Transmission of viruses from animals to humans is 'a direct result of our actions' © Roberto Westbrook

Transmission of viruses from animals to humans is ‘a direct result of our actions’

Contact between humans and wildlife ‘increases the risk of virus spillover', say researchers, while human activity responsible for wildlife population declines has enabled the spread of zoonotic viruses.

  • Close contact between humans and wildlife through hunting, trade, habitat degradation and urbanisation increases the risk of virus spillover.
  • Rodents, bats and primates have together been implicated as hosts for the 75.8 per cent of zoonotic viruses.
  • The top 10 mammalian species with the highest number of viruses shared with humans included dogs, cats, horses, pigs and cattle.
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As coronavirus continues to dominate headlines, the question of whether infectious diseases can be connected to environmental change remains.

Researchers have found that exploitation of wildlife by humans through hunting, trade, habitat degradation and urbanisation facilitates close contact between the two, increasing the risk of virus spillover.

According to a new study, many of these activities also drive wildlife population declines and the risk of extinction.

Published in the Proceedings of the Royal Society B, the research provides new evidence for assessing spillover risk in animal species. It highlights how the processes that create wildlife population declines also enable the transmission of animal viruses to humans.

Lead author Christine Kreuder Johnson is project director of USAID PREDICT and director of the EpiCenter for Disease Dynamics at the One Health Institute, a programme of the UC Davis School of Veterinary Medicine.

She said: “Spillover of viruses from animals is a direct result of our actions involving wildlife and their habitat. The consequence is they’re sharing their viruses with us.

“These actions simultaneously threaten species survival and increase the risk of spillover. In an unfortunate convergence of many factors, this brings about the kind of mess we’re in now.”

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.

They found clear trends in spillover risk that highlight how people have interacted with animals throughout history.

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Domesticated animals, including livestock and pets such as dogs and cats, have the highest number of shared viruses with humans, with eight times more zoonotic viruses compared to wild mammalian species, according to the study.

This is likely a result of our frequent close interactions with these species for centuries, researchers say.

The scientists also found wild animals that have increased in abundance and adapted well to human-dominated environments also share more viruses with people.

These include some rodent, bat and primate species that live among people, near our homes, and around our farms and crops, making them high-risk for ongoing transmission of viruses to people.

Researchers say 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.

They set out that bats have repeatedly been implicated as a source of “high consequence” pathogens, including Sars, Nipah virus, Marburg virus and ebolaviruses, the study notes.

Professor Johnson said: “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.

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“We need to find ways to co-exist safely with wildlife, as they have no shortages of viruses to give us.”

How do viruses jump from animals to humans?

Every animal species hosts unique viruses that have specifically adapted to infect it. Over time, some of these have jumped to humans – these are known as ‘zoonotic’ viruses.

As our populations grow, we move into wilder areas, which brings us into more frequent contact with animals we don’t normally have contact with. Viruses can jump from animals to humans in the same way that they can pass between humans, through close contact with body fluids like mucus, blood, faeces or urine.

Because every virus has evolved to target a particular species, it’s rare for a virus to be able to jump to another species. When this does happen, it’s by chance, and it usually requires a large amount of contact with the virus.

Initially, the virus is usually not well-suited to the new host and doesn’t spread easily. Over time, however, it can evolve in the new host to produce variants that are better adapted.

When viruses jump to a new host, a process called zoonosis, they often cause more severe disease. This is because viruses and their initial hosts have evolved together, and so the species has had time to build up resistance. A new host species, on the other hand, might not have evolved the ability to tackle the virus. For example, when we come into contact with bats and their viruses, we may develop rabies or Ebola virus disease, while the bats themselves are less affected.

It’s likely that bats were the original source of three recently emerged coronaviruses: SARS-CoV (2003), MERS-CoV (2012) and SARS-CoV-2, the cause of the 2019-20 coronavirus outbreak. All of these jumped from bats to humans via an intermediate animal; in the case of SARS-CoV-2, this may have been pangolins, but more research is needed.

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