More than 20 people have been infected by a rare strain of Salmonella in Denmark, with half of them needing hospital treatment.

The Statens Serum Institut (SSI) is investigating the outbreak that has sickened 23 people since May. Eleven people needed hospital treatment but no deaths have been reported.

Patients were reported to the agency from May 31 to June 25. The 13 women and 10 men are aged eight to 87 and live across the country. Most ill people live in Hovedstaden (12), Sjælland and Nordjylland have four cases each, two come from Syddanmark and one from Midtjylland.

“It is a rare type of Salmonella that we have not previously had in Denmark. We are currently interviewing patients and collecting information from their purchases to track down which food can be the culprit,” said Luise Müller an epidemiologist at Statens Serum Institut.

Search for source of infection

Interviews with ill people showed they had not been traveling before they became ill, they do not know each other, and they have not participated in joint events. At least eight people who needed hospital treatment had been discharged at the time of the interviews.

Findings point to a common food sold throughout the country as the source of infection, according to SSI.

Laura Espenhain, an epidemiologist at Statens Serum Institut, told Food Safety News that no travel, common event or other epidemiological link has been identified.

“No clear hypothesis have come from interviewing patients. We are now collecting information from receipts from food purchases, to see if a product, which we have not asked about in the interview, appears,” she said.

“We have some weeks lag between sampling and whole genome sequencing result, so we cannot rule out that the outbreak is not still ongoing.”

Rare Salmonella strain

SSI is investigating the outbreak with the Danish Veterinary and Food Administration (Fødevarestyrelsen) and National Food Institute, Technical University of Denmark to find the source of infection.

Salmonella Coeln is rare in Denmark with only two to four registered cases per year, which are usually linked to travel abroad.

Whole genome sequencing found the strains currently involved are closely related to each other and of sequence type 1995.

Espenhain added SSI has notified the European Centre for Disease Prevention and Control (ECDC) and other countries through the Epidemic Intelligence Information System (EPIS) and there is no indication an outbreak with the same strain is ongoing in other countries.

Last year, the Austrian Agency for Health and Food Safety (AGES) noted an increase in cases of infection with Salmonella Coeln in several provinces, particularly in August and September. The rise was attributed to Salmonella Coeln cluster type 1768.

In 2018, 75 Salmonella Coeln cases were confirmed at the National Reference Center for Salmonella compared to 52 the year before and 32 in 2016.

AGES investigated the suspected foodborne outbreak with the Federal Ministry of Labour, Social Affairs, Health and Consumer Protection (BMASGK) and state authorities but was unable to identify the source.