Deadly viruses are spilling over from animals as humans keep destroying their habitat, says study
Processes that create wildlife population declines also enable the transmission of animal viruses to humans
The exploitation of wildlife through hunting, trade, and human activities such as farming and urbanization leading to habitat destruction increased human-animal interaction and facilitated the jump of dangerous viruses like COVID-19 from animals to humans. Many experts believe that the COVID-19 virus was passed from bats to a mystery animal species that then passed it on to people.
A recent study provides new evidence for assessing spillover risk in animal species and highlights how the processes that create wildlife population declines also enable the transmission of animal viruses to humans. Spillover of viruses from animals is a direct result of human actions involving wildlife and their habitat, according to the researchers from the University of California (UC) Davis, US and the University of Melbourne, Australia.
The consequence, explain the experts, is that animals are sharing their viruses with humans.
“Exploitation of wildlife by humans through hunting, trade, habitat degradation, and urbanization facilitates close contact between wildlife and humans, which increases the risk of virus spillover, Many of these same activities also drive wildlife population declines and the risk of extinction,” says the study published in the journal Proceedings of the Royal Society B.
The scientists assembled a large dataset of the 142 known viruses that spill over from animals to humans and the species that have been implicated as potential hosts. Using the IUCN Red List of Threatened Species, they examined patterns in those species’ abundance, extinction risks and underlying causes for species declines. The data show clear trends in spillover risk that highlight how people have interacted with animals throughout history.
Domesticated species, primates and bats were identified as having more zoonotic viruses than other species. Among threatened wildlife species, those with population reductions owing to exploitation and loss of habitat shared more viruses with humans.
A key finding is that domesticated animals, including livestock, have shared the highest number of viruses with humans, with eight times more zoonotic viruses compared to wild other mammalian species. “This is likely a result of our frequent close interactions with these species for centuries,” says the study.
The top ten mammalian species with the highest number of viruses shared with humans included eight domesticated species: pigs, cattle, horses, sheep, dogs, goats, cats and camels.
Researchers also found that wild animals that have increased in abundance and have even expanded their range by adapting well to human-dominated environments share more viruses with people. These include some rodent, bat and primate species that live among people, near homes, around farms and crops, making them high-risk for ongoing transmission of viruses to people.
The analysis shows that rodents, bats, and primates have together been implicated as hosts for the majority or 75.8% of zoonotic viruses described to date. “These orders represent 72.7% of all terrestrial mammal species. As a group, domesticated mammals host 50% of the zoonotic virus richness but represent only 12 species,” says the study.
At the other end of the spectrum are threatened and endangered species, says the team. This includes animals whose population declines were connected to hunting, wildlife trade and decreases in habitat quality. According to the researchers, these species were predicted to host twice as many zoonotic viruses compared to threatened species that had populations decreasing for other reasons.
“Threatened and endangered species also tend to be highly managed and directly monitored by humans trying to bring about their population recovery, which also puts them into greater contact with people. Bats repeatedly have been implicated as a source of high consequence pathogens, including SARS, Nipah virus, Marburg virus, and Ebola viruses,” says the team.
A previous study found that bats are the natural reservoirs of a number of high-impact viral zoonoses: they host at least 61 viruses that can infect humans. Scientists from China had also warned in 2019 that it is highly likely that future coronavirus outbreaks will originate from bats.
The researchers of the current study caution that infectious diseases from wildlife have emerged at an increased pace within the last century, and are likely to continue to emerge, given expected increases in population growth and landscape change. They recommend surveillance strategies that integrate animal and human health in monitoring for emerging infectious diseases and consider environmental change that is likely to intensify close proximity animal-human interactions in the near future.
“We need to be really attentive to how we interact with wildlife and the activities that bring humans and wildlife together. We obviously don’t want pandemics of this scale. We need to find ways to co-exist safely with wildlife, as they have no shortages of viruses to give us,” says lead author Christine Kreuder Johnson, director of the EpiCenter for Disease Dynamics at the One Health Institute, a program of the UC Davis School of Veterinary Medicine, in the analysis.