END-VOC researchers analysed nearly 60,000 viral genomes and found that there are about twice as many jumps from humans to animals than vice versa
END-VOC researchers have published a surprising finding after analysing nearly 60 thousand viral genomes: viral jumps from humans to other animals (anthroponosis) are more frequent than from animals to humans (zoonosis). Even more common are jumps from animal to animal host that do not involve humans. These findings will help us better understand viral evolution and better prepare for future outbreaks of new diseases.
Three out of every four emerging human infectious diseases are caused by viruses that normally circulate in animal reservoirs. Given the enormous impact of zoonotic diseases such as COVID-19, Ebola, or influenza, research efforts have mostly focused on viruses that jump from other animals to humans (zoonosis), while jumps in the other direction (anthroponosis) have received far less attention. But anthroponotic viruses also pose a threat: they can cause fatal diseases in endangered species, impact food security, or become more pathogenic for humans after evolving in animal reservoirs.
“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 biodiversity conservation efforts,” says François Balloux, UCL researcher and work package leader in END-VOC.
More jumps from humans to other vertebrates than vice versa
Balloux and his UCL colleagues Lucy van Dorp and Cedric Tan analysed almost 60,000 viral genomes that have been deposited on public databases to date. They reconstructed the evolutionary histories (or phylogenetic trees) of vertebrate-associated viruses across 32 families, which allowed them to identify putative host jumps, as well as their directionality. Among possible jumps involving human hosts, 64% went in the human to animal direction, while 36% went in the other direction.
“For SARS-CoV-2, we reconstructed a fair number of jumps from humans to other animals, but also several subsequent jumps from animals back into humans,” says Balloux. This has clear implications for the emergence of new viral variants. For MERS, another coronavirus believed to jump regularly from camels to humans, the research team found evidence of considerable human to human transmission, as well as human to camel spillovers. “These findings are in line with reports suggesting that MERS-CoV circulates at higher rates in humans than generally acknowledged in the Arabic Peninsula” says Tan.
The findings also indicate that host jumps are associated with an increase in new mutations, although viruses that infect a wide variety of hosts show weaker signals of this adaptive process, likely because they target host cell machinery that is more conserved across different animal species.
Huge gaps in viral surveillance
Global genomic surveillance is key for preparing to outbreaks of new diseases. But the analysis also revealed massive gaps in the genomics data currently available: the great majority of the 12 million viral sequences on public databases (roughly 86%) were human-associated; the rest were mostly associated to domestic animals, while only a small fraction were associated with wild vertebrate species. Most samples were collected from the USA and China, whereas samples from Africa, Asia and Southern America were highly underrepresented.
“About 79% of host jumps that we identified were between non-human animals. But this is likely to be a massive underestimate as our surveillance focuses on viruses that infect humans and, to some extent, domestic animals,” says Balloux.
Reference
Cedric CS Tan, Lucy van Dorp and François Balloux. The evolutionary drivers and correlates of viral host jumps. Nat Ecol Evol (2024). https://doi.org/10.1038/s41559-024-02353-4
