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Global chicken numbers have increased seven-fold since the 1960s, but with the rise of the bird population has come faster-paced, wider-reaching farming practices that experts now fear have fueled a more than 100-fold increase in the spread of the world's most common foodborne bacteria. The researchers behind the findings hope their conclusions, announced on July 27, prompt a rethink of how the poultry industry, and its modern practices, tackle pathogens before they reach the dinner table.
"Our study provides compelling evidence that the rapid expansion of intensive poultry production has altered the ecology and evolution of Campylobacter jejuni, the leading bacterial cause of foodborne gastroenteritis worldwide," Sam Sheppard, senior researcher on the new study, told Newsweek. "By concentrating billions of chickens in high-density production systems, we've inadvertently created ideal conditions for the bacterium to spread, diversify, adapt and, increasingly, move between domestic poultry and wild birds."
Dr. Steven Goldberg, chief medical officer at HealthTrack, an outpatient infectious disease diagnosis service, told Newsweek that Campylobacter can cause diarrhea, sickness and stomach illness. It caused about 1.9 million foodborne illnesses and 13,000 hospitalizations in the U.S. in 2019. At the same time, there are approximately 31 billion chickens worldwide, making up about 70 percent of all bird biomass on Earth.
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A Global Surge in Poultry, and in Pathogens
The research, led by Ineos Oxford Institute for Antimicrobial Research (IOI) at the University of Oxford, examined nearly 2,800 bacterial genomes gathered from chickens and wild birds across 30 countries, including the U.S., from 1979 to 2024. The team found that the explosive growth of the global poultry industry has created near-perfect conditions for Campylobacter to spread, mix and evolve.
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That scale has turned commercial flocks into what the study describes as ecological "pathogen sponges," capable of absorbing and amplifying bacterial strains drawn from multiple sources. Mathematical modeling in the study showed that once poultry populations cross a certain threshold, strains that originated in wild birds can become self-sustaining inside commercial flocks—even when those strains are, at first, poorly suited to their new host.
Genomic analysis also revealed specific changes in the bacterium linked to its adaptation to farmed environments, including genes tied to antimicrobial resistance, tolerance of oxidative stress, metal acquisition and motility—all traits that help Campylobacter survive and spread within these modern production systems.
What This Means for Public Health
In the U.K. alone, where the research team is based, Campylobacter causes more than 3.5 times as many gastroenteritis cases each year as all other monitored foodborne bacteria combined. A previous IOI study found that 80 percent of human Campylobacter infections in Oxfordshire, England, were linked to poultry meat, and that a significant share of those infections were resistant to antibiotics.
That resistance is what worries researchers most. Rising rates of antimicrobial resistance are making Campylobacter infections progressively harder to treat, turning a routine case of food poisoning into a serious medical concern.
Even so, Sheppard was clear that the findings are not cause for panic.
"There is no immediate reason for the public to be alarmed," he said. "Campylobacter infection is already a well-recognized food safety issue, and the risks can be greatly reduced through good kitchen hygiene, avoiding cross-contamination, and thoroughly cooking poultry.
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"However, the findings are a reminder that intensive livestock production doesn't just increase food production—it can also create new ecological opportunities for pathogens to evolve and spread," he added.
Campylobacter Increase Across Countries
- In England, where the researchers are based, the number of reported Campylobacter cases rose from 60,055 in 2023 to 70,352 cases in 2024, which makes for an increase of 10,297 cases, with a resulting rate rise from 104.1 to 121.9 reports per 100,000 people.
- In the U.S., Campylobacter causes the most bacterial diarrheal illness. The Centers for Disease Control and Prevention (CDC) estimates that 1.5 million people get ill from it every year.
- In France, Campylobacter infections are on the rise, with 2023 data showing a higher incidence rate in children under 10 and in men.
- In Canada, 145,000 people are affected by food-borne illness per year.
- In Australia, campylobacteriosis is the most commonly detected foodborne infection. About 40,000 infections were reported in 2024.
A Call for a New Industry Standard
Sheppard pointed to the poultry industry's success in curbing Salmonella through vaccination and coordinated biosecurity efforts as a model for what could be done with Campylobacter.
"Similar ambition should now be directed towards Campylobacter, which remains the most common bacterial cause of gastroenteritis in many countries," he said. "Enhanced farm biosecurity can reduce Campylobacter in poultry flocks, but ultimately we should be aiming higher than simply managing contamination.
"We've become accustomed to a containment level 2 pathogen entering millions of domestic kitchens every day, but familiarity shouldn't be mistaken for acceptability," Sheppard added.
Backyard Flocks Offer a Different Lesson
For smaller-scale poultry keepers, the findings reinforce practices many already follow.
Elam Miller, owner of the Hen House Collection, a chicken coop maker who tracks poultry health trends, told Newsweek the study "is no reason to panic," but added that it confirms what backyard flock owners have long suspected.
"We know that bacteria spreads easier in crowded conditions," Miller said. "It makes sense that bacteria adapt faster in those spaces."
Miller said good flock management, including clean water, adequate space, free-flowing air and low stress, helps keep birds healthier, even if it cannot eliminate the bacteria entirely.
"Despite the bacteria being found in food, that does not mean you have to stop eating chicken or raising livestock altogether," Miller said. "Instead, it encourages maintaining food safety measures and healthy upkeep in the coops to lower the risk of spreading the bacteria."
Reference
Kyne, J. O., Penman, B. S., Kelly, D. J., Sheppard, S. K. (2026). Accelerating Campylobacter zoonosis in the Anthropocene.
Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2609969123.
Contact Newsweek editors on this story: Kara Dolman and Emma Lee-Sang
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