A Yersinia outbreak in Norway has been solved and declared over just days after it was announced.

Folkehelseinstituttet (the Norwegian Institute of Public Health) said the national outbreak of Yersinia enterocolitica O3 was likely caused by a pre-mixed salad product.

Less than a week earlier, the agency had announced an investigation into a recent rise in Yersinia infections.

The outbreak started in mid-November and involved 10 confirmed patients. All of them are women aged 11 to 59 years old who became ill in mid to late November with the last case on Nov. 26. Three people live in Viken, two each in Oslo, Innlandet, and Vestland, and one in Telemark and Vestfold.

Detailed analysis ongoing
An investigation included local officials, the Veterinary Institute, and Mattilsynet (the Norwegian Food Safety Authority).

Analysis of patient interviews by Folkehelseinstituttet found that everyone said they had eaten a fresh-cut salad product in the week before illness. Investigations are ongoing to try and find exactly which raw material in the salad was contaminated.

Lettuce has a short shelf life and the implicated salad products are probably no longer on the market, according to authorities.

Every year, between 40 and 80 cases of yersiniosis are reported to Folkehelseinstituttet with 60 percent to 80 percent of these infected domestically.

In the spring of 2018 and 2020, outbreaks of Yersinia enterocolitica were investigated and linked to salad products. In 2018, at least 18 people across the country fell ill. The outbreak this year involved 23 people in May with an imported salad with spinach suspected as the source.

Yersiniosis is an infection caused by the bacterium Yersinia enterocolitica. The most common symptoms in children are diarrhea, fever, and abdominal pain. In older children and adults, right-sided abdominal pain and fever could be the main symptoms. They typically develop four to seven days after exposure and lasts one to three weeks.