A fresh scientific investigation shows that we, as humans, pass on more viruses to animals than they transmit to us. This unexpected revelation comes following an in-depth analysis of all publicly available viral genome sequences.
Disease-carrying agents that have made the leap from animals to humans have been responsible for some of the most lethal illnesses known to mankind. For instance, the HIV virus originated from chimpanzees, while bats are believed to be the source of the virus causing the COVID-19 pandemic.
However, this recent study published in the journal Nature Ecology & Evolution reveals that this viral exchange is not a one-way street. Rather, it appears that humans transmit approximately double the number of viruses to animals than we receive from them.
The research team scrutinized almost 12 million viral genomes in their study. The team detected nearly 3,000 instances of virus transmission from one species to another.
Of these instances, 79% involved a virus moving from one animal species to another. The remaining 21% involved humans. Interestingly, of these human-involved transmissions, 64% were from humans to animals (anthroponosis) and 36% were from animals to humans (zoonosis).
Affected Animals
The animals on the receiving end of anthroponosis included a variety of species, such as:
- Pets – cats and dogs
- Domesticated animals – pigs, horses, and cattle
- Birds – chickens and ducks
- Primates – chimpanzees, gorillas, and howler monkeys
- Wild animals – raccoons, the black-tufted marmoset, and the African soft-furred mouse.
Wild animals were particularly susceptible to human-to-animal transmission than the reverse.
“This really highlights our enormous impact on the environment and the animals around us,” commented Cedric Tan, lead author of the study and a doctoral student at the University College London Genetics Institute.
Animals and humans play host to countless microbes capable of jumping to another species through close contact. The study took into account viral transmissions involving all vertebrate groups: mammals, birds, reptiles, amphibians, and fish.
“Viruses can jump between different species via the same modes of transmission that apply to humans,” Tan explained. These modes include direct contact with infected fluids or bites from other species.
However, for a virus to invade a new host, it must either already have the necessary biological toolkit or acquire host-specific adaptations to enter the new host species’ cells and exploit their resources.
Throughout history, pandemics that have claimed millions of lives have been caused by pathogens like viruses, bacteria, and fungi that crossed over to humans from animals. As such, zoonosis has been a primary concern in relation to dangerous emerging infectious diseases.
“The vast majority of pathogens circulating in humans have been acquired from animals at some point in time,” noted Francois Balloux, a computational biologist and co-author of the study.
Not all species-to-species transmissions have significant outcomes. In most cases, the virus is poorly adapted to the new host, leading to no onward transmission.
However, in some instances, the virus can circulate, causing an outbreak, an epidemic, or even establishing itself as an endemic pathogen. While small zoonotic disease outbreaks may be fairly common, full-blown epidemics are rare events, evolutionarily speaking.
The most significant current threat is the bird flu H5N1, which is circulating in wild birds. The severity of recent host jumps is primarily due to the host species not having pre-existing immunity to the novel disease.
Historically, diseases like the 14th-century Black Death, caused by a bacterium usually found in wild rodents, have resulted in mass fatalities. Modern threats, such as the Ebola virus, have similarly originated 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,” Tan mentioned. However, it’s important to note that during the COVID-19 pandemic, outbreaks were also documented in farmed minks that had been infected by humans.
