We are the infectors: Humans pass on viruses to animals twice than the other way around!

It is a well-established fact that some of the deadliest diseases that have afflicted humans actually originated from pathogens that leapt from animals to humans. 

For instance, the virus behind AIDS originated from chimpanzees, and even COVID-19 that forced people the world over to isolate for almost two years is believed to have come from bats. However, a new study has revealed that this transmission is not a one-way street and humans may be responsible for infecting animals more than they infect us.

Analysing millions of viral genome sequences, scientists found that humans actually pass on more viruses to animals—about twice as many—as they receive from them.

The study published in the journal Nature Ecology & Evolution, as per a Reuters report, examined nearly 12 million virus genomes and identified almost 3,000 instances of viruses crossing the species barrier. 

Around 79 per cent of these transmissions or about 4/5th were instances of a virus moving from one animal species to another, while the remaining 21 per cent involved humans. 

Surprisingly, within this 21 per cent, 64 per cent (around 2/3rd) were human-to-animal transmissions (anthroponosis), and only 36 per cent were animal-to-human transmissions (zoonosis).

Human infections were found to plague a variety of animal species, including pets like cats and dogs, domesticated animals like pigs, horses, and cattle, birds like chickens and ducks, primates such as chimpanzees, gorillas, and howler monkeys. Infections also plagued other wild animals like raccoons, the black-tufted marmoset, and the African soft-furred mouse.

What's more! Wild animals, in particular, were more likely to experience human-to-animal transmission.

Cedric Tan, lead author of the study and a doctoral student in computational biology at the University College London Genetics Institute, noted, "This really highlights our enormous impact on the environment and the animals around us."

Humans and animals host countless microbes that can jump to another species through close contact. 

The study examined viral transmissions involving all vertebrate groups: mammals, birds, reptiles, amphibians, and fish.

Tan explained, "Viruses can jump between different species via the same modes of transmission that apply to humans, including direct contact with infected fluids, or getting bitten by other species, amongst others."

"However, before a virus can jump into a new host, it must either already possess the biological toolkit, or acquire host-specific adaptations, to enter the cells of the new host species and exploit their resources," he added.

Francois Balloux, director of the UCL Genetics Institute and study co-author, added, "The vast majority of pathogens circulating in humans have been acquired from animals at some point in time."

He added that many species-to-species transmissions are inconsequential, and "lead nowhere". However, in other cases, "the virus can start circulating, causing a disease outbreak, an epidemic, a pandemic or even establishes itself into an endemic pathogen." 

(With inputs from agencies)