Industrial-scale poultry farming has significantly contributed to the proliferation and evolution of Campylobacter bacteria, the leading bacterial cause of diarrhoeal illness globally, according to a recent study by researchers from the University of Oxford. Analyzing nearly 2,800 bacterial genomes from chickens and wild birds across 30 countries between 1979 and 2024, the team documented a dramatic rise in the transfer and mixing of Campylobacter strains, driven largely by the intensification of chicken farming.

Campylobacter is responsible for a majority of bacterial gastroenteritis cases, particularly linked to the consumption of undercooked chicken. It can cause symptoms ranging from diarrhoea to more severe complications such as muscle weakness and paralysis. While infection commonly resolves without medical intervention, some cases require antibiotic treatment. The emergence of antimicrobial resistance (AMR) in these bacteria, making some antibiotics less effective, has raised concerns over treatment challenges in the future.

Researchers found that the global chicken population has expanded roughly sevenfold since the 1960s, now numbering around 27 to 31 billion birds and representing about 70% of the planet’s avian biomass. This explosion in poultry numbers has led to increased interaction between Campylobacter strains from chickens and wild birds. Prior to chicken domestication around 5,000 years ago, Campylobacter strains were typically restricted to individual bird species, but cross-species transmission has increased approximately 100-fold since 1900.

The study showed that intensive poultry farming environments serve as hotspots for bacterial evolution. Chickens, often raised in crowded sheds where antibiotics are routinely used to promote growth and control disease, have become hosts to rapidly evolving Campylobacter populations. These conditions facilitate the exchange and recombination of different bacterial strains, including those harboring genes conferring antimicrobial resistance.

Lead researchers emphasized that this genetic mixing increases the risk of creating new Campylobacter variants that could pose heightened threats to human health. “The scale is what’s bad,” said one scientist, highlighting how dramatic changes to ecosystems through large-scale chicken production may fuel pathogen evolution and pose significant public health risks globally.

In the United Kingdom, for example, an estimated 60 to 80 percent of Campylobacter infections originate from contaminated poultry meat. Although proper cooking can eliminate the bacteria, contamination through handling or cross-contact with foods such as fresh vegetables remains a common pathway for infection.

Experts warn that the rapid growth in global chicken farming must be managed more sustainably to reduce the emergence of drug-resistant and potentially more virulent bacterial strains. Addressing antimicrobial use practices and improving biosecurity measures in poultry production could be essential steps to curbing the spread and evolution of Campylobacter bacteria.