Health & Science: Intensive poultry farming may be helping harmful bacteria evolve in ways that make them more persistent, resistant to antibiotics and better able to spread between species, according to a new study examining the evolution of Campylobacter jejuni. The findings have raised fresh concerns about the public-health consequences of highly concentrated animal farming.
Researchers from the Universities of Oxford and Sheffield analyzed genetic information from 2,747 samples of Campylobacter jejuni collected from chickens and wild birds. Their findings suggest that the rapid expansion and intensification of poultry farming has significantly changed how these bacteria circulate and evolve.
Campylobacter jejuni is an important cause of bacterial food poisoning and can lead to intestinal illness and diarrhea in people, often following consumption of contaminated poultry. The study found that chicken-associated strains have evolved rapidly and acquired genetic characteristics that can help them survive and adapt in new environments.
One of the most concerning findings was the dramatic increase in transmission between chickens and wild birds. Researchers found that transmission between the two populations has increased by more than 100-fold since 1900, coinciding with the development and expansion of modern poultry farming.
Scientists suggest that densely populated farms can create ideal conditions for bacteria to circulate. Large numbers of animals living in close proximity provide pathogens with repeated opportunities to spread, reproduce and adapt.
Researchers described chickens as acting like “sponges” for different bacterial strains because they can acquire bacteria from their surroundings and from contact with other animals. This mixing may provide pathogens with more opportunities to develop characteristics that allow them to infect new hosts.
Another major concern is antimicrobial resistance. The study identified chicken-adapted bacterial strains carrying genetic traits associated with resistance to antimicrobial drugs. Some strains were resistant to tetracycline, an antibiotic used in both animal farming and human medicine.
The researchers also identified evidence that some bacteria had developed greater tolerance to oxygen-rich environments. According to the study’s authors, this characteristic could potentially help bacteria survive outside animals, including on food, and increase opportunities for human infection.
The findings add to broader concerns about the extensive use of antibiotics in intensive animal agriculture. When antimicrobial medicines are used frequently, bacteria can be exposed repeatedly to these drugs, creating conditions in which resistant strains may survive and multiply.
The issue extends beyond Campylobacter. A report cited by Sentient Media found that in 2025, around 73 percent of poultry samples that tested positive for Salmonella were resistant to at least one antibiotic considered highly important for human medicine, compared with 48 percent a decade earlier.
Researchers emphasize that stronger biosecurity measures could help reduce the risks. Better separation between wild birds and farmed poultry, improved hygiene and stronger disease surveillance may reduce opportunities for pathogens to move between populations.
The study highlights a broader challenge created by industrial farming: when very large numbers of animals are raised together, infectious organisms can encounter many potential hosts in a relatively small area. This can increase opportunities for pathogens to spread and evolve.
Experts say protecting public health will require attention not only to food safety after animals reach processing facilities but also to conditions on farms where pathogens can emerge and adapt.
The findings do not mean that every factory farm will produce dangerous bacteria or that eating poultry inevitably leads to infection. Rather, they point to the need for stronger controls around animal health, antimicrobial use, farm biosecurity and food safety.
As global demand for poultry continues to grow, scientists say understanding how farming practices influence pathogen evolution will become increasingly important. The study provides another warning that how animals are raised can influence the evolution and spread of bacteria that ultimately affect human health.
