Risk of Virus Spillover From Animals to Humans Increases Through Wildlife Exploitation and Domestication

Animal domestication and human exploitation of wildlife—via hunting, trade, habitat degradation and urbanization—is increasing the risk of animal viruses being transmitted to humans, a team of scientists has said.

According to the research published in the journal Proceedings of the Royal Society B, the processes leading to declines in wildlife populations are also facilitating the transmission of animal viruses to humans. This process is known as "virus spillover"—as occurred with the novel coronavirus that has recently swept across the globe.

"Viruses jump species when there is close enough contact to enable transmission between an infected animal and a susceptible person. Animals in close contact can share viruses with humans by respiratory droplets, or contact with feces, urine or blood," Christine Johnson, lead author of the study from the University of California, Davis, told Newsweek.

"Certain zoonotic viruses (vector-borne viruses) have adapted to transmit between hosts by a vector, such as a mosquito or a tick," she said.

To investigate the drivers of animal-human virus transmission, Johnson and her colleagues put together a dataset of the 142 viruses that are known to have spilled over into people and the species that have been implicated as hosts.

The team then examined trends in the abundance of these species, their risk of extinction and the factors that may be driving population declines among these animals, where applicable.

"We wanted to further investigate what causes pathogen spillover. Lots of studies have looked at this from the virus perspective; quite a few have also investigated host factors, but none had yet looked at whether trends in species abundance were driving spillover risk," Johnson said.

The researchers say the results provide a global view of spillover risk, linking trends in species abundance to the likelihood that they share viruses with us.

"Spillover risk scales up with increasing global abundance of species," Johnson said. "Disease transmission has been especially common from domesticated species and wildlife that have adapted to the way we've changed the landscape."

According to the study, domesticated animals—including livestock—have been the largest source of virus spillover to date, which is perhaps not surprising given their large numbers and our frequent close interaction with them.

Another significant source were wild animals that have been increasing in abundance and have adapted well to environments where humans dominate, such as some rodents, bats and primate species living near people.

Finally, the findings show that human-driven activities that have led to losses in wildlife habitats have increased opportunities for animal-human interactions, leading to a higher risk of virus spillover.

"We also found that some of the same human activities that caused species declines have also led to transmission of zoonotic viruses from animals to people, namely human exploitation of wildlife—i.e. hunting, or the wildlife trade—and human encroachment into natural habitat," Johnson said.

"We provide evidence of how human activities have driven spillover risk from wildlife. Viruses in animals can be either pre-adapted or be evolving to infect humans. Spillover of new viruses are fortunately rare events but it's probably more common than we realize," she said.

Indonesian market, dead bats
Dead bats for sale hang in an Indonesian market. University of California, Davis

According to Johnson, virus spillover and subsequent epidemics in people are the result of many ecological and epidemiological factors, most of which are usually identified after an outbreak has occurred.

"Humans have drastically changed the planet and nearly a third of all vertebrate species are threatened or endangered," Johnson said. "Exploitation of wildlife, through hunting, capture, and the wildlife trade, typically involves very close contact that facilitates disease transmission. Live wild animals sold in markets where animals and people mix in high density and close contact present the perfect opportunity for host jumping between diverse and different species that would normally never come together in the natural world."

"As natural habitat is diminished, wildlife come into closer contact with people. Wildlife also shift their distributions to accommodate anthropogenic activities and modification of the natural landscape. This has hastened disease emergence from wildlife and put us at risk of pandemics," she said.

The researchers say the latest data could help authorities prepare for pandemics and prevent outbreaks of disease.

"We hope we can shift thinking from pandemic response to pandemic prevention. Disease emergence that occurs anywhere can affect us all and we need to all understand the impact we are having on the natural world to find more sustainable ways to co-exist," Johnson said.

The latest study was published on the same day that more than 200 organizations from across the world—including World Animal Protection and Humane Society International—issued a letter to the World Health Organization urging them to endorse a permanent ban on live wildlife markets and the use of wildlife in traditional medicine. The letter notes that the ongoing COVID-19 pandemic has been associated with a wildlife market in Wuhan, China

"The current COVID-19 pandemic has illustrated just how deadly the wildlife trade can be, not just for the wild animals involved, but also for people throughout the world. COVID-19 has killed thousands of people and will likely have lasting negative impacts on local and global economies," Teresa Telecky, vice president of wildlife at Humane Society International, said in a statement.

"It is a tipping point that governments globally must not ignore. Wildlife markets worldwide are a petri dish for the next global pandemic, so governments across the globe must act to permanently ban the wildlife trade, including for food, medicine, fur, pets and other reasons. Governments must also help those traders involved to find new livelihoods as quickly as possible."