City birds are teeming with antibiotic-resistant bacteria, study finds
Ducks and crows in the city may offer us a connection to nature, but scientists have discovered that wild birds living near people are more likely to carry bacteria resistant to key antibiotics.
Antimicrobial resistance (AMR) is largely caused by the overuse of medicines, such as antibiotics, in humans and livestock.
The issue is very worrying: according to 2019 dataGlobally, approximately 4.95 million deaths have been associated with bacterial AMR, of which 1.27 million were directly caused by such resistance.
Wild bird species commonly found in urban areas are reservoirs of bacteria resistant to a wide range of drugs, researchers say.
“What we are essentially seeing are genes that confer resistance to antimicrobials that would be used to treat human infections,” said Prof Samuel Sheppard, co-author of the study from the Ineos Oxford Institute.
The team said their findings were important because wild birds have the capacity to travel considerable distances. Sheppard said a major concern was that these birds could pass on antimicrobial-resistant bacteria to captive birds destined for human consumption, such as those kept in poultry farms.
Sheppard and colleagues describe in the journal Current Biology how they analyzed the genomes of bacteria found in 700 samples of bird droppings from 30 wild bird species in Canada, Finland, Italy, Lithuania, Japan, Sweden, the United Kingdom and the United States.
The team specifically looked at the presence of different strains of Campylobacter jejuni – a type of bacteria that is ubiquitous in birds as a natural part of their gut microbiome. Such bacteria are a major cause of human gastroenteritis, although antibiotics are generally only used in severe cases.
Sheppard added that in general any wild bird may be expected to possess a single strain of C.jejunispecific to that species.
However, the team found that wild birds found in urban areas were much more likely to be from C.jejuni than those who live far from man.
In addition, the strains found in urban species contain about three times as many genes known to lead to antimicrobial resistance. These genes are also associated with resistance to a wider range of antimicrobials.
The authors suggest that wild birds can acquire antimicrobial-resistant bacteria in a number of ways. For example, seagulls and crows lurk near garbage dumps, while ducks and geese can acquire the bacteria from rivers and lakes contaminated by human sewage.
According to Dr Thomas Van Boeckel, an expert in antimicrobial resistance at ETH Zurich who was not involved in the study, the research is unique because it focuses on the impact of human use of antimicrobials on animals.
“What are the implications of this for the birds? We don’t really know, but it seems that we humans are responsible for this change,” he said.
Dr Danna Gifford from the University of Manchester added that the findings could have implications for human health.
“Although alarming, the risk of direct transmission of resistance from urban birds to humans is unclear. However, transmission from poultry to humans is well documented,” she said. “As urban development encroaches on agricultural land, increasing contact between urban birds and poultry raises significant concerns about indirect transmission through the food chain.”
Dr Andrew Singer from the UK Centre for Ecology and Hydrology said more samples were needed to ensure the results were accurate, but precautions could be taken.
“The obvious place to start is to ensure that birds do not congregate in our landfills, wastewater treatment plants and animal manure piles, where both pathogens and AMR are abundant,” he said. “Furthermore, we must also eliminate the discharge of untreated sewage into our rivers, which exposes all river-using wildlife – and people – to human-borne pathogens and AMR.”