The rapid global expansion of industrial poultry farming is accelerating the spread and evolution of Campylobacter — the world’s leading bacterial cause of diarrhoeal disease, according to a new study.
Published in the Proceedings of the National Academy of Sciences (PNAS) on July 27, 2026, the study by University of Oxford analysed nearly 2,800 bacterial genomes collected from chickens and wild birds across 30 countries between 1979 and 2024.
The research led by Oakem J Kyne found that industrial poultry production has increased the movement of Campylobacter strains between wild birds and commercial chickens by more than 100-fold, creating ideal conditions for the bacteria to evolve and acquire traits such as antimicrobial resistance (AMR).
Intensive livestock production has done more than amplify pathogen abundance, it has reshaped the evolutionary landscape in which zoonotic bacteria emerge. By creating dense, globally connected host populations, industrial farming increases spillover opportunities, accelerates host switching, and expands pathogen effective population sizes; enhancing adaptive potential.
Traits selected for success in livestock, such as stress tolerance and transmission efficiency, can inadvertently promote human infection. More broadly, anthropogenic ecosystem change can generate powerful evolutionary feedback loops, transforming local microbes into globally disseminated zoonotic threats and fundamentally altering the trajectory of pathogen emergence.
Model simulations indicated that expanding, high-density chicken populations could act as ecological “pathogen sponges,” absorbing and amplifying diverse strains while sustaining high prevalence and coinfection rates.
Genome-wide association analyses show that strains adapted to the chicken niche frequently acquire genes linked to oxidative stress, metal homeostasis, motility, and antimicrobial resistance. Additionally oxidative stress responses could facilitate food chain transmission and tetracycline resistance may inadvertently increase their ability to persist in humans. More than half of the chicken-associated loci were linked to oxidative stress response, protection, or sensing.
“Findings provide new evidence that human-driven environmental change can increase the spread of infectious diseases. As chicken populations have grown, bacteria that were once largely confined to wild birds have gained far more opportunities to enter poultry flocks, spread and become established,” said Samuel K Sheppard, one of the study's authors.
Another study in March 2026 by scientists from the University of Oxford found rising AMR in diarrhoeal disease. The study estimated that 80 per cent of human Campylobacter infections in Oxfordshire, UK are associated with poultry meat, and that many of these infections are resistant to antibiotics.
The rise of poultry is largely a modern phenomenon with the number of chickens increasing sevenfold since the 1960s to roughly 31 billion birds today.
Understanding how increasingly intensive farming practices and global livestock trade networks influence the emergence of zoonotic pathogens is critical to safeguarding human health. Each year, around 550 million people—including 220 million children under five—suffer from foodborne diarrhea, with Campylobacter being the most common bacterial cause.