Humans pass more viruses to other animals than we catch from them

Humans pass on more viruses to domestic and wild animals than they catch from them, a new study suggests.

University College London (UCL) researchers analysed all publicly available viral genome sequences, to see where viruses have jumped from one host to infect another vertebrate species – animals with a backbone and skeleton.

Understanding how and why viruses evolve to jump into different hosts may help experts figure out how new viral diseases emerge in humans and animals.

Most infectious diseases are caused by viruses circulating in animals, and when these cross over into humans – a process known as zoonosis – they can cause disease outbreaks, epidemics and pandemics such as Ebola, flu or Covid-19.

Experts suggest that given the large impact of these diseases on public health, humans have generally been considered as a sink for viruses rather than a source, with human-to-animal transmission of viruses receiving far less attention.

In the new study scientists found that roughly twice as many host jumps were inferred to be from humans to other animals (known as anthroponosis) rather than the other way round.

Additionally, they found even more animal-to-animal host jumps that did not involve humans.

According to the scientists, their findings highlight the underappreciated fact that human viruses often spread from humans into wild and domestic animals.

Co-author Professor Francois Balloux, UCL Genetics Institute, said: “We should consider humans just as one node in a vast network of hosts endlessly exchanging pathogens, rather than a sink for zoonotic bugs.

“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.”

For the study, the research team developed and applied tools they used to analyse the nearly 12 million viral genomes that exist in public databases.

They reconstructed the evolutionary histories and past host jumps of viruses across 32 viral families, and looked for which parts of the viral genomes acquired mutations during host jumps.

The study found that, on average, viral host jumps are associated with an increase in genetic changes, or mutations in viruses, reflecting how viruses must adapt to better exploit their new hosts.

Further, viruses that already infect many different animals show weaker signals of this adaptive process, suggesting that viruses with broader host ranges may possess traits that make them more capable of infecting a diverse range of hosts.

Lead author, PhD student Cedric Tan, UCL Genetics Institute and Francis Crick Institute, said: “When animals catch viruses from humans, this can not only harm the animal and potentially pose a conservation threat to the species, but it may also cause new problems for humans by impacting food security 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.

“Additionally, 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.

“Understanding how and why viruses evolve to jump into different hosts across the wider tree of life may help us figure out how new viral diseases emerge in humans and animals.”

The findings are published in Nature Ecology and Evolution.