A One Health status on surveillance, outbreak investigation and action plans for Campylobacter in Denmark

In Denmark, between 3,740 and 5,389 human Campylobacter cases have been registered yearly in the past five years (See Table A1). In 2022, the number of registered Campylobacter cases was 5,142, 28% of which were travel-related. Since 2019, a subset of human samples (around 10-15%) have been sent routinely to Statens Serum Institut for subtyping by whole genome sequencing in order to detect clusters and outbreaks. Figure 5.1 shows the number of domestic cases from 2018 to 2022, the proportion of cases in clusters, and clusters with a food match.
In 2022, the threshold for investigation of clusters was changed to examine genetic clusters of human cases down to five cases. This was done to gain further knowledge of their aetiology. Since this surveillance setup only captures the tip of the iceberg, it is estimated that whenever a cluster of five cases is detected, this covers an outbreak of a least 40-50 cases.
From 2018 to 2022, 62 genetic clusters were detected with five C. jejuni cases or more constituting 32% (849 / 2,683) of the sequenced human isolates. Twenty-six clusters comprised ten or more cases constituting 23% (618 / 2,683) of the sequenced human isolates. The largest cluster contained 124 cases (spanning 2018-2022) and was described in Anonymous 2020 [1].
In 2022, nineteen genetic clusters with five or more cases were detected (Figure 5.2) comprising 26% (192 / 737) of the sequenced cases. For nine of these clusters a food match was detected within the same or the previous year. Food matches were linked to Danish produced chicken meat, except from one that was linked to imported chicken meat. In total, 25% (184 / 737) of the sequenced human isolates in 2022, was matched to a source.
Since chicken meat is consumed by the majority of the Danish population, and it is hard to remember brand or type of meat, the interviews of cases in the genetic clusters do not add much to the aetiology. The exception is point source outbreaks with a specific source e.g. the suspected milk outbreak in Bornholm in 2021 [2].