Encroaching on wildlife and natural habitats has made us vulnerable to deadly pandemics such as COVID-19.
An Asian palm civet paces its cage at a market in Denpasar, Bali. Susceptible to coronaviruses such as the one that causes COVID-19, civets could be the “intermediate host” from which the virus spilled over to humans. (Photo by Luke Massey/NPL/Minden Pictures)
A YEAR AGO, THE WORLD as we knew it was about to come to an end. It was the world—sometimes called the "Before Times”—where workers commuted to offices, children shuttled to schools and we spent many of our leisure hours in restaurants, bars, churches, movie theaters, sports stadiums and other crowded spaces.
In March, all of that ground to a halt when COVID-19—a devastating disease caused by a newly discovered coronavirus—began rampaging across the United States as it already had throughout much of the world. Just two weeks after the World Health Organization declared the disease a pandemic on March 11, it had infected more than 1 million people in 171 countries across six continents, killing at least 51,000. In this country, where more than 4,300 had died by the end of the month, nearly 10 million people also had lost their livelihoods as businesses shuttered.
Most of us, understandably, were shocked by the sudden arrival of this catastrophe. But experts who study infectious disease were not so surprised. A pandemic such as COVID-19 “was something we’d been warning the world about for decades,” says Jonathan Epstein, a disease ecologist and vice president for science and outreach at EcoHealth Alliance in New York City. Scientists’ warnings had grown more urgent in recent years with the emergence of SARS and MERS, two lethal respiratory illnesses with pandemic potential that also are caused by coronaviruses.
Like SARS, MERS and many other deadly maladies—from Ebola, AIDS and Zika to pandemic flu, Lyme disease and rabies—COVID-19 is a zoonotic disease, caused by a pathogen that originally lived in animals. According to the United Nations Environment Programme—which recently co-published a scientific assessment of zoonotic diseases, or zoonoses—60 percent of the 1,400 microbes currently known to infect humans originated in wild or domestic animals. If one looks at emerging, or new, infectious diseases, that proportion jumps to 75 percent.
Scientists say that SARS-CoV-2—the virus that causes COVID-19—almost certainly originated in horseshoe bats from Asia, perhaps in China where the outbreak began in the city of Wuhan in late 2019. Based on several years surveying bat caves in the country and analyzing viral samples collected from the animals, researchers from EcoHealth Alliance and their Chinese colleagues announced in February that they had identified a virus from the intermediate horseshoe bat, Rhinolophus affinis, that shares 96 percent of its genome with SARS-CoV-2—the closest genetic match so far.
Although bats are hunted and eaten by people throughout the region, Epstein says the 4 percent difference between the two genomes suggests the virus had been circulating and evolving in bats for decades—and perhaps in other species—before it infected the first human. Likely candidates for such “intermediate hosts” include pangolins, palm civets, raccoon dogs and bamboo rats, all vulnerable to coronaviruses and either hunted or commercially raised for food or fur. Yet today, more than a year after the first known human case, Epstein says “we may never know exactly how this virus spilled over from animals to people.”
Zoonotic diseases have likely afflicted humans since our earliest days on the planet. Perhaps the most infamous, bubonic plague, is caused by a bacterium that lives in rats and is transmitted to people through flea bites. The plague, also known as the Black Death, killed an estimated 25 million people across Europe, or roughly a third of the continent’s entire population, during the 1300s.
While we’ve lived with such scourges for centuries, “the rate of zoonotic disease outbreaks has increased statistically in the past few decades,” Epstein says. It’s not difficult to see why. Viruses and other pathogens have always circulated naturally in animals, but as the human population explodes, “we’ve created more and larger interfaces between wildlife and people,” says Christian Walzer, executive director of health for the Wildlife Conservation Society in New York City. And once one human picks up a zoonotic pathogen, he adds, it potentially can spread to the opposite side of the world in a matter of hours. “More than 700 people a month fly from Wuhan to New York,” notes Walzer.
The most explicit interspecies contacts occur when humans kill animals, then consume or handle the meat, hides and other parts. Scientists have traced the AIDS pandemic back to such encounters in Africa during the early 1900s, when hunters picked up at least four simian immunodeficiency viruses from apes that eventually evolved to become the human immunodeficiency virus, or HIV.
By far the single riskiest interfaces are food markets—also called “wet markets”—that sell live animals. Poorly regulated and common across much of Africa and Southeast Asia, such markets typically feature multiple species of both wild and domestic animals housed in filthy, overcrowded cages packed tightly together. Many of these creatures are in poor health and under stress, conditions ripe for new infections. With saliva, urine and feces flowing freely—mixing with blood if animals are butchered on site—viruses and other pathogens can easily recombine and pass among different species, including humans. Walzer calls the markets “cauldrons of contagion.”
