Humans spread more viruses to other animals than they give to us

Animals such as rats are often regarded as disease carriers. But when it comes to the spreading of illnesses, it turns out other animals have more reason to fear us than we them.

An analysis of viral genomes has found that when viruses move between humans and other animals, in 64 per cent of cases it is humans infecting other animals – rather than the reverse.

“We give more viruses to animals than they give to us,” says Cedric Tan at University College London. For instance, after the SARS-CoV-2 virus jumped from bats to humans, possibly via another species, humans passed it on to many other species.

Tan and his colleagues have been using a global database of sequenced viruses to study how they jump between species. There are nearly 12 million sequences in the database, but many are incomplete or lack data on when they were collected and from what host species.

So the researchers narrowed down the 12 million to roughly 60,000 high quality sequences with full accompanying data. They then created “family trees” for related viruses.

Altogether, they identified nearly 13,000 viral lineages and 3000 jumps between species. Of the 599 jumps involving humans, most were from humans to other animals rather than vice versa.

The team wasn’t expecting this, but in retrospect it makes sense, says Tan. “Our population size is huge. And our global distribution is basically everywhere.”

In other words, a virus spreading among humans will have numerous opportunities to jump to many other species all around the world, whereas a virus circulating in a non-human species that is limited to one region will have far fewer opportunities.

SARS-CoV-2, MERS-CoV and flu viruses were those most commonly passed to other animals via humans, the study found. This is in line with other studies showing, for instance, that SARS-CoV-2 has spread from people to pets, zoo animals, farmed animals such as mink and to wild animals such as white-tailed deer.

However, even when SARS-CoV-2, MERS-CoV and flu viruses were excluded from the analysis, the team found that 54 per cent of jumps were from humans to other animals.

The spread of viruses from humans to other species is a threat to many endangered animals, says Tan. For instance, several wild chimpanzees have died in Uganda due to outbreaks of human metapneumovirus and human respirovirus.

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