Zoonotic viruses fill us with dread—and rightly so. Such viruses have been known to spread from animals to human hosts, transmitting deadly diseases such as Ebola, rabies, and bird flu. And according to virologists and other scientists with relevant expertise, the SARS-CoV-2 virus, the source of the COVID-19 pandemic, is believed to have originated in bats, though pangolins are suspected intermediaries.

Little wonder, then, that when we think of species-jumping vertebrate-infecting viruses, we usually imagine them jumping toward us. But according to a new study from University College London (UCL) researchers, these viruses are more likely to jump in the other direction—away from us and toward other vertebrate species.

The study appeared in Nature Ecology & Evolution, in an article titled, “The evolutionary drivers and correlates of viral host jumps.”

“[We] harnessed the entirety of publicly available viral genomic data, employing a comprehensive suite of network and phylogenetic analyses to investigate the evolutionary mechanisms underpinning recent viral host jumps,” the article’s authors wrote. “Surprisingly, we find that humans are as much a source as a sink for viral spillover events, insofar as we infer more viral host jumps from humans to other animals than from animals to humans.”

The UCL researchers analyzed the nearly 12 million viral genomes that have been deposited on public databases to date. Leveraging this data, they reconstructed the evolutionary histories and past host jumps of viruses across 32 viral families, and they looked for which parts of the viral genomes acquired mutations during host jumps.

The researchers 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. This pattern was consistent throughout most viral families considered. Additionally, they found even more animal-to-animal host jumps, that did not involve humans.

The researchers’ work highlights the highly relevant and largely underappreciated fact that human viruses frequently spread from humans into wild and domestic animals.

“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 Francois Balloux, PhD, chair in computational systems biology 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.”

The findings also show that, on average, 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. 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 inherently more capable of infecting a diverse range of hosts, whereas other viruses may require more extensive adaptations to infect a new host species.

According to the article’s lead author, Cedric Tan, a doctoral student at UCL and the Francis Crick Institute, the consequences of humans transmitting viruses to animals can be dire. “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.”

Cell entry is generally seen as the first step for a virus to infect a host. However, the team found that many of the adaptations associated with host jumps were not found in the viral proteins that enable them to attach to and enter host cells, which points to viral host adaptation being a complex process that remains to be fully understood.

“Our research was made possible only by the countless research teams that have openly shared their data via public databases,” added co-author Lucy van Dorp, PhD, a senior research fellow at UCL. “The key challenge, moving forward, is to integrate the knowledge and tools from diverse disciplines including genomics, epidemiology, and ecology to enhance our understanding of host jumps.”

Previous articleTumor-Like Vasculature of TB Granulomas Inspires New Therapeutic Combos
Next articleCulprit behind Psoriasis Revealed