Industrial chicken farming is accelerating the spread of campylobacter, the most common bacterial cause of diarrhoea worldwide, scientists have found. The study reveals a dramatic increase in the exchange of campylobacter strains between wild birds and chickens as farming has intensified, creating conditions for the bacteria to mix, spread and acquire new traits. Antimicrobial resistance traits have also risen, raising concerns about treatment of serious infections.
Global chicken population surge
Since the 1960s, global chicken numbers have increased seven-fold to approximately 27 billion birds, accounting for about 70% of all bird biomass on Earth. Intensive breeding has led to “super growing” chickens that reach adult size and are slaughtered at around five weeks old. The industrialisation has made animal protein affordable at scale but also presents new health risks.
Campylobacter risks
Campylobacter causes more than three times as many gastroenteritis cases in the UK each year as all other monitored foodborne bacteria combined. An estimated 60-80% of infections are caught from raw chicken. While cooking kills the pathogen, cross-contamination is a major risk. Most people recover without treatment, but antibiotics are sometimes needed, and infection raises the risk of irritable bowel syndrome.
Evolutionary analysis
The study, published in the journal Proceedings of the National Academy of Sciences, analysed nearly 2,800 bacterial genomes from chickens and wild birds in 30 countries between 1979 and 2024. Scientists reconstructed evolutionary trees, finding that before chicken domestication 5,000 years ago, campylobacter strains were tightly linked to individual bird species. As chicken numbers increased, cross-infection rose dramatically, with an estimated 100-fold increase in strain transitions between chickens and wild birds since 1900 compared with pre-domestication – a trend that has continued steeply in recent decades.
Expert warnings
“As the chicken population has exploded, they’ve increasingly picked up strains from different wild birds and become a cauldron of bacterial evolution,” said Prof Sam Sheppard of the University of Oxford, senior author of the paper. “Lots of strains come together and hybridise. We do not want that, because that’s where new Frankenstein monster bugs emerge.” He added that cooking kills pathogens, “but just a tiny splash and it’s on your lettuce leaves or your hands or your knife.”
Sheppard warned: “By adjusting the world’s ecosystems in this dramatic way, it leads to the evolution of pathogens within those birds. That’s a massive risk to us. We can’t continue to produce chickens at this huge capacity.”
Antimicrobial resistance
The researchers identified genetic changes in campylobacter associated with adaptation to chicken environments, including genes for antimicrobial resistance. These help bacteria survive in modern poultry systems where antibiotics are routinely used to encourage rapid growth or control disease in densely packed sheds.
Maya Pardo, legal coordinator at Communities Against Factory Farming (CAFF), said: “Intensive chicken farming has spread out of control in the UK, and the 100-fold increase in the movement of campylobacter is the latest in a long list of devastating consequences. While communities are desperately calling for no more factory farms, the government is absurdly looking to relax planning rules to build yet more of these disease-ridden industrial facilities.”
