Officials in Denmark are investigating an outbreak of E. coli that has sickened 10 people.

Statens Serum Institut (SSI) reports the E. coli O157:H7 outbreak began in May and the source of infection is not yet known.

Eight people had bloody diarrhea, which is a typical symptom of infection with this type of Shiga toxin-producing E. coli (STEC). Six have been hospitalized but no cases have developed hemolytic uremic syndrome (HUS), a type of kidney failure associated with E. coli infection. The people that needed hospital treatment have been discharged and no deaths have been reported.

Those sickened live across Denmark, they have not been traveling or shared common events in the time before becoming ill, primarily during the first two weeks of June. Four people are ill in Midtjylland, three in Hovedstaden, two in Syddanmark and one in Nordjylland.

Patient demographics provide clues 

Patients are six men and four women with a median age of 29. Half of them are between 16 and 37 years old, with a complete range of 8 to 63 years old. Investigators say E. coli O157:H7 often infects younger people, often less than five years old

Laura Espenhain, an epidemiologist at Statens Serum Institut, told Food Safety News this could be a possible clue in finding the source.

“Potentially, and we are taking the age profile into account when hypothesizing what the source could be. For now, unfortunately, we do not have a clear hypothesis,” she said.

“The last patients became ill in the first weeks of June. We cannot rule out that the outbreak is still ongoing since there is some weeks lag between when a patient provides a sample and until we have the sequencing result.

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

SSI is investigating the outbreak with the Danish Veterinary and Food Administration (Fødevarestyrelsen) and National Food Institute, Technical University of Denmark. The agencies are also investigating 23 people that have been infected by a rare strain of Salmonella with almost half of them needing hospital treatment.

Past outbreaks and STEC in Denmark

To find what may have made the E. coli patients sick, interviews are underway with them or their relatives to obtain information about relevant foods, animal contact and other exposures.

The outbreak strain has the serotype O157:H7 and encodes the Shiga toxin (Stx) 1a and Stx2a genes. Stx2a is more often associated with severe disease and HUS. Thanks to whole genome sequencing, it has been found that isolates of the 10 cases are closely related and of the sequence type 11.

Since 2014 Denmark has seen about 40 STEC O157 infections each year, with a range from 33 to 51.

The last outbreak with STEC O157:H7 in Denmark was in 2012 when 13 cases were reported over a six-week period. In that outbreak, patients were younger, with a median age of 14 and range of 3 to 68 years old than in the new outbreak, and eight of the 13 developed HUS. The 2012 outbreak investigation pointed to ground, or minced, beef, probably from the same batch, as the cause of infection.

The number of people in Denmark found to have become infected by STEC nearly doubled from 281 in 2014 to 495 in 2018 and the largest increase was seen from 2017 to 2018. STEC isolates must be submitted to SSI wher they have been whole-genome sequenced since 2014.

“PCR diagnostics and more potential patients being examined for STEC explain some or maybe all of the increase, but we nevertheless have initiated several activities to better understand the transmission routes, including scientific studies, among those a large case-control study of risk factors for sporadic infections,” said Espenhain.