Denmark recorded its highest number of Campylobacter infections ever in 2019, according to a report by the Statens Serum Institut.

Campylobacter remains the bacterium that most frequently causes gastrointestinal infection in the Danish population.

The Statens Serum Institut (SSI) report shows the number of patients is increasing. This past year, 5,389 patients were diagnosed in Denmark and in 2018, 4,547 cases were recorded.

There was a high incidence among young adults in their 20s in 2018 and 2019 and the rate among elderly people older than  85 and particularly in men was higher than previously observed. Among young children of 0 to 4 years, the incidence was lower than previously seen.

Infections abroad
Of all cases in 2018, 1,514 (33 percent) had been acquired abroad. This figure was 1,928 (36 percent) in 2019. The share of people infected abroad exceeded previous numbers as it was 47 percent in 2018 and 56 percent in 2019 based on figures from the national microbiology database. Data from previous years, which were based on interviews and a sample of patients, indicated a third of all patients become infected abroad.

In 2019, Turkey accounted for the most registered infections with 192, followed by Spain at 191, Thailand with 90, Indonesia at 85, India with 60, France at 50 and Morocco with 49.

For the first time Campylobacter isolates were collected from a sample of Danish patients and routinely analyzed by whole genome sequencing (WGS) in 2019. This revealed that various small and one large outbreak occurred. These outbreaks were mainly due to chicken meat produced in Denmark.

SSI started monitoring Campylobacter infections based on WGS of Campylobacter jejuni and Campylobacter coli isolates. With three departments of clinical microbiology (DCMs), all isolates from culture-positive patients in 2019 were submitted from DCM Aalborg along with a monthly sample from DCM Odense and DCM Slagelse. SSI received a small number of isolates from other DCMs.

Insight into outbreaks
“Previously, it was assumed that Campylobacter only very rarely causes disease outbreaks. But our results demonstrate that many Campylobacter outbreaks occur in the Danish population — and that these outbreaks are predominantly caused by foods,” said Eva Møller Nielsen, who leads SSI’s work on genome sequencing of Campylobacter.

Introduction of PCR diagnostics on feces material affects the number of positive cases recorded, according to SSI but this cannot explain all the increase.

More than 90 percent of the 668 submitted isolates were Campylobacter jejuni and the rest were Campylobacter coli. Results from WGS analysis showed half of them formed part of a cluster. Isolates formed a large number of small clusters with two to four patients and 14 larger clusters of more than five patients.

A total of 88 people had a Campylobacter jejuni infection with the same clone belonging to sequence type ST122. Patients with this type were recorded throughout the year with more in February to August. The ST122 clone made up 13 percent of all WGS-typed Campylobacter isolates in 2019. This type was also found in chicken meat from a slaughterhouse belonging to HKScan in Vinderup.

At the same time, the Danish Veterinary and Food Administration (Foedevarestyrelsen) performed WGS on Campylobacter isolates from chicken meat.

With this agency and Denmark’s Technical University, SSI compared Campylobacter genome sequences from patients and chicken meat. Approximately one third of patients had a Campylobacter isolate that was identical to bacteria detected in chicken.

The outbreak with the ST122-clone matched isolates from chicken meat produced at a single abattoir and was traced to one producer but more firms may have been affected.

Monitoring based on WGS is continued in 2020 with a similar sample of Campylobacter from patients diagnosed at four of the country’s DCMs and continuous analyses of samples from chicken meat and other relevant food sources.

The objective is to detect larger nationwide outbreaks, enhance understanding of the contribution of different food sources to the number of infections and to help efforts to reduce foodborne Campylobacter infections.

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