Clonal, regionally clustered spread-rather than vertical transmission-explains surges in fluoroquinolone‑resistant Campylobacter jejuni in Finnish broilers.

BACKGROUND

Campylobacter jejuni is a major zoonotic foodborne pathogen, with poultry recognized as an important reservoir for human infection. Although antimicrobial use in Finnish broiler production is minimal, sporadic increases in fluoroquinolone- and tetracycline-resistant C. jejuni have been detected. We investigated the population structure, antimicrobial resistance, and transmission pathways of C. jejuni in Finnish poultry production.

RESULTS

A total of 333 C. jejuni isolates collected from breeder (n = 145) and broiler (n = 188) flocks during 2014-2019 were characterized using whole-genome sequencing, antimicrobial resistance profiling, multilocus sequence typing (MLST), core-genome MLST (cgMLST), and phylogenomic analysis. Resistance to fluoroquinolones and tetracycline was concentrated in specific years and was strongly associated with particular lineages, especially ST-206 clonal complex (CC), whereas the predominant ST-45 CC was largely susceptible. Genomic clustering identified several resistant clones circulating among geographically proximate broiler farms, indicating regional horizontal dissemination. Resistant isolates from breeders were uncommon, and no genomic evidence supported vertical transmission of resistant strains from breeder to broiler flocks. Instead, resistance peaks were primarily attributable to clonal expansion of successful resistant lineages.

CONCLUSIONS

Transient increases in fluoroquinolone- and tetracycline-resistant C. jejuni in Finnish broiler production were driven mainly by regional clonal horizontal spread rather than vertical transmission from breeders to broilers. These findings highlight the value of genomic surveillance for detecting and tracking antimicrobial-resistant Campylobacter lineages and support the integration of molecular epidemiology into poultry surveillance and control programmes.