- Issue: Fall 2020
- Author: Jesse Marcus
In the 14th century, flea-infested rats carried the bubonic plague across western Asia and Europe, wiping out 60% of the human population. In the spring of 1918, a particularly deadly strain of avian influenza known as the Spanish flu (thought to have originated in the United States) began to spread across the world, killing at least 50 million people. And sometime in the early 20th century, HIV likely jumped from primates to humans in the forests of Central Africa, precipitating the global AIDS crisis that has claimed more than 32 million lives and impacted the health of another 37 million currently living with the disease.
These three diseases sparked the worst pandemics in human history, killing more people than all the wars of the 20th century combined. And while they differ from one another in myriad ways, they have one critical thing in common: All are zoonotic diseases, or zoonoses, meaning they originated in animals before jumping the species barrier to humans.
“The coronavirus that causes COVID-19 joins a long and growing list of animal pathogens that threaten human health and prosperity,” says WWF president and CEO Carter Roberts. “The tragic story of zoonotic diseases is, at its core, the story of our broken relationship with nature.”
If humanity wants to rewrite that narrative, we must rethink the link between public health and the health of the natural world.
Zoonoses: A primer
To begin with, viruses are not all bad; they are a natural part of the world’s biodiversity and have an important role to play in the natural order of things. The issue at hand, however, is what makes a particular virus, or pathogen, dangerous to humans, and why.
“You want to know how species A attacks species B,” says Princeton University professor Andrew Dobson, one of the world’s top disease ecologists working on zoonotic diseases. “[The species] could be lions and zebras, it could be ferrets and mice. But it could also be viruses and their hosts.”
Even the most ravenous lion can kill only so many zebras in its lifetime. After all, the big cats spend up to 20 hours a day sleeping off their last meal. Likewise, a mosquito carrying malaria or a tick carrying Lyme disease must first bite an infected host, then spend some downtime digesting its meal before feeding again and transmitting the pathogen to another host.
But other diseases—including COVID-19—don’t suffer from the same constraints. Unlike a lion or a mosquito, a virus doesn’t have to take a break as it moves between hosts; its spread is limited only by the number of susceptible organisms in the vicinity. For example, the more people around you when you cough, the more people you’re going to infect. “That’s what makes pathogens more dangerous than predators,” says Dobson. “Predators can be satiated.”
The good news is that most pathogens that inhabit animals pose little threat to people. In order for a zoonosis to “spill over” to people and lead to an outbreak, an epidemic, or even a pandemic, it has to not only jump the interspecies barrier but also cross two other critical barriers: the intrahuman (overcoming a host’s immune system) and the interhuman (spreading to other people).
These are significant hurdles for the majority of pathogens. Many of the world’s most menacing zoonoses active today, such as malaria and Lyme disease, lack the capability for human-to-human transmission and can infect a person only through an animal vector like a mosquito. Others, like monkeypox, are transmissible but don’t have the staying power to sustain an epidemic.
The bad news is that novel zoonoses—meaning those that have not been previously identified—are emerging with increasing frequency. Three to four infectious disease outbreaks—those diseases that aren’t contained and exceed normal expectations of cases—occur each year, and up to 76% are zoonotic. Before COVID-19, just six zoonotic diseases took countless lives and together caused US$80 billion in economic damage. COVID-19 has already killed hundreds of thousands of people and is on track to destroy trillions of dollars in wealth.
Understanding what drives zoonotic disease
Today, many factors conspire to facilitate the emergence and spread of zoonoses, from increased global travel to inadequate public health mechanisms. But when it comes to the emergence of infectious zoonotic diseases, three root causes—direct drivers of zoonotic outbreaks—pose outsized risks.
Human encroachment into natural habitats
Scientific evidence indicates that the leading driver of emerging diseases is our rapid rollback of nature, as people clear forests, grasslands, and other critical ecosystems to make way for settlements, infrastructure, and commercial agriculture.
History shows that when local communities’ rights are recognized, and when they have a voice in the management of their own resources, nature and people can thrive.
Pasture and cropland already occupy around 50% of Earth’s habitable land, much of it centered in heavily developed countries. So as human activity transforms what remains of the world’s intact wilderness into a patchwork of forests, farms, and ranches—much of it to meet the growing demands of global trade and overconsumption on the part of the most developed nations—people and domestic animals come into closer and more frequent contact with wildlife, heightening the risk of exposure to pathogens.
