Swedish officials are investigating the source of infection as part of a national outbreak of Yersinia enterocolitica.

The Public Health Agency (Folkhälsomyndigheten) reported there had been an increase in the number of cases of Yersinia in the country since the start of March. Since the outbreak strain of Yersinia enterocolitica type O3 had been found in different parts of the country, it is likely a foodborne infection, added the agency.

A total of 26 cases have been confirmed and another 30 infections are being investigated. onset dates for confirmed cases range between Feb. 10 to April 3. There have been no deaths reported and information on hospitalization is not collected at national level. Confirmed cases have a median age of 28 years (range six to 62 years old), 60 percent are women and they live in different areas from the county of Halland in the south to Västerbotten in the north.

Infection with Yersinia is relatively rare in Sweden with between 200 to 300 cases reported annually. In 2004 more than 800 were recorded.

A spokeswoman for Folkhälsomyndigheten told Food Safety News that cases have been confirmed as belonging to the outbreak thanks to the use of whole genomic sequencing (WGS).

“When the number of cases increased a request was sent out to the local clinical microbiological laboratories to send in Yersinia enterocolitica isolates to the Public Health Agency of Sweden for typing using WGS. WGS could clearly either confirm or exclude cases,” she said.

“Patients have been interviewed and most cases have also completed a trawling questionnaire. A case control study is ongoing and we are also collecting receipts from grocery stores with the hope of finding a common denominator.

“It is too early to say (if the outbreak is over or not) since we still await WGS results from possible cases with more recent onset of disease. The incubation period is three to seven days, generally under 10 days.”

One link being investigated is from sampling during an official food control in Latvia in mid-March, which found Yersinia in snack carrots also sold in Sweden. The batch has already been sold out and is presumed to be no longer available to buy. The raw carrots came from Sweden and were packed in Denmark.

The Folkhälsomyndigheten spokeswoman said the only link to the snack carrots is the timing and it has no information as yet that several cases ate this type of carrot. The agency is continuing to investigate the source of infection with affected municipalities and the National Food Agency (Livsmedelsverket).

Raw or undercooked meat consumption is often the cause of infection. Direct transmission from other animals such as pets or through contaminated food or drink is also possible. After an incubation period of three to seven days, symptoms includes fever, diarrhea and abdominal pain in the right lower part of the abdomen.