Humans transmit twice as many viruses to animals as vice versa

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Virus transmission from humans to animals: Are we infecting our pets?

Photo: Violeta Stoimenova / Getty Images

Many dangerous infectious diseases have been transmitted from animals to humans in the past. The coronavirus is the most recent known example, but far from the first. Conversely, in research, it has so far played little role which viruses humans transmit to the animal world - and what they do with them.

According to a new study, there are such so-called jumps from humans to animals on a large scale, reports a research team in the journal “Nature Ecology & Evolution”. This means that people pass on more viruses to domestic and wild animals than they catch from them.

This process, known as spillover, has so far been studied primarily in humans: Ebola, flu and Covid-19 have their origins in the animal kingdom, and measles and smallpox are also so-called zoonoses. Factors such as increasing population density and human penetration into previously undeveloped areas increase the risk of such transmissions.

The group led by Cedric Tan from University College London (UCL) has now examined almost twelve million viral genetic sequences in order to reconstruct which viruses have spread from one host to another vertebrate species. According to the results, there were about twice as many jumps from humans to animals (called anthroponosis) than from animals to humans.

Of the 599 identified host jumps, 64 percent were considered anthroponotic. For example, Sars-CoV-2 and Influenza A have been transmitted several times to farm animals or wild animals living in captivity. Host jumps from animal to animal were also often detected.

“We should view humans as just a node in a vast network of hosts endlessly exchanging pathogens, rather than as a sink for zoonotic pathogens,” explained UCL co-author Francois Balloux.

For some animal species, humans pose an existential risk

If a disease is transmitted from humans to animals, this poses a possible risk to the survival of this species, according to the study. "For example, the human-transmitted metapneumovirus has

caused fatal outbreaks of respiratory disease in chimpanzees in captivity."

Such a spillover in a livestock population could also have an impact on food safety - for example if many animals had to be killed in order to contain the further spread. In addition, viruses often change when they jump from host to host. This shows that viruses have to adapt in order to be able to make good use of new hosts.

For viruses that already infect many animal species, the changes with a new jump are smaller - a virus with a broad host range then apparently already has many of the properties necessary for adaptation, the researchers conclude.

However, the overall picture could change significantly if the genome of significantly more viruses is recorded, say the study authors. The previous sequences only represent a tiny fraction of the existing vertebrate viruses. In particular, the extent of the jumps between animal species is still largely underestimated. If more attention is paid to these transmissions, it could possibly help prevent future pandemics.

sug/dpa