersinia enterocolitica, 33 lived in Stockholm, Västra Götaland and Halland. Isolates from 24 of these cases were typed by whole genome sequencing and 16 outbreak patients with clustered isolates of sequence type 18 could be identified.

Of these 16, eleven were women and five were men aged 7 to 34 years old. All fell ill between Jan. 4 and 18.

Managing resources
Rikard Dryselius, a microbiologist at Folkhälsomyndigheten (Public Health Agency of Sweden), said collection and typing of Yersinia isolates is not done routinely in the country.

“Instead, individual counties can ask for help with typing Yersinia when they see a need. Alternatively, we may ask the laboratories to send in isolates if we see an increase in cases nationally that should be investigated,” he said.

“Due to the strained situation for infection control units and clinical laboratories during the pandemic, we chose to request interviews/questionnaire responses and collection of isolates only from the counties wher the increase was highest. It is therefore very likely that there were more than 16 outbreak cases but the lack of additional isolates to sequence means that we cannot confirm this.”

Folkhälsomyndigheten, infection control units in the Stockholm, Västra Götaland and Halland regions and Livsmedelsverket (Swedish Food Agency) investigated the incident.

Health officials interviewed sick people and compiled questionnaire responses about what and wher they ate before illness onset.

A case-case study wher questionnaire responses from outbreak cases were compared with answers from people with Yersinia infection that did not belong to the outbreak was done.

Restaurant and iceberg lettuce link
The investigators found the outbreak cases had, to a greater extent, visited a certain restaurant chain. Follow-up interviews about what they had eaten led to iceberg lettuce being suspected as the source of infection.

According to the trawling questionnaire sent to outbreak cases, seven out of 12 mentioned they had visited the restaurant chain before falling ill while seven out of 72 cases with yersiniosis who responded to the same questionnaire in 2019 mentioned that they had done so.

The suspicion of a contaminated batch of iceberg lettuce distributed to a restaurant chain was strengthened by the outbreak timeline, which suggested that a product with a wide geographical spread and a limited shelf life was the underlying cause.

Sweden has no significant production of iceberg lettuce at this time of year and attempts are being made to trace the country of origin.

Meanwhile, investigations are ongoing to find the source of a Salmonella Enteritidis outbreak in the country. It is not related to the Salmonella outbreak in Norway linked to meat from Germany.

The first person fell ill at the end of December 2020. There are 25 confirmed patients across 14 regions in southern and central Sweden.

Almost three quarters of patients are children younger than 10 years of age and four are older than 70.