Discerning Environmental Pathways of Campylobacter Transmission

Objectives: To determine the contributions of different animal and environmental sources of human campylobacteriosis and the associated source-specific risk factors. Methods: 1,417 Campylobacter jejuni/coli isolates collected in the Netherlands in 2017-2019 were whole-genome sequenced, including isolates from human cases (n=280), broilers/turkeys (n=238), laying hens (n=56), cattle (n=158), veal calves (n=49), sheep/goats (n=111), pigs (n=110), dogs/cats (n=100), wild birds (n=62), and surface water (n=253). Case exposure data were collected using questionnaires. Source attribution was performed using STRUCTURE based on core-genome multilocus sequence typing (cgMLST). Source-specific risk factors were identified using regression analysis with the attributions as outcome. Results: Human cases were mostly attributed to broilers/turkeys (54.5%), surface water (17.2%), and cattle (12.3%). When pets were included as potential source, 19.5% of cases were attributed to pets. Of the several associations identified, never consuming chicken meat, as well as frequent chicken meat consumption, were risk factors for infections attributable to broilers/turkeys. Consuming unpasteurized milk increased the risk for infections attributable to cattle. Risk factors for infections attributable to surface water/wild birds were open water swimming, fishing, contact with dog faeces, and game meat consumption. Conclusions: Poultry and cattle were the main livestock sources of human campylobacteriosis, while surface water and pets were important non-livestock sources. Livestock sources were only partially consistent with foodborne transmission, as significant effects of frequency and alternative pathways of exposure were involved.