Intensive farming practices, including overuse of antibiotics, high animal numbers, and low genetic diversity, increase the risk of animal pathogens transferring to humans. That warning comes from an international team of researchers led by the Universities of Bath and Sheffield in the UK, working with support from the United States Department of Agriculture (USDA).

The research, published in the journalProceedings of the National Academy of Sciences, looks at the evolution ofCampylobacter jejuni. According to apress releasefrom the University of Bath, this bacterium is the leading cause of gastroenteritis in high-income countries.

Facts aboutCampylobacter, as outlined in the release:
  • Carried in the feces of chickens, pigs, cattle and wild animals;
  • Estimated to be present in the feces of 20% cattle worldwide;
  • Transferred to humans from eating contaminated meat and poultry;
  • Causes bloody diarrhea in humans;
  • Causes serious illness in patients with underlying health issues and can cause lasting damage, though not as dangerous as typhoid, cholera or E.coli;
  • An estimated 1 in 7 people suffer from an infection at some point in their life;
  • Causes three times more cases than E. coli, Salmonella and listeria combined;
  • Very resistant to antibiotics due to the use of antibiotics in farming.
The research team studied the genetic evolution ofCampylobacter jejuni. What they found: Cattle-specific strains of the bacterium emerged at the same time as a dramatic rise in cattle numbers in the 20th Century. The study authors suggest that changes in cattle diet, anatomy, and physiology triggered gene transfer between general and cattle-specific strains with significant gene gain and loss. As is explained in the release, this helped the bacterium cross the species barrier and infect humans. Add in the increased movement of animals globally, they say, and intensive farming practices have provided the "perfect environment" in which to spread globally through trade networks.
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“There are an estimated 1.5 billion cattle on Earth, each producing around 30 kg of manure each day; if roughly 20% of these are carrying Campylobacter, that amounts to a huge potential public health risk," explained Professor Sam Sheppard, Director of Bioinformatics from the Milner Centre for Evolution at the University of Bath, in the release. “Over the past few decades, there have been several viruses and pathogenic bacteria that have switched species from wild animals to humans: HIV started in monkeys; H5N1 came from birds; now COVID-19 is suspected to have come from bats."

Sheppard added that research suggests that environmental change and increased contact with farm animals are additional factors that have caused bacterial infections to cross over to humans. “I think this is a wake-up call to be more responsible about farming methods, so we can reduce the risk of outbreaks of problematic pathogens in the future.”
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In the study, the authors note: "Further understanding of the genetic and functional basis of host adaptation, particularly to livestock that constitute the majority of mammal biomass on Earth, is important for the development of novel strategies, interventions, and therapies to combat the increasing risk of pathogens with the capacity to spread from livestock to humans." The hope going forward, the researchers say, is that this study can help scientists predict potential problems so those problems can be prevented before they result in another epidemic.