Researchers at the University of Oxford have identified a worrying link between the development of industrial poultry farming and the evolution of Campylobacter bacteria — the world’s leading cause of diarrhoeal diseases. According to the study, the main route of human infection is the consumption of raw chicken meat, which accounts for 60–80 % of all infection cases. Even after heat treatment, the risk remains due to cross‑contamination: bacteria can transfer to other foods, skin, or hands. For this reason, UK authorities recommended as early as 2014 not to wash raw chicken carcasses.
The intensification of poultry farming has led to a sharp increase in the exchange of Campylobacter strains between wild birds and domestic chickens. This creates favourable conditions for the bacteria to acquire new traits — in particular, resistance to antibiotics.
Professor Sam Sheppard, the study’s senior author, emphasises that the growing chicken population is increasingly becoming infected with strains from various species of wild birds, turning into a kind of “cauldron” where strains mix and hybridise. This is how new, more dangerous bacterial variants emerge, making the diseases they cause harder to treat.
The link between the evolution of the infection and the growth of chicken production is supported by statistics: since the 1960s, the global chicken population has increased sevenfold, reaching approximately 27 billion individuals — about 70 % of the total bird biomass on Earth.
As part of the study, scientists analysed nearly 2,800 bacterial genomes collected from chickens and wild birds in 30 countries (including the UK, the USA, and China) between 1979 and 2024. This data enabled them to reconstruct the evolutionary trees of the strains and estimate the rate of their transmission between different hosts. Before chickens were domesticated (around 5,000 years ago), Campylobacter strains were typically associated with specific bird species, and cross‑species transmission was rare. However, as the chicken population grew, the situation changed dramatically: experts estimate that since 1900, the number of strain transfers from chickens to wild birds has increased a hundredfold compared to the pre‑domestication period, and this trend has intensified notably in recent decades.
According to Professor Sheppard, the main problem is the scale of the changes: the radical transformation of ecosystems promotes the development of pathogenic microorganisms in birds, posing serious risks to human health. The scientist believes that further scaling up chicken production is unacceptable.
In addition, the researchers found genetic changes in Campylobacter linked to adaptation to modern poultry farming conditions. These include genes conferring resistance to antimicrobial drugs. They help the bacteria survive in systems where antibiotics are used regularly — both to stimulate growth and to prevent and treat diseases.
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