RELEASED from quarantine in a hotel in Wuhan, China, this January, Peter Daszak made for the wildlife market linked to the first cases of a mystery pneumonia in the closing days of 2019. Back then, the Huanan seafood market was a jostling scrum of stalls selling not just seafood, but all manner of domestic and exotic wild animals, the living cheek by jowl with the dead.
It is now an empty shell, closed since the first cluster of cases of what morphed into the covid-19 pandemic. Daszak, a zoologist, visited earlier this year as a member of the World Health Organization-backed team sent to investigate the origins of the virus causing that illness, SARS-CoV-2, and assess what role the now-infamous market might have played.
No one yet knows, and hypotheses will take years to test. But it is clear that the Huanan outbreak was just a symptom of a sickness, not a cause of it. For two decades, evidence has been building of the link between how we encroach on, degrade and exploit the natural world and the risk of “zoonoses” – animal diseases that spill over into humans.
Some of those links are still fuzzy, and there are competing views on how important each is. “It’s big and complex, and there are quite a lot of unknowns,” says Christian Walzer at the Wildlife Conservation Society in New York. But we know enough to say one thing: if we don’t act on what we have already learned, the costs to human health and wealth of pandemics such as covid-19 will just keep on recurring.
That certainty comes not least because we do now know that a lot of candidate diseases are out there.”The last 15 years has seen a real explosion in the understanding of how many potential pathogens there are,” says Walzer. A UN biodiversity panel report last year estimated that there are 1.7 million undiscovered viruses in animals. Not all will acquire the traits they need to infect us (see “How diseases jump to humans”). But about five new infectious diseases in people are identified every year, and 70 per cent of emerging diseases are caused by microbes of animal origin.
One major factor driving these developments is the growing human population and rising living standards. Both of these fuel our need for land, and so our encroachments into nature’s habitats. Every new road or mine, or area of forest cleared for agriculture, increases the chance that people come into contact with species carrying potential spillover diseases. “We have these edges of destruction, the disturbed areas, often only several hundred metres in. You trap for food, hunt, you collect firewood, you may be driving livestock in,” says Walzer. “You’re creating this interface, and that does increase the contact rate.” In that sense, the increasing risk of zoonoses is essentially just a numbers game.
According to research by Daszak, who works at the EcoHealth Alliance in New York, and his colleagues, highly biodiverse tropical regions, where land use is often changing rapidly, are hotspots for emerging zoonotic diseases. The most frequent spillover occurs around people’s homes and fields. For some diseases, we know in some detail how our activities helped it occur. One of the best described instances is the Nipah virus. Its jump from bats to pigs to people in 1998 followed years of the intensification of pig farming and deforestation in Malaysia that increased the interface between the species. There is still no vaccine for Nipah, which kills up to 75 per cent of people it infects. Another example is the Hendra virus in Australia, where deforestation is linked to bats carrying the virus reaching people and horses. SARS-CoV-2, originally also thought to have arisen in a bat, may one day be added to the list.
The growing interface with other animals created by our expanding cities, farms and extractive industries is just one aspect of the story, however. Another, perhaps even more important one, is how human encroachment is changing the composition of ecosystems. “It’s this idea of how habitat degradation is leading to sick landscapes,” says David Redding at the Zoological Society of London.
Case studies suggest that as we disturb habitats, we alter the community of animals in ways that increase disease risk. Take Lyme disease, which is carried by ticks found across the northern hemisphere. As forests become degraded, they support fewer larger-bodied species that are better at removing ticks, while smaller tick-friendly species such as rodents thrive. More hosts means more ticks to bite us, and more risk of the disease spreading.
Despite such compelling evidence, Redding was initially sceptical that the sick-landscape hypothesis was generally applicable. But last year, he and his colleagues examined data on disturbed and undisturbed ecosystems across the world, gathered as part of a project looking at emerging diseases called PREDICT. What they found dispelled his doubts: disturbed landscapes such as cities and plantations that have replaced natural forest have both a higher proportion of disease-carrying species and greater numbers of them too.
Redding hasn’t yet been able to quantify what that means for the risk of people becoming infected. Kimberly Fornace at the London School of Hygiene & Tropical Medicine is involved in one effort to find out, using GPS collars on macaques and GPS tracking on human volunteers to map the emergence of a zoonotic malaria in Malaysian Borneo.
“We’ve found exposure to infected mosquitoes is often higher at forest edges, where mosquito, macaque and human habitats overlap,” she says.
But we should be wary of a simplistic idea that biodiverse habitats are a hotbed of pathogens all waiting to jump to us. “The problem is that story doesn’t wash, it’s wrong,” says Felicia Keesing at Bard College in Annandale, New York. “It assumes all the elements of biodiversity are equally dangerous to us. They are not.”
