More viruses are transmitted from humans to animals than vice versa

Animals such as rats are often considered carriers of the disease. But when it comes to the spread of disease, it turns out that other animals have more reason to fear us than we do.

Analysis of the viral genome found that when viruses move between humans and other animals, in 64% of cases they are transmitted from humans to other animals, rather than vice versa.

“We give more viruses to animals than they give us,” he says. Cedric Tan At University College London. For example, after the SARS-CoV-2 virus passed from bats to humans, likely through another species, humans passed the virus on to many other species.

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

So the researchers narrowed down the 12 million to about 60,000 high-quality sequences with complete accompanying data. They then created a “family tree” of related viruses.

In total, approximately 13,000 virus lineages and 3,000 jumps between species were identified. Of the 599 jumps involving humans, most were from humans to other animals, not the other way around.

Tan says the team didn't expect this, but in retrospect it makes sense. “Our population size is huge. And our global footprint is basically everywhere.”

In other words, a virus that circulates among humans has many opportunities to spread to many other species around the world, whereas a virus that circulates among non-human species confined to a single region does not. That's far less.

Studies have found that SARS-CoV-2, MERS-CoV, and influenza viruses are the viruses most commonly transmitted by humans to other animals. This is consistent with other studies showing, for example, that SARS-CoV-2 spread from humans to pets, zoo animals, domestic animals such as mink, and wild animals such as white-tailed deer.

However, even when SARS-CoV-2, MERS-CoV, and influenza viruses were excluded from the analysis, the researchers found that 54 percent of infections were from humans to other animals.

The spread of viruses from humans to other species is a threat to many endangered animal species, Tan said. For example, outbreaks of human metapneumovirus and human respirovirus have killed several wild chimpanzees in Uganda.