Much less obvious, human encroachment on wildlife habitat also significantly increases our vulnerability to zoonotic disease. Results of a recent study, published in August 2020 in Nature, reveal one reason why. Analyzing a database of 6,801 ecological systems worldwide—ranging from undisturbed to completely dominated by humans—scientists from University College London and their colleagues discovered that the greater the degree of human disturbance, the higher the abundance of 376 wildlife species—primarily passerine birds, bats and rodents—that are known hosts to zoonotic pathogens. At the same time, the abundance of larger mammals, including predators that prey on disease hosts, declines. In a commentary accompanying the report, disease ecologists Richard Ostfeld of the Cary Institute of Ecosystem Studies and Felicia Keesing of Bard College note that the study shows “the greatest zoonotic threats arise where natural areas have been converted to croplands, pastures and urban areas.”
Ostfeld and Keesing have seen evidence of that firsthand during two decades studying Lyme disease transmission in New York’s Hudson Valley. Where development has cut the valley’s forests into small fragments, Keesing says, “populations of white-footed mice boom because their predators and competitors have disappeared.” She and Ostfeld have found that white-footed mice not only host more of the ticks that transmit Lyme, but they also are more likely than other mammals to infect ticks with the bacterium that causes the disease. The ticks, in turn, pass it to people.
Today, the lion’s share of habitat loss and degradation takes place in the tropics, especially the old-growth tropical rainforests legendary for their diversity of wildlife—each species housing a unique microbial biome that includes pathogens. Walzer cites a recent study concluding that the world’s mammals and birds host roughly 1.7 million undiscovered viruses, some 700,000 with “zoonotic potential.”
The risk of viral spillover is heightened by the piecemeal pattern of tropical forest loss, which can quickly expand the area of direct contact between humans and other species. “If you build a road through virgin forest from point A to point B,” says Walzer, “you are establishing all along the length of that road new interfaces, or edges, with wildlife habitat.” People then can enter the forest to hunt, collect wood, grow crops or raise livestock—the latter known as “mixing bowls” where viral strains can combine to become more deadly.
Sometimes, we force wild animals to flee the forest and enter our world. That’s what happened in Malaysia during the late 1990s, when decades of deforestation and human encroachment coupled with drought sent fruit bats out of rainforests in search of food. They found it along forest edges where newly arrived industrial pig farmers also grew fruit trees. Feeding on mangoes and other fruits, the bats dropped food scraps and excrement into pig pens below. Over time, these tens of thousands of domestic animals picked up a bat virus, Nipah, they eventually passed to farmers who tended them. The Nipah virus killed hundreds of people.
If the reasons behind an uptick in zoonotic disease are clear, reversing this trajectory is not so straightforward. The most-obvious and achievable action, experts say, would be to end all commercial trade in wildlife for human consumption—both legal and illegal—especially of birds and mammals, which harbor the greatest number of zoonotic pathogens. According to Walzer, “this highly risky enterprise exists today primarily to meet demands of the wealthy in large urban centers.” Beyond public health benefits, he adds, banning commercial trade could help many imperiled species and their habitats as well as subsistence hunters who rely on declining populations of wild animals for protein. In an encouraging sign, China last February prohibited all sales of wildlife for food, though it’s unclear if the ban is permanent or will be strictly enforced. And to have a real impact, other nations that are major players in this multibillion-dollar business would need to enact similar bans.
Unfortunately, other major drivers of zoonotic disease outbreaks—specifically the destruction and degradation of natural habitats—are global crises conservationists have been battling for decades with only limited success. To make a financial argument to take more-rigorous action in the wake of COVID-19, an international, multidisciplinary team of more than a dozen experts recently estimated the total costs of cutting tropical deforestation in half, along with curbing the wild meat trade and taking other measures that would reduce pathogen spillover from wildlife. Their results? “We could do all of that for $20 to $30 billion a year, or just 2 percent of the economic and mortality costs of the pandemic in 2020 alone,” says Stuart Pimm, a Duke University conservation biologist and co-author of the report published in July 2020 in Science.
On February 1, 2021, when this article was posted , COVID-19 was still raging worldwide. During the previous year, it had infected at least 102.6 million people—killing more than 2.2 million—and had cost the global economy trillions of dollars in losses. Will this undeniable human disaster finally provide an impetus for actions that stop future zoonotic pandemics before they start? Perhaps. On the one hand, “people have short memories,” says Epstein, “but this is different than previous disease outbreaks.” Whether you’ve been sick, lost a loved one, suffered financial hardship or simply had your world turned upside down, he says, “every single person on the planet has been impacted in some way by this virus.”
The National Wildlife Federation has long supported efforts to prevent, monitor and control wildlife diseases that, as the current pandemic makes painfully clear, also can pose grave dangers to humans. Last summer, Mike Leahy, NWF’s director of wildlife, hunting and fishing policy, and his colleagues submitted recommendations to Congress on steps it can take to bolster the U.S. response to wildlife disease, including better regulating imports of potentially risky species, more funding for wildlife disease research and surveillance and federal legislation giving government the authority to declare wildlife disease emergencies. They also stressed the need to restore wildlife habitat. “By focusing more resources on healthy wildlife populations,” Leahy says, “you are reducing the risk not just to wildlife but to people.”
Laura Tangley is senior editor of National Wildlife.
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