A large proportion of Campylobacter infections are not sporadic and can be linked to outbreaks, according to a study in Denmark.

Campylobacter outbreaks are rarely reported, which may reflect limitations of surveillance testing as molecular typing is not routinely performed.

Researchers whole genome sequenced 1,509 Campylobacter jejuni isolates from 774 patients and 735 food or animal sources in Denmark from 2015 to 2017 to determine the frequency of genetic clusters among patients and to find links to isolates from poultry meat, broiler chickens, cattle, pigs, and dogs. The 774 Campylobacter jejuni isolates represented about 10 percent of cases during the period.

The work, published in Emerging Infectious Diseases, found numerous clusters as 366 of the 774 clinical isolates formed 104 clusters of more than two isolates. A total of 41 patient clusters representing 54 percent of patients matched a potential source, mostly domestic chickens or broilers.

Finding source of small clusters
Clinical isolates were supplied by four clinical microbiological laboratories: Aalborg University Hospital, Slagelse Hospital, Hvidovre University Hospital, and Odense University Hospital. These labs diagnosed about 60 percent of the Campylobacter cases in Denmark.

Clinical isolates represented 673 domestically acquired infections, 60 travel-associated infections, and 41 in people of unknown travel status. A large fraction of clinical isolates matched those from domestic broilers and chicken or imported chicken meat, confirming chicken is a major source of human infections.

Campylobacter jejuni isolates from food and animals were collected by the Danish Veterinary and Food Administration. A total of 735 isolates represented animals from Denmark including 27 pigs, 214 cattle, and 150 broilers or retail meat such as 172 domestic chicken, 111 imported chicken, nine imported turkey, four domestic duck, and 22 imported duck.

A total of 82 clusters were small with two to four clinical isolates and 22 large clusters consisted of five to 17 clinical isolates. only two were not matched to a source. In contrast, among the small clusters, less than a third matched a source isolate.

Researchers said this indicates domestic chicken meat and broilers are not dominant sources of small clusters and suggests they may arise from imported food, food with a lower contamination load, small batches of less widely distributed food, or non-food sources such as direct animal contact or environmental exposures.

WGS and epidemiology
Scientists say WGS is a valuable tool for improved surveillance and outbreak detection of Campylobacter but a limitation is detection of multi-strain outbreaks.

Researchers found two examples for which distinct genetic clusters could be linked by epidemiologic data. The first was an outbreak in which 14 isolates were sequenced as part of an investigation. These isolates split into two distinct clusters of eight and six clinical isolates, respectively. The multi-strain outbreak involved 100 schoolchildren served unpasteurized milk at a dairy farm.

In the second, a sample from a patient was positive for two strains belonging to distinct clusters. The two clusters were observed for the same period of 2016, and each matched isolates from domestic chicken meat and broilers within a few weeks and contained clinical isolates from the four regions. The two isolates from broilers representing each of the clusters were from the same slaughterhouse; the broilers originated from two geographically close farms.

A total of 75 clusters contained isolates of both clinical and food or animal origin. Clinical isolates most often matched isolates from domestic chicken meat, broilers, or both, corresponding to 25 percent of all clinical isolates in the study.

The chicken meat and broiler isolates in the same cluster were often linked to the same slaughterhouse, and some came from the same farm. However, it was also common for cluster isolates to originate from different farms.

Clusters often originated with the detection of a food or animal isolate, followed by the presence of several clinical isolates over a few months. In some instances, the cluster type disappeared for months and later reappeared in new patients.

The work has led to continuous surveillance based on sampling of 10 percent of the patients being started in Denmark to detect large outbreaks and link them to food or animal sources to reduce the incidence of Campylobacter infections.