A UK-wide genomics study has found increasing antimicrobial resistance (AMR) in Campylobacter isolates from human infections, alongside continued evidence that chicken meat is the dominant source of illness.

The research, conducted by the UK Food Standards Agency (FSA) in collaboration with the University of Oxford, the Moredun Institute, and the Quadram Institute, was funded by the national Pathogen Surveillance in Agriculture, Food, and the Environment (PATH-SAFE) program.

Campylobacter isolates from human cases and agri-food sources were analyzed using whole genome sequencing (WGS) to characterize resistance patterns and transmission pathways. Human clinical isolates were collected between April 2019 and December 2024, and agri-food, animal, and environmental isolates were retrieved from long-term storage collections held by the project’s participating university institutions.

Fluoroquinolone and Tetracycline Resistance Increased

The researchers reported that resistance to fluoroquinolones and tetracyclines in Campylobacter jejuni isolates from humans has risen since UK FSA’s previous study in 2015–2018. Fluoroquinolone resistance increased from 35.8 percent to 55.2 percent in 2024, while tetracycline resistance rose from 32.6 percent to 48.2 percent.

Macrolide and aminoglycoside resistance remained comparatively low among human isolates, and multidrug resistance was rare.

However, co-resistance to both fluoroquinolones and tetracyclines has also increased over time, with 42.5 percent of C. jejuni isolates in 2024 predicted to be resistant to both antimicrobial classes. Concerningly, three clonal complexes commonly found only in poultry (ST-353, ST-354, and ST-464) showed resistance levels approaching 100 percent.

Further research is needed to understand the persistence of these highly resistant, poultry-associated lineages, especially since antimicrobial stewardship on farms has continued to improve significantly over time.

Aside from poultry, AMR prevalence was generally low among isolates from ruminants and wild birds, with less than 5 percent resistance to fluoroquinolones and tetracycline. The findings suggest these sources pose a relatively low risk for AMR Campylobacter infections in humans. However, the detection of livestock-associated resistant strains in wild birds highlighted the potential for environmental spillover and the need for continued biosecurity controls.

Chicken Remains Primary Source of Infection

Source attribution modeling, which used machine learning and genomic data, predicted that more than 80 percent of human campylobacteriosis cases (including C. jejuni and C. coli infections) originated from chicken; respectively, 86.7 percent and 79.7 percent of human C. jejuni and C. coli cases were predicted to come from chicken meat. This is consistent with the findings of global source attribution models from 25 studies published between 2001 and 2017, which estimate 60–80 percent of human campylobacteriosis is derived from chicken meat.

However, the present study was not able to determine specific transmission routes, such as undercooked poultry versus cross-contamination.

Other sources, including cattle, sheep, wild birds, and backyard chickens, were also found to carry Campylobacter strains capable of causing human illness, although their overall contribution appeared to be lower than that of chicken meat. Still, the contribution from ruminants is also considerable (predicted to cause 11.5 percent of C. jejuni infections and 17.5 percent of C. coli infections), shown to be particularly important in rural or agricultural regions in previous studies.

Public Health and Disease Surveillance Implications

The study aimed to address gaps in surveillance by including underrepresented sources such as wild birds and free-range poultry, and to support a “One Health” approach to understanding Campylobacter transmission across humans, animals, and the environment.

Overall, FSA suggests that incorporation of WGS and artificial intelligence (AI)-based attribution frameworks into public health surveillance could support more targeted control measures across the food production chain.

Campylobacter remains the leading cause of bacterial gastroenteritis in the UK, with an estimated annual economic burden exceeding £700 million. To address this important public health threat, the researchers concluded that continued monitoring of AMR trends and improved understanding of transmission pathways are critical for informing intervention strategies.