From chikungunya to echinococcosis to bird flu, animals can transmit many diseases to humans. But the reverse is also true. In fact, UK research shows that people, too, pass viruses on to wild and domestic animals.
Researchers at University College London made this discovery after analysing close to 12 million viral genomes deposited in public databases. They were able to reconstruct the trajectories of viruses that jumped from one host to another vertebrate species, within the framework of around 30 viral families.
Human health is closely linked to that of wildlife and ecosystems. Viruses, bacteria and parasites can take many twists and turns before infecting our species, thus becoming what are commonly known as zoonoses.
When these pathogens cross the barrier between animals and humans, they can trigger epidemics and even pandemics, as was the case with Covid-19.
Until now, zoonotic transmission was thought to occur at the expense of humans, who have generally been seen as a “sink” for pathogens rather than a source. But the authors of this research found that humans frequently spread viruses to wild, farmed and domestic animals.
In fact, they identified twice as many human-to-animal infections than the other way around. “When animals catch viruses from humans, this can not only harm the animal and potentially pose a conservation threat to the species – it may also cause new problems for humans by impacting food security,” lead author Cedric Tan said.
He cited, for example, “if large numbers of livestock need to be culled to prevent an epidemic, as has been happening over recent years with the H5N1 bird-flu strain”.
This phenomenon is all the more worrying given that, with each change of host from one species to another, viral genomes are modified to better adapt to their new host.
“If a virus carried by humans infects a new animal species, the virus might continue to thrive even if eradicated among humans, or even evolve new adaptations before it winds up infecting humans again,” Tan added.
The scientists hope that their findings, published in the journal Nature Ecology & Evolution, will encourage more research into the impact of human-transmitted viruses on wildlife.
“By surveying and monitoring transmission of viruses between animals and humans, in either direction, we can better understand viral evolution and hopefully be more prepared for future outbreaks and epidemics of novel illnesses, while also aiding conservation efforts,” co-author Francois Balloux noted.
Hopefully, this will enable us to anticipate emerging infections before any of them becomes a pandemic.