The explosion in global chicken numbers – now 27 billion birds, a seven-fold increase since the 1960s – is turning poultry farms into a cauldron of bacterial evolution, scientists have warned. A new study has found that intensive chicken farming has dramatically accelerated the spread of campylobacter, the world's most common bacterial cause of diarrhoea.
Campylobacter already 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 human infections originate from raw chicken. Most people recover without treatment, but some require antibiotics, and infection can increase the risk of irritable bowel syndrome.
“Industrial chicken farming has accelerated the evolution of campylobacter bacteria, raising fears of hard-to-treat infections, a study warns.”
The latest research, published in the Proceedings of the National Academy of Sciences, analysed nearly 2,800 bacterial genomes collected from chickens and wild birds across 30 countries, including the UK, US and China, between 1979 and the present. It found that as farming intensified, campylobacter strains from wild birds and chickens began mixing at an unprecedented rate, creating ideal conditions for the bacteria to acquire new traits.
“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, the study’s senior author. “Lots of strains come together and hybridise. We do not want that, because that’s where new Frankenstein monster bugs emerge.”
The study also detected a rise in antimicrobial resistance traits in circulating campylobacter strains, raising fears that serious infections in people may become harder to treat. Some antibiotics previously used to treat campylobacter are no longer effective.
Chickens now account for about 70% of all bird biomass on Earth. Intensive breeding has produced “super-growing” birds that reach adult size and are slaughtered at around five weeks old. While industrialised chicken farming has delivered affordable animal protein at scale, it also presents new health risks.
Sheppard stressed that cooking kills the pathogen, “but just a tiny splash and it’s on your lettuce leaves or your hands or your knife.” The findings highlight a growing tension between the demand for cheap chicken and the hidden cost of bacterial evolution.
