Human Transmit Twice As Many Viruses To Animals Than They Give Us, Study Says

After a huge new analysis of viral genomes, scientists discovered that humans transmit more viruses to domestic and wild animals than we catch from them.

Anthroponosis Virus

The researchers examined over 12 million virus genomes and discovered roughly 3,000 cases of viruses crossing from one species to another to determine where viruses jumped from one host to infect another vertebrate species.

Of those, 79% involved a virus traveling from one animal species to another animal species. The remaining 21% involved people.

Experts said that 64% were human-to-animal transmissions called anthroponosis, whereas 36% were animal-to-human transmissions known as zoonosis.

The researchers discovered that almost twice as many host jumps were assumed to be from humans to other animals. This tendency was constant across the majority of viral families studied.

In addition, they discovered more animal-to-animal host leaps that did not involve humans.

The animals affected by anthroponosis include pets like cats and dogs, domesticated animals such as pigs, horses, and cattle, birds like chickens and ducks, primates like chimpanzees, gorillas, and howler monkeys, and other wild animals like raccoons, the black-tufted marmoset, and the African soft-furred mouse.

Wild animals, for instance, were far more likely to be infected by humans than the reverse.

"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 conservation efforts," said Professor Francois Balloux of UCL Genetics Institute.

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Genetic Changes

The findings also reveal that, on average, viral host jumps are associated with an increase in genetic changes, or mutations in viruses, compared to their continuing evolution alongside a single host species, demonstrating how viruses must adapt to better exploit their new hosts.

Furthermore, viruses that already infect a variety of animals show weaker signs of this adaptive process, implying that viruses with broader host ranges may have characteristics that make them inherently more capable of infecting a diverse range of hosts, whereas other viruses may require more extensive adaptations to infect a new host species.

Lead author, Ph.D. student Cedric Tan (UCL Genetics Institute and Francis Crick Institute), stressed that when animals catch viruses from humans, this can not only harm the animal and potentially pose a conservation threat to the species, but it may also cause new problems for humans by affecting food security if large numbers of livestock need to be culled to prevent an epidemic, as has happened in recent years with the H5N1 bird flu strain.

"If a virus carried by humans infects a new animal species, the virus might continue to thrive even if eradicated among humans, or even evolve new adaptations before it winds up infecting humans again," he added.

Understanding how and why viruses develop to jump into other hosts across the larger tree of life will help us figure out how novel viral illnesses emerge in humans and animals.

Cell entry is widely regarded as the first stage for a virus to enter a host. However, the scientists discovered that many of the changes linked with host jumps were not found in the viral proteins that allow them to adhere to and enter host cells, indicating that viral host adaptation is a complex process that remains to be fully understood.

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