If every wild and domesticated bird on Earth were placed on a scale, chickens would account for about 70 percent of the total biomass. Their global population has risen sevenfold since the 1960s to roughly 31 billion, and new research suggests that expansion has created an enormous, unplanned evolutionary experiment inside the birds themselves.
Industrial poultry production appears to have altered how Campylobacter jejuni moves between bird species and evolves. The bacterium is the world’s most common bacterial cause of diarrhea, and researchers trace an estimated 60 to 80 percent of human C. jejuni infections to poultry. A study published in the Proceedings of the National Academy of Sciences estimated that host transitions involving chickens and wild birds have increased more than 100-fold since 1900 compared with pre-domestication levels.
“Industrial farming has created one of the largest animal habitats on the planet. Our findings provide new evidence that human-driven environmental change can increase the spread of infectious diseases,” said senior study author Sam Sheppard in a press release.
How Chickens Became Pathogen Sponges
Researchers reached that conclusion after analyzing 2,747 bacterial genomes collected from chickens and wild birds across 30 countries between 1979 and 2024.
Wild bird species have traditionally carried distinct bacterial populations, shaped by generations of evolving separately from one another. That pattern is breaking down. Genomic analysis showed that chickens have been colonized by strains descended from several wild bird lineages that previously remained separate.
Researchers describe the effect as something like a sponge. As chicken populations have ballooned, they've soaked up diverse bacterial strains from multiple wild sources, amplifying and mixing them together inside densely packed flocks in a way that simply wasn't possible before industrial farming existed.
To understand why scale matters, researchers modeled transmission between linked wild bird and chicken populations. Below a certain chicken population size, wild-bird strains occasionally spilled over but failed to become established in chickens.
How Growing Chicken Populations Help Campylobacter Take Hold
Once the modeled chicken population grew past that threshold, the pattern flipped. Wild-bird-adapted strains became self-sustaining inside chickens, and the number of infected chickens outnumbered infected wild birds carrying the same strains.
The model does not establish that today’s global poultry population has crossed a single real-world threshold. Instead, it demonstrates how expanding chicken populations can allow strains from wild birds to persist and spread in poultry.
New Traits Are Emerging, Including Drug Resistance
Researchers found that chicken-associated strains were more likely to carry genetic traits involved in tolerating oxidative stress, acquiring metals, and moving through the gut. Some also carried tetO, a gene associated with resistance to the antibiotic tetracycline.
That resistance gene became more common among chicken-associated samples collected after 2015. Researchers said its prevalence may reflect greater exposure to antimicrobial drugs in poultry than in wild birds.
The effective population size of these chicken-adapted bacterial strains grew by 50 to 200 times over the past several decades, much faster than the chicken population itself, suggesting that factors beyond host numbers are accelerating their evolution.
“As Campylobacter strains adapt to life in poultry, they can acquire traits that help them survive in challenging environments, including traits linked to antimicrobial resistance. Understanding how farming practices influence bacterial evolution is an important step towards reducing the burden of foodborne disease,” first author Oakem Kyne said in the press release.
In the U.K., Campylobacter causes more than 3.5 times as many gastroenteritis cases each year as all other monitored foodborne bacteria combined. Rising antimicrobial resistance is also making some infections more difficult to treat.
What this study shows is that the scale of modern chicken farming has made that threat more common while actively shaping the bacteria into something new, one enormous, crowded flock at a time.
Read More: Ancient Chickens Shine Light on Evolution of Major Poultry Disease
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- This article references information from a study published in the Proceedings of the National Academy of Sciences:AcceleratingCampylobacterzoonosis in the Anthropocene