A lot has been written about the possibility of humans contracting diseases from animals as we push into wild places: Ebola and the virus behind the recent COVID-19 pandemic just to name two. But what about the ways in which humans are infecting animals, and the possibility that will boomerang back to us?
Bird poop suggests that could pose a problem as well. The gut microbes of wild birds that frequent urban areas, such as crows, starlings and thrushes, bear signs of antibiotic resistance that could pose a problem for people and the animals we depend on, according to a paper recently published in Current Biology.
“Wild bird populations in cities are reservoirs of bacteria resistant to many important human antibiotics,” said Samuel Sheppard, an Oxford University scientist who led the research. “There is an urgent need to understand how human activity is influencing the spread of zoonotic diseases and AMR (antimicrobial resistance).”
To look for an anthropic signature in the microbes inhabiting wild birds, Sheppard and his collaborators zeroed in on Campylobacter jejuni, a bacteria commonly found in birds’ digestive systems. In people, this it can cause diarrhea, and is often tied to eating raw or undercooked poultry.
Typically, a wild bird species will have its own genetically distinct strain of Campylobacter. But when the scientist sifted through the genomes of 700 types of the bacteria from 30 bird species spread over eight countries, they discovered something unusual. Some types of birds had been colonized by several different strains of the bacteria. And it didn’t happen at random.
The birds with the biggest bacterial variety were usually ones that lived in closer proximity to humans. American crows had three times as many versions of the bacteria as the overall average. Blackbirds and song thrushes had twice as many.
A similar pattern emerged when the scientists looked for genetic markers of antibiotic resistance in the Campylobacter. Urban birds such as crows and thrushes, as well as starlings, had higher rates of bacteria with signs of resistance to widely-used antibiotics. The same was true with water birds, including gulls, ducks and geese that might encounter water-borne contaminants.
The work echoes previous discoveries about links between loss of wild habitat and avian diseases, such as the finding that birds at California farms with less nearby wild habitat had higher rates of disease-causing microbes in their feces.
The new research points to the potential for urban settings to seed birds with pathogens that can withstand modern medicines. Those birds can, in turn, transmit these microbes to people or other valuable animals. “Wild birds have potential to transfer AMR over long distances to livestock raised for meat consumption and companion animals such as pets,” said Sheppard. “This can have economic implications for agriculture, animal welfare, and food security.”
Monitoring of such bird-borne pathogens in cities could help scientists learn still more about the pathways such diseases can follow and the role of antibiotic usage on their proliferation. So, the next time you see a guano-splattered ledge on a city building, remember, it could hold more clues than you think about how humans are leaving their imprint on the wild.
Mourkas, et. al. “Proximity to humans is associated with antimicrobial-resistant enteric pathogens in wild bird microbiomes.” Current Biology. Sept. 9, 2024.
Photo by Holger Selover-Stephan via flickr