Some 70% of the world’s forests already lie within roughly half a mile of human-modified land, and it’s in these fringes of wilderness where the likelihood of interspecies transmission is greatest. Climate change compounds the problem, as shifting precipitation patterns and rising temperatures push insects carrying diseases like malaria, Zika, and West Nile into new areas.
At more extreme levels of habitat loss, a perverse scenario arises. The risk of a person coming into contact with an infected animal decreases because the shrinking natural habitat can no longer sustain large populations of wildlife. But the risk of any given spillover turning into a full-fledged epidemic increases, because a pathogen that manages to beat the odds and infect a person now has a much larger human population through which to spread.
Sale and consumption of high-risk wildlife
Some animals, including bats, rodents, primates, carnivores, birds, and ungulates (hoofed mammals) are at especially high risk of carrying diseases, and conservationists and public health experts have long warned that the global wildlife trade can act as a superhighway for infectious diseases.
In Asia, the largest destination for the wildlife trade, markets with open-air stalls sell animal products and live animals for multiple uses, including for food and traditional medicines tied to health and culture, as well as for pets and luxury items. Many such markets operate illegally, but even in legal markets there may be illegally sourced species that are more likely to circumvent sanitary checks and other regulations designed to detect and prevent zoonotic diseases. The fact that many species hail from remote areas of the world exacerbates the risk because they’re more likely to carry diseases that people have never encountered and for which there is no known treatment or vaccine.
Severe acute respiratory syndrome, or SARS, likely arose from a wildlife market in southern China in 2002, having possibly spread from bats to palm civets and raccoon dogs and then on to humans. Evidence suggests the coronavirus that causes COVID-19 followed a similar trajectory, almost certainly originating in bats and then likely mutating via intermediate host species, before spilling over to humans.
Wild animal meat is also widely sold throughout impoverished rural areas around the world, where many people depend on it for their food and livelihoods. Meanwhile, the harvest of some wildlife for overseas markets creates another challenge by depleting the wildlife populations that local people rely on for subsistence.
Expansion and intensification of livestock production
Demand for animal protein—which has long been driven by overconsumption in the developed world—is projected to increase by more than 70% between 2010 and 2050. In developing countries, much of that growth is necessary to meet people’s basic nutritional needs.
Rapid expansion and intensification of livestock production to meet that demand could expose more people and livestock to pathogens from wildlife. Poultry, cattle, pigs, horses, and camels are all known for being susceptible to diseases from wild animals.
The Nipah virus, referred to as “the worst disease no one has ever heard of,” first spilled over to humans in Malaysia in 1998 when—due in part to deforestation of their natural habitat—infected bats fed on mango trees overhanging crowded pig pens. The bats dropped half-eaten fruit infected with their saliva; the pigs ate the fruit and transmitted the disease to farmers. Other notable zoonotic disease outbreaks, such as that of H1N1 (swine flu) in 2009, have arisen from concentrated feeding operations.
Administering low levels of antibiotics to livestock reduces the likelihood of such disease transmission. But it contributes to another problem: antimicrobial resistance—one of the biggest threats to global health, food security, and development today—resulting in approximately 700,000 human deaths every year.
Prescriptions for an ailing planet
Fortunately, conservation interventions have the potential to significantly reduce the risk of emerging diseases by addressing the key drivers described above.
Moreover, because these drivers are also root causes of climate change and biodiversity loss, protecting nature through these interventions has the potential to offer a host of benefits for conservation and human health. By keeping wildlife pathogens from crossing that first interspecies barrier to infect people and livestock, we can help stop potential pandemics before they begin.
Conserve natural habitats
First, we need to stem—and reverse as much as possible—the loss of natural habitats in disease hot spots, with a focus on keeping intensive agriculture, roads, and urban areas away from forests.
Many large multinational companies, like Walmart and McDonald’s, are already working with WWF and other conservation groups to eliminate deforestation and conversion from their global supply chains, but we desperately need more companies to join their efforts. Moreover, it’s no longer sufficient for companies merely to reduce their negative impacts on the environment. They should also invest in restoring degraded lands and rehabilitating degraded soils—investments that yield significant benefits for companies’ bottom lines as well as for nature and communities.
