How 31 Billion Chickens Are Driving the Evolution of a Major Foodborne Pathogen

Chickens now make up roughly 70 percent of all bird biomass, with a global population that has skyrocketed to about 31 billion since the 1960s. This explosive growth has inadvertently set off an extensive evolutionary process within the birds themselves, particularly concerning the Campylobacter jejuni bacterium, which is the leading cause of foodborne diarrhea globally.

Researchers have found that industrial poultry production has significantly influenced how this pathogen moves between bird species and evolves. In fact, the study published in the Proceedings of the National Academy of Sciences suggests that host transitions involving chickens and wild birds have increased over 100 times since 1900, compared to pre-domestication levels.

Traditionally, wild bird species carried distinct bacterial populations, shaped by generations of evolving independently. However, the analysis of 2,747 bacterial genomes collected from chickens and wild birds across 30 countries between 1979 and 2024 has revealed that chickens have been colonized by strains descended from several wild bird lineages.

This pattern breaking down is akin to a sponge, where chicken populations have absorbed diverse bacterial strains from multiple wild sources, amplifying and mixing them together inside densely packed flocks in a manner that was previously impossible.

The research model indicates that once the chicken population surpasses a certain threshold, the pattern shifts. Strains adapted to wild birds become self-sustaining inside chickens, and the number of infected chickens outnumbers infected wild birds carrying the same strains. While the model does not confirm that today's global poultry population has crossed a specific real-world threshold, it demonstrates how expanding chicken populations can allow wild bird strains to persist and spread within poultry.

Interestingly, the bacterial strains found in chickens have more often carried genetic traits associated with tolerating oxidative stress, acquiring metals, and moving through the gut. Additionally, some of these strains carry the tetO gene, which is linked to antibiotic resistance, particularly to the antibiotic tetracycline. This resistance gene has become more prevalent among chicken-associated samples collected after 2015, possibly due to increased exposure to antimicrobial drugs in poultry compared to wild birds.

The effective population size of these chicken-adapted bacterial strains has grown by 50 to 200 times over the past few decades, far exceeding the growth of the chicken population itself. This rapid evolution suggests that factors beyond just host numbers are driving the development of these strains. In the UK, Campylobacter causes more than three and a half times as many gastroenteritis cases annually as all other monitored foodborne bacteria combined.

Moreover, rising antimicrobial resistance is making some infections more challenging to treat. This study underscores how the scale of modern chicken farming is not only increasing the prevalence of this threat but is also actively shaping the bacteria into something entirely new, all within one massive, crowded flock.