Yersinia outbreak in Norway is under investigation after the Norwegian Institute of Public Health (Folkehelseinstituttet, FHI) confirmed nine cases of gastrointestinal infection and identified six further suspected cases. The source of infection remains unknown, and health authorities say more people may still be affected.
FHI has opened an outbreak investigation with the Norwegian Food Safety Authority (Mattilsynet), the Norwegian Veterinary Institute (Veterinærinstituttet) and municipal medical officers in the affected areas. The aim is to identify the source, assess the scale of the outbreak and prevent further transmission.
Yersinia cases spread across several Norwegian counties
The 15 people included in the outbreak were tested between 26 May and 15 June. They are aged between 10 and 75, with a median age of 29. Eleven are women.
The infected people live in Trøndelag, Vestfold, Telemark, Østfold, Innlandet, Rogaland and Vestland. According to FHI, the confirmed cases involve Yersinia enterocolitica O3, a bacterial strain that can cause gastrointestinal illness.
Laboratory analysis shows that the confirmed cases share the same genotype. This suggests that the people may have been infected from a common source, although authorities have not yet identified it.
“The source of infection is currently unknown, and we therefore cannot rule out that more people may become infected in the outbreak,” said senior medical officer Silje Bruland Lavoll at FHI.
Health authorities are tracing food and exposure histories
FHI said the people included in the outbreak are being interviewed about what they ate, drank and were exposed to before becoming ill.
The reference laboratory at FHI is also analysing bacterial samples on an ongoing basis to determine whether new cases belong to the same outbreak.
Such investigations can be complex. Authorities often need to compare patient interviews, laboratory results and food distribution patterns. FHI also stressed that it is not always possible to identify the exact source of infection.
What Yersinia infection is and how it spreads
Yersinia bacteria can spread between animals and humans. Pigs are considered an important reservoir, but people are usually infected through contaminated food or water. Transmission can also occur through contact with infected animals or, in some cases, from person to person when hand hygiene is poor.
Common symptoms include diarrhoea, abdominal pain and fever. In some cases, abdominal pain can be severe and resemble appendicitis. The infection usually lasts one to two weeks, and symptoms generally appear three to seven days after exposure.
Most people recover without treatment. However, FHI advises people with long-lasting or severe symptoms, including bloody diarrhoea, high fever or signs of dehydration, to contact a doctor. This is particularly important for young children, older people, pregnant women and people with weakened immune systems.
Norway has seen previous food-linked Yersinia outbreaks
Norway has recorded several larger yersiniosis outbreaks in recent years. Previous investigations have sometimes pointed to short-shelf-life food products, including salad products, while other outbreaks have been linked to pork products.
In 2022, a national outbreak of Yersinia enterocolitica O3 involved nine confirmed cases across six counties. FHI later said interviews indicated that the probable source was a type of salad or an ingredient in a salad mix, although no specific product was identified.
Another outbreak in 2020 involved 25 cases and was suspected to be linked to pre-washed salad products containing baby spinach or spinach. In 2022, a separate outbreak connected to an internat school in Agder and other cases was linked to pork products, including pork neck and traditional pork-based food.
These previous cases do not identify the source of the current outbreak. They do, however, show why Norwegian authorities are focusing on food histories, laboratory comparisons and possible distribution chains.
Food hygiene advice remains central
FHI recommends good hand hygiene, especially before cooking and eating, after using the toilet and after contact with animals.
People should keep raw food separate from ready-to-eat food, wash knives and cutting boards after contact with raw meat, and ensure that minced meat, pork and poultry are cooked thoroughly. Fruit, berries, salad and vegetables that are eaten raw should be rinsed well.
People with diarrhoea should avoid preparing food for others.
The investigation is expected to continue as FHI analyses new samples and interviews those affected. The next update from Norwegian authorities is scheduled for Tuesday, 30 June.
