Intensive farming techniques could increase the likelihood of pathogens becoming a public health risk, according to researchers.

An international team led by the Universities of Bath and Sheffield in England, investigated the evolution of Campylobacter jejuni, a bacterium carried by cattle.

Scientists studied the genetic evolution of the pathogen and found that cattle-specific strains emerged at the same time as a rise in cattle numbers in the 20th Century.

They used large isolate collections and comparative genomics techniques, linked to phenotype studies, to understand the timescale and genomic adaptations associated with the rise of Campylobacter jejuni in cattle, which are the most prolific agricultural mammal.

Crossing the species barrier
Funding for the study, published in the journal Proceedings of the National Academy of Sciences, was provided by the Medical Research Council, Wellcome Trust, Food Standards Agency, Biotechnology and Biological Sciences Research Council, U.S. Department of Agriculture’s Agricultural Research Service and Food Standards Scotland.

Researchers suggested that changes in cattle diet, anatomy and physiology triggered gene transfer between general and cattle-specific strains with significant gene gain and loss. This helped the bacterium to cross the species barrier and infect humans, making for a major public health problem.

Professor Sam Sheppard from the Milner Centre for Evolution at the University of Bath, said in recent decades, there have been several viruses and pathogenic bacteria that have switched species from wild animals to humans.

“There are an estimated 1.5 billion cattle on Earth, each producing around 30 kilograms of manure each day; if roughly 20 percent of these are carrying Campylobacter, that amounts to a huge potential public health risk. Our work shows that environmental change and increased contact with farm animals has caused bacterial infections to cross over to humans too. 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.”

Expansion into a new host species is typically thought to be accompanied by evolutionary specialization and gradual divergence from the ancestral population. This process may be quicker in intensive farming where frequent animal contact, high animal numbers, and low genetic diversity provide opportunities for pathogens to evolve.

Farming practices and trade
Scientists said increasingly intensive farming practices and global trade networks mean the shifting nature of livestock pathogens presents a major public health threat.

Researchers did a population-genomic analysis of 1,198 Campylobacter jejuni isolates chosen to represent known diversity to describe the nature and timescale of adaptation to cattle. The isolates came from 18 different sources belonging to 36 clonal complexes.

The most dramatic change to the natural host niche of Campylobacter jejuni has occurred because of intensive livestock production favoring sublineages associated with agricultural animal hosts, such as cattle, according to the study.

Dated phylogeny estimated emergence of the ST-61 complex to have occurred in 1859. Much of the ST-61 sublineage diversification was more recent from 1925 to 1965, and there was evidence population expansion coincided with intensification of cattle farming in the 20th century.

“New farming techniques, improved livestock breeding, and higher stocking densities led to amplified food production, but the host population explosion and global transmission networks have also fundamentally changed the cattle niche, potentially favoring strains that are adapted to specialist lineages,” according to researchers.

Further understanding of the genetic and functional basis of host adaptation could help develop novel strategies, interventions, and therapies to combat the increasing risk of pathogens with capacity to spread from livestock to humans.

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