Intensive Chicken Farming Fuels Diarrhoea Bacteria Spread

Intensive chicken farming methods are accelerating the global spread of campylobacter, the most frequent bacterial agent of diarrhoea, according to recent scientific findings. The study reveals a substantial increase in the transfer of campylobacter strains between wild bird populations and domestic chickens, driven by the intensified nature of modern agriculture.

Conditions Favouring Bacterial Evolution

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The intensive conditions within modern chicken farms provide an ideal environment for campylobacter to mix, spread, and develop new characteristics, the research indicates. This includes a rise in antimicrobial resistance traits within circulating strains, which could make treating serious human infections more challenging. Professor Sam Sheppard of the University of Oxford, a senior author of the study, explained that the exploding chicken population has led to increased acquisition of strains from various wild birds, creating a 'cauldron of bacterial evolution' where new hybrid strains can emerge.

Campylobacter causes more gastroenteritis cases in the UK annually than all other monitored foodborne bacteria combined. It is estimated that 60-80% of human infections originate from raw chicken. While most individuals recover without medical intervention, some require antibiotics, and infection can increase the risk of developing irritable bowel syndrome. The growing resistance of bacteria to drugs means that some antibiotics previously used for campylobacter infections are becoming less effective.

Scale of Production and Health Risks

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The study suggests that the unprecedented scale of chicken production is fuelling the evolution of campylobacter. Global chicken numbers have increased seven-fold since the 1960s, reaching approximately 27 billion birds and constituting about 70% of all bird biomass on Earth. This industrialisation has provided affordable protein but also introduced new health hazards. The research analysed almost 2,800 bacterial genomes from chickens and wild birds across 30 countries between 1979 and 2024.

Analysis of this data allowed scientists to trace the evolutionary paths of different strains and the frequency of exchange between hosts. Before chicken domestication around 5,000 years ago, campylobacter strains were largely specific to particular bird species. However, with the rise in chicken numbers, there has been an estimated 100-fold increase in strain transitions between chickens and wild birds since 1900, a trend that has continued rapidly in recent decades. Professor Sheppard stated that the immense scale of production, which significantly alters global ecosystems, poses a substantial risk to human health and cannot be sustained.

Researchers also identified genetic changes in campylobacter associated with adaptation to the chicken environment, including genes linked to antimicrobial resistance. These adaptations help bacteria survive in poultry production systems where antibiotics are commonly used to promote growth or manage disease in crowded conditions. Maya Pardo, legal coordinator at Communities Against Factory Farming (CAFF), commented that the 100-fold increase in campylobacter movement is a severe consequence of uncontrolled intensive chicken farming in the UK, occurring while the government considers relaxing planning rules for more such facilities.

Next Steps

The study was published in the journal Proceedings of the National Academy of Sciences. Further research will likely focus on mitigation strategies to reduce the spread of campylobacter and antimicrobial resistance within poultry production and its interfaces with wild bird populations.