Danish officials are investigating an outbreak of Yersinia after 18 people fell ill in March.

Statens Serum Institut, a public health research institute, reported the source of infection is still unknown. Cases in the outbreak in Denmark have been linked to more than 60 infections in Sweden.

The 18 confirmed Yersinia enterocolitica cases live across the country. Ten of those ill are women and eight are men aged from two to 74 years old. Most patients are between 20 and 30 years old. One person needed hospital treatment. Cases have been recorded in Hovedstaden, Sjaelland, Syddanmark, Midtjylland, and Nordjylland.

The link between Danish and Swedish cases

The cause of the outbreak is Yersinia enterocolitica, serotype O3, biotype 4. Whole genome sequencing has shown that all patients are infected with the same bacterial strain.

Denmark sees around 400 Yersinia enterocolitica cases a year with 366 infections reported last year.

An outbreak is also ongoing in Sweden, so Danish officials are cooperating with Swedish authorities to find the source of infection which is suspected to be a food.

Luise Müller, an epidemiologist at Statens Serum Institut (SSI), told Food Safety News that illness onset dates range from March 3 to 19.

“The national laboratory at SSI noticed an increase in Yersinia serotype 3 biotype 4 and at the same time Sweden notified us about the Swedish outbreak. Subtyping with whole-genome-sequencing was then performed confirming the Danish outbreak and linking it to the Swedish cases,” she said.

“We are currently interviewing patients and if relevant we will make a case-control study. We are also working together with Folkhälsomyndigheten to compile all patient data and find common food items consumed by cases both in Sweden and Denmark. It is too early to say if the outbreak is still ongoing since we have some delay in the typing of samples due to the Easter holiday.”

In Denmark, SSI, DTU Food Institute and the Danish Veterinary and Food Administration (Fødevarestyrelsen) are involved in the investigation.

Snack carrots ruled out as a source

As part of the Swedish outbreak investigation, snack carrots produced in Denmark were identified as a possible source of infection as Latvian tests detected Yersinia in them. However, Müller said this link has now been ruled out.

“Latvia notified via the Rapid alert System for Food and Feed a finding of Yersinia in carrots produced in Denmark. Further typing in Latvia this week, however, revealed that this was a non-pathogenic Yersinia different from the outbreak strain,” she said.

The Public Health Agency of Sweden (Folkhälsomyndigheten) previously told Food Safety News there had been an increase in cases of Yersinia since the start of March. A total of 37 cases who live across the country are confirmed and another 26 are being investigated.

Authorities in Denmark and Sweden launched an urgent inquiry on April 26 on the European Centre for Disease Prevention and Control (ECDC) platform sharing the biotype, serotype and sequence data. So far, no other countries have reported any cases related to the outbreak.

Raw or undercooked meat consumption is often the cause of infection. Outbreaks have also been caused by contaminated ready-to-eat vegetables. After an incubation period of three to seven days, symptoms include fever, diarrhea and abdominal pain in the right lower part of the abdomen.