Humans Spread More Viruses to Animals, Study Indicates


Some of the deadliest diseases to strike humanity have resulted from germs that spread from animals to humans. For example, the virus that causes AIDS spreads from chimps to humans. Many specialists believe that the virus that caused the COVID-19 pandemic came from bats.

However, as a new study demonstrates, this trade has not been one-way. An examination of all publicly accessible viral genome sequences revealed a surprise finding: humans give animals approximately twice as many viruses as they give us.

The researchers examined over 12 million virus genomes and discovered roughly 3,000 cases of viruses crossing from one species to another. Of those, 79% involved a virus that spread from one animal species to another. The remaining 21% involved people. 64% were human-to-animal transmissions (anthroponosis), whereas 36% were animal-to-human transmissions (zoonosis).

Anthroponosis affected pets like cats and dogs, domesticated animals like pigs, horses, and cattle, birds like chickens and ducks, primates like chimps, 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.

“This highlights our enormous impact on the environment and the animals around us,” said Cedric Tan, a doctorate student in computational biology at the University College London Genetics Institute and lead author of the study, which was published this week in Nature Ecology & Evolution.

People and animals are hosts to an enormous number of microorganisms, which can spread to other species through close contact. The study examined viral transmissions across all vertebrate groups, including mammals, birds, reptiles, amphibians, and fish.

“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,” Tan went on to say.

“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 said.

Over the millennia, diseases such as viruses, bacteria, and fungi that spread from animals to humans have triggered pandemics that have killed millions of people. Zoonosis has been the key concern in terms of deadly new infectious illnesses.

“The vast majority of pathogens circulating in humans have been acquired from animals at some point in time,” said computational biologist and research co-author Francois Balloux, director of the UCL Genetics Institute.

“The current greatest threat is likely avian flu H5N1, which is circulating in wild birds. The fundamental reason recent host jumps can be so damaging is that the host species population lacks pre-existing immunity to the novel disease,” Balloux explained.

The 14th-century Black Death, which killed millions of people in Europe, Asia, the Middle East, and North Africa due to the bacterial disease bubonic plague, was caused by a bacterium that normally circulates in wild rats.

Present-day concerns, such as the Ebola virus, arose from animals.

“It is largely believed that SARS-CoV-2, the agent of the COVID-19 pandemic, likely originated in horseshoe bats and jumped into humans,” he stated.

SARS-CoV-2 outbreaks were, however, observed during the epidemic in farmed minks infected by humans.

Many species-to-species transmissions are insignificant.

“In most cases, such infections lead nowhere, as the virus is poorly adapted and there is no onward transmission in the new host,” Balloux said in a statement.

“In some cases, the virus can spread, causing a disease outbreak, epidemic, pandemic, or even establishing itself as an endemic pathogen.” Small zoonotic disease outbreaks are probably rather common, even if we miss the vast majority of them, but full-fledged epidemics are unusual events in evolutionary terms,” Balloux stated.

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