We have bred so many chickens, packed them together so densely and pushed their bodies to grow so quickly that we are changing the evolution of bacteria.
New research has found that industrial chicken farming is accelerating the spread and evolution of campylobacter, the most common bacterial cause of diarrhoea in the world. Campylobacter already causes more than 3 times as many cases of gastroenteritis in the UK each year as all other monitored foodborne bacteria combined. An estimated 60 to 80% of human campylobacter infections are caught from raw chicken. And we are creating increasingly favourable conditions for the bacteria responsible.
We Created This Problem
Before chickens were domesticated around 5,000 years ago, different strains of campylobacter tended to remain associated with individual bird species. Cross-infection was relatively uncommon. Then humans changed the ecosystem. Researchers analysed nearly 2,800 bacterial genomes collected from chickens and wild birds across 30 countries between 1979 and 2024. They estimated that movements of campylobacter strains between chickens and wild birds have increased around 100-fold since 1900 compared with before domestication.
Since the 1960s, the global chicken population has increased seven-fold to around 27 billion birds. Chickens now account for approximately 70% of all bird biomass on Earth. Human demand for chicken flesh has become so enormous that the overwhelming majority of bird biomass on this planet is now made up of one species we selectively breed, confine and kill.
The average person now eats around 6 times more chicken than they did in 1961, and bacteria are taking advantage of the artificial ecosystem we created.
Professor Sam Sheppard, one of the researchers behind the campylobacter study, described chickens as becoming a “cauldron of bacterial evolution”.
When billions of genetically similar birds are continually bred, transported and confined in enormous populations, bacteria have countless opportunities to circulate, encounter different strains and acquire new traits. Apparently turning the planet into one enormous chicken factory has consequences.
Who knew?
Five Weeks From Birth To Death
Modern chicken production is built around speed. Some chickens have been selectively bred to grow so rapidly that they can reach slaughter weight at around 5 weeks old. Their bodies have not evolved to grow like this. Humans deliberately engineered these birds around one economic objective: producing as much flesh as possible, as quickly and cheaply as possible.
Conventional European systems examined in another study could house around 20.7 birds per square metre, with approximately 90,000 chickens housed on a single farm. Disease spreading under those conditions should surprise absolutely nobody. Neither should the industry’s reliance on antibiotics.
The campylobacter researchers identified genetic changes associated with adaptation to chicken farming, including genes linked to antimicrobial resistance. Campylobacter infection occasionally requires antibiotic treatment. As resistance increases, drugs that once worked can become ineffective. The system is therefore capable of creating both the disease risk and conditions that make some infections more difficult to treat.
The Antibiotics Tell Their Own Story
We already have evidence that less intensive production reduces antibiotic use dramatically. Research comparing conventional chickens with slower-growing birds raised under European Chicken Commitment standards found that slower growth and lower stocking densities could reduce antibiotic use by as much as 85%.
In the Netherlands in 2024, 96% of slower-growing flocks received no antibiotics at all, compared with 77% of fast-growing flocks. The enormous antibiotic use associated with intensive farming is not some unavoidable feature of chickens existing. It is connected to how humans breed and confine them.
Slow their growth. Give them more space. Disease pressure falls. Antibiotic use falls. But there is another problem.
Those slower-growing chickens consume more feed and take longer to reach slaughter weight. Producing the same quantity of flesh under European Chicken Commitment standards was associated with greenhouse gas emissions between 6% and 11% higher, depending on the production stage, while land use increased 14% and phosphorus excretion increased 37% per kilogram. So we arrive at the absurd debate animal agriculture creates again and again. Should we breed chickens who suffer more efficiently, or chickens who consume more resources before we kill them? Perhaps there is a third option.
Stop breeding billions of chickens to kill in the first place.
Cheap Chicken Is Not Cheap
The defence of factory farming inevitably comes back to price. We are told intensive farming is necessary because people need affordable food. Except cheap chicken is cheap largely because many of the real costs never appear on the supermarket price label.
A report examining factory farming of pigs and chickens estimated that the system costs UK taxpayers more than £1.2 billion every year. Environmental pollution alone accounted for an estimated £518 million, including £458 million associated with ammonia air pollution and £60 million from river pollution caused by manure. Indirect agricultural subsidies supporting pig and chicken production were estimated at another £269 million. Industrialisation was also estimated to have cost around 14,000 farming jobs compared with a scenario in which the sector had not continued industrialising after 2000.
Then there is public health.
Around two million people in the UK were estimated to live within two kilometres of a megafarm. Higher respiratory disease associated with living near these facilities produced an estimated mortality cost of almost £92 million annually. And even this £1.2 billion estimate did not include antimicrobial resistance, climate change, or healthcare costs associated with excessive meat consumption. Chicken is not cheap. Someone else is simply paying part of the bill. Usually animals, taxpayers, rural communities, ecosystems and future generations.
More Chicken Means More Risk
Global meat supply has risen from around 25kg per person to 47kg over the past 60 years. Chicken consumption has increased particularly dramatically. And this expansion is expected to continue. That means more chickens being bred. More feed being grown. More waste being produced. More enormous sheds. More animals living in conditions where disease can spread. More interactions between industrial chicken populations, bacteria and the wider environment.
Scientists studying campylobacter could hardly have been clearer.
“The scale is what’s bad,” Sheppard said.
But governments continue allowing factory farming to expand while debates about chicken production frequently focus on tinkering with the system.
Give them slightly more space.
Breed them slightly more slowly.
Use fewer antibiotics.
Reduce pollution.
Improve biosecurity.
Every one of those measures may reduce a particular harm. None addresses why we are breeding tens of billions of chickens in the first place.
There is no requirement for humans to consume six times more chicken than previous generations did. This is not an unavoidable natural phenomenon. We created an industrial system based on breeding an unimaginable number of sentient individuals, forcing their bodies to grow at extraordinary speeds and killing them after only weeks of life.
Now we are discovering that turning chickens into biological production units on this scale also creates an extraordinary playground for bacterial evolution. The solution cannot forever be finding increasingly sophisticated ways to manage the consequences of animal exploitation. Sometimes the simplest answer really is the obvious one.
Stop exploiting animals.
Chickens are the most abused of all animals on the planet 🐓🥀