An outbreak of Yersinia in Denmark and Sweden with more than 50 cases has been linked to fresh spinach.

Statens Serum Institut, a public health research institute in Denmark, reports 20 people have been infected in the country. One person needed hospital treatment. The Public Health Agency of Sweden (Folkhälsomyndigheten) has recorded 37 confirmed cases.

In March, 20 cases of Yersinia enterocolitica were found in Denmark. There were 11 women and nine men aged 2 to 74 years old, with most cases aged 20 to 30. Patients were distributed throughout Denmark in Hovedstaden, Sjaelland, Syddanmark, Midtjylland, and Nordjylland.

The link to spinach was based on a case control study and the traceback investigation, which indicated spinach from Italy was responsible. In March, most fresh spinach in Denmark comes from Spain or Italy. No specific batch of product was found to be the source of the outbreak and no product testing was conducted.

After interviews with patients, Statens Serum Institut did a study in which healthy people of the same gender and age, and who lived in the same municipality as those sick, were asked if they had eaten certain foods that many of the Yersinia patients ate. The study showed patients had consumed fresh spinach to a far greater extent than the control people.

The investigation found spinach was bought in Netto and a supermarket chain in Sweden. Danish officials said the implicated product is no longer on the market because the country had not seen any cases since March and given duration of the outbreak it was likely only one batch that was contaminated.

The cause of the outbreak was Yersinia enterocolitica, serotype O3, biotype 4. Whole genome sequencing found all patients were infected with the same bacterial strain.

“Although the outbreak is over, we can use this knowledge to prevent it from happening again. It is also a good reason to remind consumers that leafy greens always must be washed thoroughly before eating,” said Luise Müller, an epidemiologist from Statens Serum Institut.

Denmark sees about 400 Yersinia enterocolitica cases a year, with 366 having been reported in 2018.

In Sweden, the increase in Yersinia infections started in March and the 37 cases were from across the country. Seven men and 20 women with an age range from 6 to 62 years fell ill.

Swedish officials said they were not able to analyse food samples since no case had spinach left at home and their case-control study did not identify a specific food item.

Infection with Yersinia is relatively rare in Sweden, with between 200 to 300 cases reported annually.

Previous outbreaks have been caused by raw or undercooked meat consumption and contaminated ready-to-eat vegetables. After an incubation period of three to seven days, symptoms includes fever, diarrhea and abdominal pain in the right lower part of the abdomen.

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