Governments around the world can also help by establishing and funding more conservation areas, including those managed by Indigenous communities, and by better managing those that already exist—with an equal commitment to ensuring the rights, livelihoods, and welfare of the people who share that space. When well-managed, such areas create sustainable buffers between people and potential animal pathogens. They also provide benefits to local communities, including jobs in tourism and potentially higher-quality air and water.
Creating and maintaining such benefits and developing conservation interventions in collaboration with those communities—particularly Indigenous peoples and other traditional custodians who have been historically marginalized or oppressed—are critically important.
History shows that when local communities’ rights are recognized, and when they have a voice in the management of their own resources, nature and people can thrive. For example, WWF’s Earth for Life initiative aims to help governments bring all those stakeholders together to permanently finance the protection of places like the Amazon rain forest, which delivers a vast array of ecosystem service benefits, including disease regulation.
Curb illegal and high-risk wildlife trade
Second, we need to permanently close high-risk markets and take steps to reduce consumer demand for high-risk wildlife products.
WWF has long advocated for governments to take action against the multibillion-dollar illegal wildlife trade. And as mounting evidence points to a wildlife market as the source of the COVID-19 outbreak, more experts and policy-makers are joining the chorus of voices calling for governments across the globe to immediately shut down markets that sell high-risk species for human consumption.
But while clear regulations and strong enforcement to combat high-risk wildlife trade would put a significant dent in the problem, the trade will continue to thrive so long as consumer demand exists. That’s why it’s so important to both work with and educate people regarding the dire threats the illegal and unregulated wildlife trade poses to people and wildlife—and to find shared solutions to change consumer behavior. Happily, a recent WWF public opinion survey indicates that many people in key regions of East and Southeast Asia are receptive to change: 93% of respondents say they would support national bans on illegal and unregulated wildlife markets.
Diversify people’s sources of protein
Third, we need to support equitable and reliable access to food for vulnerable communities.
Today, roughly 820 million people around the world are chronically undernourished, and over 2 billion suffer from micronutrient deficiencies. For those who lack alternative sources of protein, losing access to wild meat could result in devastating food insecurity.
At WWF, we know that any solution must be driven by science and also locally developed through direct and collaborative partnerships with the people most immediately affected. It is abundantly clear that the impacts of COVID-19 on vulnerable communities go far beyond today’s public health emergency. We can’t simply say “close the markets” without better understanding what those markets mean to, and provide for, the people who rely on them. So we know that our response, and the solutions we propose to prevent future pandemics, will be developed in partnership with those who depend on nature for survival or who are grappling with deeply rooted traditions.
Building a better future
On many fronts, this is a time of great challenge.
The list of Earth’s latest symptoms is long and foreboding: wildfires raging across California, the Amazon, Australia, and elsewhere; coastal communities hammered by storm surges; megacities shrouded with hazardous smog; crops wilting in the heat. And now we must add a global pandemic to that list.
At WWF, we know that any solution must be driven by science and also locally developed through direct and collaborative partnerships with the people most immediately affected.
For many people, the impacts of shrinking natural habitats, biodiversity loss, and climate change seem far away—tragic spectacles beamed to their TV sets from somewhere else. COVID-19 has done away with that false sense of security. It has brought the real impacts of environmental degradation into our neighborhoods and into our homes. And it has exposed glaring flaws in our relationship with the one world that we all call home.
Addressing zoonoses at their source—preventing habitat loss and degradation while stemming the direct exploitation of wildlife—could be one of the most cost-effective means of averting zoonotic disease epidemics and sustaining our own health.
This effort must be just one plank of a new platform that connects the health of nature to human health, whether it’s uncoupling our energy systems from the fossil fuels and firewood that cause respiratory illness; or addressing how climate change shifts animal and human movements, opening up new vectors for disease; or identifying where, in the ways we produce and consume food, we are putting ourselves at greater risk than need be.
“We all want to do our part to reduce the risk of future pandemics,” confirms WWF chief scientist Dr. Rebecca Shaw, who is helping define the scientific underpinnings of WWF’s plans. “As a conservation organization,” she adds, “we must remain focused on addressing the root causes, which lie at the intersection of humans and the natural world.”
“We need to cast aside the myth of infinite growth on a finite planet,” says Carter Roberts, “and instead learn to strike a better balance between people and nature.”
A glossary of scientific words and phrases related to global health