Five groups of animals are the most likely to carry zoonotic diseases that spill over into humans, says Keesing. They are rats, bats, primates, carnivores such as cats and dogs, and cloven-hoofed animals such as sheep, goats, cows and camels. Camels were the source of infection for the coronavirus behind Middle East respiratory syndrome, or MERS, which has seen sporadic outbreaks since its first appearance in 2012.
Why it is these five groups is still being explored. With primates, it is probably because they are close relatives to us and the same pathogens can more easily infect us. For carnivores and hoofed animals, it may be due to our proximity to them. With rats and bats, it could partly be because of their large number.
“More often than not, a pathogen has passed through more than one animal”
Blaming any one animal or group of animals for a zoonosis can be difficult. Take bats, often mentioned as the ultimate source of SARS-CoV-2. “You’ve been told a story about bats being the most dangerous group,” says Keesing. Although another virus in horseshoe bats is 96 per cent identical to SARS-CoV-2’s genome, the missing 4 per cent is a reminder that the “vast majority of pathogens come from lots of hosts”. More often than not, a pathogen has passed through, and mutated in, more than one animal before it arrives in humans. Different species coming together, sometimes because of humans, is where disease risk often shoots up.
The coronavirus SARS-CoV-1, which caused the 2002-04 severe acute respiratory syndrome (SARS) outbreak, illustrates her point. The virus has been traced back to cave-dwelling horseshoe bats, but there is no direct evidence of transmission from bats to humans. One possibility, uncovered by a team of researchers including Daszak in 2017, is that masked palm civets at a wildlife market in Guangdong, China, provided an intermediary.
The stories of SARS-CoV-1 and SARS-CoV-2 suggest that it isn’t just our encroachment into nature’s space that increases the risk of zoonosis, but how we increasingly trade in and transport wildlife over large distances (see “Market sources”). Better regulation of the legal international trade in animals would help stem the risk of pandemics, says John Scanlon, the former head of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Secretariat, which oversees that trade. He is calling for amendments to the CITES treaty, which came into force in 1975, that build animal health into its decision-making, as well as a new global agreement to tackle the illegal trade. “Spillover can happen in any country, and to succeed we need to take a global approach,” says Scanlon, now at the End Wildlife Crime initiative.
It isn’t as if any of these things are new. In the wake of the SARS outbreak, conservationists and health experts drew up the “Manhattan Principles”, a list of 12 recommendations for preventing future zoonotic epidemics, based on the realisation that the health of humans, other animals and ecosystems are interrelated. Walzer was part of a team that in 2019 began updating those principles, in an effort that just predated covid-19. “Although the wording of the principles might seem prescient, this pandemic was predicted and largely inevitable, and will happen again if decisive actions are not taken,” they write in the resulting paper, which has just been published.
If there is any silver lining to the covid-19 pandemic, it might be that we learn to take seriously the idea that humans aren’t apart from the natural environment, but a part of it, says Steve Unwin at the University of Birmingham, UK. He is a veterinarian who works with wildlife regularly infected by pathogens from humans. “We often put humans at the end of a chain, but we are part of a web of spillover. Pathogens don’t know directions,” he says. That is well understood in scientific circles, but we will only get better at reducing the risk of pandemics if this realisation spreads to governments, too, he says.
Beyond that, we need to fundamentally rethink our interactions with animals and how we are changing their habitats. “Nature is unpredictable. We tend to get very narrow – you know, we’ve got to stop SARS-CoV-3,” says Redding. “But actually, we’ve got to think broadly about how we interact with animals and how we disturb landscapes.” His research suggests that misplaced responses to the covid-19 pandemic – such as a backlash against bats that led to their culling – would simply disturb environments and make matters worse. A simple positive step, however, would be to stop our removal of predators that check the numbers of disease-carrying species such as rodents.
“The covid-19 pandemic was inevitable and will happen again if decisive actions aren’t taken to protect nature”
Ecological restoration – creating more protected areas, allowing forests to regrow – is also key. The UN has designated this the Decade on Ecosystem Restoration. Now we need governments to step up and take action to make that happen. “One of the big frontiers is to what degree we can restore habitats,” says Keesing.
Decisions that individuals make in all parts of the world can play their part, too. As Daszak points out, encroachment of people into biodiverse areas is largely to supply demand for goods in richer countries, be it palm oil from Malaysia and Indonesia or meat from cattle ranches in South America. “We are sleepwalking into these disasters. We just need to open our eyes about the decisions that we make,” says Redding. “People consuming make decisions that impact the likelihood of future spillovers.”
