“We Are the Infectors”: Humans Transmit Viruses to Animals Twice as Often as Vice Versa!

It’s widely known that some of the most dangerous diseases affecting humans originated from pathogens that crossed over from animals. For example, the virus responsible for AIDS came from chimpanzees, and COVID-19, which led to global lockdowns for nearly two years, is believed to have originated from bats. However, a recent study suggests that this transmission isn’t a one-sided affair, and humans might be transmitting diseases to animals more frequently than the other way around.

Remarkable Study Findings
By analyzing millions of viral genome sequences, researchers discovered that humans actually transmit more viruses to animals—approximately twice as many—as they receive from them.

Published in the journal Nature Ecology & Evolution, the study examined nearly 12 million virus genomes and identified close to 3,000 instances of viruses crossing species barriers.

Around 79 percent of these transmissions, or roughly four-fifths, involved a virus moving from one animal species to another, while the remaining 21 percent involved humans.

Surprisingly, within this 21 percent, 64 percent (about two-thirds) were human-to-animal transmissions (anthroponosis), with only 36 percent being animal-to-human transmissions (zoonosis).

Which Animals are Affected by Human Infections?
Human infections were found to impact various animal species, including pets like cats and dogs, domesticated animals such as pigs, horses, and cattle, birds like chickens and ducks, and primates like chimpanzees, gorillas, and howler monkeys. Infections also extended to other wildlife like raccoons, the black-tufted marmoset, and the African soft-furred mouse.

Interestingly, wild animals were more susceptible to human-to-animal transmission.

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

How Infections Cross Species Barriers
Both humans and animals harbor numerous microbes that can jump to another species through close contact.

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

Tan explained, “Viruses can jump between different species through similar modes of transmission observed in humans, including direct contact with infected fluids or being bitten by other species.”

“However, before a virus can infect a new host, it must either possess the necessary biological tools 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 co-author of the study, noted, “The majority of pathogens circulating in humans originated from animals at some point.”

He further explained that while many species-to-species transmissions are inconsequential, in other cases, “the virus can start circulating, leading to a disease outbreak, epidemic, pandemic, or even establishing itself as an endemic pathogen.”