HQ Team
March 27, 2024: Humans transmit more viruses in domestic and wild animals than the other way around, scientists found after analyzing 12 million viral genomes from public databases.
Scientists from the University College London reconstructed the publicly available viral genome sequences to find out where the viruses had dumped from one host to infect other vertebrate species.
When these viruses cross over from animals into humans, a process known as zoonosis, they can cause disease outbreaks, epidemics and pandemics such as Ebola, flu or Covid-19. Most emerging and re-emerging infectious diseases are caused by viruses circulating in animals.
Given the impact of zoonotic 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, according to a university statement.
New adaptations
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, said Lead author, PhD student Cedric Tan of UCL Genetics Institute and Francis Crick Institute.
The study, published in Nature Ecology and Evolution, 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 scientists found that roughly twice as many host jumps were inferred to be from humans to other animals, known as anthroponosis. This pattern was consistent throughout most viral families considered. Additionally, they found even more animal-to-animal host jumps, that did not involve humans.
“We should consider humans just as one node in a vast network of hosts endlessly exchanging pathogens, rather than a sink for zoonotic bugs,” said Co-author Professor Francois Balloux at the UCL Genetics Institute.
“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.”
Frequent spread by humans
According to the statement, it was a “high and largely under-appreciated fact” that human viruses frequently spread from humans into wild and domestic animals.
Viral host jumps are associated with an increase in genetic changes or mutations in viruses, relative to their continued evolution alongside just one host animal, reflecting how viruses must adapt to better exploit their new hosts.
Viruses that already infect different animals showed weaker signals of the adaption process, suggesting that viruses with broader host ranges may possess traits that make them inherently more capable of infecting a diverse range of hosts.
Other viruses may require more extensive adaptations to infect a new host species and many of the adaptations associated with host jumps “remain to be fully understood.”
“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,” lead author, Tan said.
“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.”
