Background: Campylobacter spp. infections are the leading cause of foodborne gastroenteritis in high-income countries, including Australia. Campylobacter colonises a variety of mammalian and avian hosts that are reservoirs for human campylobacteriosis. Though most Australian outbreak investigations implicate chicken meat, the proportions of sporadic cases attributable to different animal reservoirs are unknown.
Methods: Campylobacter isolates were obtained from notified human cases, and raw meat and offal from the major livestock in Australia: chickens, pigs, and ruminants (cattle and sheep) between 2017 and 2019. Isolates were speciated, with sequence types determined using multi-locus sequence genotyping. We used Bayesian source attribution models to estimate the proportion of human cases attributable to each livestock source by comparing the frequency of sequence types in cases and each animal source. We employed a model comparison approach with ten base models and explored adjusting these for age, gender, jurisdiction, rurality, and season. Four of the ten base models included an ‘unsampled’ source to estimate the proportion of cases attributable to wild, feral, or domestic animal reservoirs not sampled in our study.
Results: We included 612 food and 710 human case isolates. The best fitting models attributed >80% of Campylobacter cases to chickens, with a greater proportion of Campylobacter coli (>84%) than Campylobacter jejuni (>77%). The best fitting model that included an unsampled source attributed 14% (95% CrI: 0.3-32%) to the unsampled source and only 2% to ruminants (95% CrI: 0.3-12%) and 2% to pigs (95% CrI: 0.2-11%.) The best fitting model that did not include an unsampled source attributed 12% to ruminants (95%CrI: 1.3-33%) and 6% to pigs (95%CrI: 1.1-19%.) Model fit was not improved by inclusion of case covariates.
Conclusions: Chickens were the leading source of Campylobacter infections in our data and should remain the focus of interventions to reduce the burden in Australia.