He suggests shifting to more plant-based diets as one step people can take. “It’s all the choices that we make: what we eat, what we wear,” says David Quammen, author of the 2012 book Spillover. There are signs some governments in richer countries are stepping in the right direction, putting checks or bans on the import of goods that disrupt ecosystems. In November, the UK passed a law to that effect. Pressure is on the European Union to do similar.
For now, research on the links between our destruction of nature and emerging diseases is growing fast. It is also nascent and sometimes woolly: Redding likens the science to where we were in the 1970s on climate change. But we don’t need to wait: there are a multitude of other reasons to protect habitats and biodiversity, from the fresh water they provide to the carbon they store. It is rather like the point made by a famous 2009 cartoon, where a delegate at a climate summit asks: “What if it’s a big hoax and we create a better world for nothing?”. As Redding puts it: “What’s the downside of trying to better protect ecosystems and keep them more intact?”
How diseases jump to humans
The pathogens other animals host include bacteria, viruses, fungi and other assorted parasites. They can spread to humans by direct contact, for example by eating the animal.
Often, though, another host plays the role of an intermediary. Genetic analysis has suggested – although it remains far from certain – that SARS-CoV-2, the virus behind the covid-19 pandemic, may have spread to humans from an original bat host via anteater-like animals called pangolins.
These intermediate hosts can provide an opportunity for a potential pathogen to acquire the genetic traits it needs to infect people. The closest known relative to SARS-CoV-2, a coronavirus designated RaTG13 that infects the Rhinolophus affinis horseshoe bat, doesn’t appear to have a spike protein suited to bind to human receptors.
If a virus does spread to a human, the danger comes when it successfully replicates in them. Fortunately, most animal viruses can’t do this. To have the potential to unleash an epidemic, a pathogen must acquire the ability to transmit from human to human, which is what SARS-CoV-2 did.
Finally, once it has the ability to replicate in humans, a pathogen can evolve, as SARS-CoV-2 has with its new variants – or as HIV (human immunodeficiency virus) did, eventually becoming, as its name suggests, a human-only virus.
It would be hard to create a better breeding ground for diseases to jump from animals to humans than live animal markets. Coronaviruses in particular are good at recombination, in which two viruses infect the same living cell and obtain new genetic material from each other. This helps them leap from species to species. Keeping live animals close together in cages, some ill, stressed and shedding more pathogens as a result, and excreting and sneezing on each other, increases the likelihood of this.
“Wild animal markets, with wildlife brought in from across the region and often globally, are just an inherent risk,” says Christian Walzer at the Wildlife Conservation Society. “It’s just silly to do that nowadays. What we’ve created with these markets is super interfaces.”
The trade routes to the markets also contribute. The wildlife trade in China readily mixes legally and illegally captured and traded animals, both farmed and wild-caught. “Viruses exploit this pathway very well, as they did with SARS, and probably with covid,” says Peter Daszak at the EcoHealth Alliance.
Slow, long, cramped transport of live animals along trade routes seems to be high risk. Daszak and his colleagues found no coronaviruses in wild pangolins in Malaysia over a decade. But the identification of viruses in trafficked pangolins that are similar to the SARS-CoV-2 virus that causes covid-19 suggests that recombination of bat and other mammal viruses could have happened over months of transport, says Daszak. Similarly, a higher prevalence of coronaviruses has been found among bamboo rats in Vietnamese markets – at the end of trade routes – than in the farms where they are raised.
The exact route of SARS-CoV-2 to Wuhan in China isn’t yet known. “The most likely scenario for the emergence of covid-19 is a person involved in the wildlife trade got infected by bats in rural China and spread the disease through their social network in the wildlife trade, into the Huanan seafood market in Wuhan,” Daszak said before the World Health Organization’s recent mission to China. “Or perhaps it got into one of these farmed wildlife species and moved through the trade network in animals.” In an update on 9 February, the WHO team said it was likely that the virus spread from a bat via an intermediary animal to people. But that intermediary is unknown and, so far, no wildlife in China has tested positive for SARS-CoV-2.
Regardless of the details of this virus’s origins, addressing wildlife markets and trade routes, and the way animals are farmed or captured, is an effective way of reducing the likelihood of future pandemics. China and Vietnam have committed to a permanent ban of the wildlife trade and markets, but conservationists fear the problem may just get displaced to other countries.
Millions of people worldwide also rely on bushmeat for sustenance, including Indigenous peoples, and their rights and needs must be protected, says Walzer. One solution might be a ban on commercial trade focused in large cities where eating wildlife is a luxury and doesn’t constitute a nutritional necessity.
About this feature
This feature is the second in a series of five produced in association with the United Nations Environment Programme and UNEP partner agency GRID-Arendal. New Scientist retains full editorial control over, and responsibility for, the content. Part three of the series next week will look at how developing a rescue plan for nature post-covid-19 means developing a rescue plan for the people who depend on it