Industrial chicken farming is accelerating the spread of the world’s most common bacterial cause of diarrhoea, scientists have found.

The study found a dramatic increase in the exchange of campylobacter strains between wild birds and chickens as farming has intensified, creating the ideal conditions for the bacteria to mix, spread and acquire new traits. There has also been a rise in antimicrobial resistance traits in circulating strains, raising concerns that serious illness in people will be more difficult to treat.

“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 and 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.”

Campylobacter is the most common bacterial cause of diarrhoea worldwide, causing 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 campylobacter infections are caught from raw chicken.

“Cooking kills pathogens, but just a tiny splash and it’s on your lettuce leaves or your hands or your knife,” said Sheppard.

Most people will get better without treatment, but sometimes antibiotics are required and infection also raises the risk of irritable bowel syndrome. Rising rates of antimicrobial resistance, which occurs when bacteria resist drugs designed to kill them, mean that some antibiotics that were previously used to treat campylobacter infections are no longer effective.

The latest work suggests that the evolution of the infection is being fuelled by the unprecedented scale of chicken production.

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 of chicken farming has led to affordable animal protein at scale, but also presents new health risks.

The latest work, published in the journal Proceedings of the National Academy of Sciences, analysed nearly 2,800 bacterial genomes collected from chickens and wild birds in 30 countries, including the UK, US and China, between 1979 and 2024.

The dataset allowed the scientists to reconstruct the evolutionary trees of the diverse strains, including the rate of exchange between hosts. Prior to domestication of the chicken 5,000 years ago, strains of campylobacter tended to be tightly linked to individual species of birds, with little cross-infection. But as chicken numbers increased, this picture changed, with an estimated 100-fold increase in transitions of strains between chickens and wild birds since 1900 compared with pre-domestication – a trend that appears to have continued steeply in recent decades.

“The scale is what’s bad,” said Sheppard. “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.”

The researchers also identified genetic changes in campylobacter that were associated with adaptation to the chicken environment. These included genes involved in antimicrobial resistance, which can help bacteria survive and thrive in modern poultry production systems, where antibiotics are routinely given to encourage rapid growth or to keep disease under control 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.”