Viruses are more transmitted from humans to animals than vice versa

With the onset of the pandemic from COVID-19 we have all become more or less familiar with the concept of spillover, i.e. the “overflow” (literal translation) of a certain pathogen from its natural reservoir towards a new host. In other words, the so-called species leap, which we have also become accustomed to imagining in only one sense: from animals towards human beings and not the other way around. But a study published on Nature Ecology & Evolution seems to overturn this perspective. From the reconstruction of the evolutionary history of 32 families of viruses, it emerged that the cases of anthroponosis, in which the virus spreads from humans to other animals, are approximately double compared to those of zoonosis, in which the virus instead follows the trajectory opposite.

“We should consider humans as one node in the vast network of hosts that continually exchange pathogens, rather than as a catchment area for zoontic microbes,” he emphasizes François Balloux, co-author of the study and lecturer at the Genetics Institute at University College London (UCL). For the study, the group of researchers analyzed approximately 12 million viral genomes deposited in public databases, from which it was possible to reconstruct the evolutionary history and species jumps relating to 32 families of viruses. The analyzes also made it possible to identify the mutations acquired following the events of spillover.

The researchers observed that, on average, the probability of mutations increases for those viruses that have carried out at least one mutation in their evolutionary history species jump, unlike those that continued to evolve within the same host. This is also understandable on an intuitive level, given the probable need to adapt to the new context. At the same time, the analyzes revealed that viruses that basically infect many different species have a lower need for adaptation (i.e. to undergo mutations) when they make a species leap, i.e. when they start to infect a host different from those they already infected before. It is possible, the authors of the study hypothesize, that viruses that already infect many different species intrinsically possess characteristics that give them this ability without having to undergo further adaptations.

As anticipated, it then emerged that, contrary to what one might think, species jumps from human beings to other animals are even more frequent than the opposite. A fact that is definitely worthy of attention: “When animals contract viruses from humans, this can not only cause harm to the animal and potentially pose a threat to the conservation of the species, but can also cause new problems for humans, affecting safety feed if a large number of farm animals need to be culled to prevent an epidemic,” he explains Cedric Tan, first author of the study and PhD candidate at UCL’s Genetics Institute and Francis Crick Institute. “Moreover – continues the researcher -, if a virus carried by humans infects a new animal species, the virus may continue to thrive even if eradicated among humans, or even evolve new adaptations before ending up infecting humans once again “.

According to the authors of the research, studying how viruses evolve, especially after making a jump between species, is essential to better understand how new viral diseases emerge and become dangerous for humans or animals. And, indeed, it seems that there is still a lot to learn. The study has in fact highlighted that many of the mutations associated with spillover they do not necessarily concern the viral proteins involved in the fusion process with the host cell – such as the now well-known spike protein of SARS-CoV-2. This indicates that the process of adaptation of the virus to the new host is probably more complex than we think and has yet to be fully understood. “By monitoring the transmission of viruses between animals and humans, in both directions – concludes Balloux – we can better understand viral evolution and hopefully be better prepared for future outbreaks and epidemics of new diseases, while aiding efforts to storage”.

v