What do Covid-19, Ebola, Lyme and AIDS have in common? They jumped to humans from animals after we started destroying habitats and ruining ecosystems.
Credit…Illustration by Mario Hugo
By Ferris Jabr
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It might have started like this: One afternoon last year, somewhere in China’s mountainous Yunnan province, a hunter entered a limestone cave. As he stepped carefully along the slick and uneven surface, his headlamp illuminated ruffled curtains of stone and walls popcorned with kernels of calcite. He continued through a series of smaller chambers until he reached a narrow passageway that reeked of ammonia — exactly what he was hoping to find. He stretched a fine-meshed net across the passage, sat down in a relatively dry area and waited.
As dusk fell, thousands of horseshoe bats — small and agile with baroquely furrowed noses — began streaming from the cave to hunt for insects. There were so many flying so close together that some of them could not avoid the net. Once a majority of the bats were gone, the hunter untangled the dozen or so he caught, dropped them into a cloth sack and collected some fresh guano from the cave floor. The next morning he took most of the bats to vendors at a nearby wildlife market, where they were stored in cages alongside peacocks, bullfrogs, rat snakes, soft-shell turtles, mouse-deer, ferret badgers and foxes, all being sold for their meat, fur or their supposed medicinal properties. After selling the guano to farmers to be used as fertilizer, he took a few of the plumpest bats to restaurants he had been personally supplying for years.
Although he didn’t realize it, the hunter had caught much more than his quarry. Like all animals, the bats were planets unto themselves, teeming with invisible ecosystems of fungi, bacteria and viruses. Many of the viruses multiplying within the bats had circulated among their hosts for thousands of years, if not longer, using bat cells to replicate but rarely causing severe illness. Through chance mutations and the frequent exchange of genes, one virus had acquired the ability to infect the cells of certain other mammals in addition to bats, should the opportunity ever arise. When the hunter entered the limestone cave, he provided the virus with a new path to follow, one that led out of the damp crevices it had always known, out of the countryside, into the world at large.
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Perhaps the hunter was contaminated by guano in the cave, transferring the virus to his nose or mouth with an absent-minded gesture. Maybe a market vendor or cook was infected by a splatter of blood or feces when a bat was skinned and gutted, passing the virus to co-workers and customers in the subsequent days and weeks. As the many stressed and injured animals in the market bled, drooled and defecated on one another, the virus might have initially jumped from bats to another caged creature, such as a pangolin — a small scaly mammal that looks like an armadillo wearing an artichoke — hybridizing with that animal’s viruses before leaping again to humans. When the chefs, traditional healers and other buyers browsed the market, they may have inhaled infectious droplets or touched contaminated surfaces, starting new chains of infection throughout the region as they returned to their homes and workplaces.
At first, the virus might have proliferated at a rate sufficient to sustain itself, but not high enough to create noticeable clusters of infection. Eventually, through pathways of contagion linked to the trade and consumption of wildlife, the virus journeyed from villages in rural China to the city of Wuhan: a modern metropolis of more than 10 million people, each a potential host with no immunity, living in dense clusters. Soon it was moving rapidly from person to person in restaurants, offices, apartment complexes, hotels and hospitals. From there, it could have easily hopped on China’s high-speed rail network, reaching Beijing and Shanghai in under six hours. At some point in late 2019 or early 2020, the virus discovered a new way to travel: It boarded a 747.
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There is much we don’t know about the origins of the ongoing pandemic and some details that we may never learn. Though genetic sequencing currently indicates that horseshoe bats are the ultimate source of SARS-CoV-2, it’s possible another animal will eventually prove to be the vector. Bats may have initially infected livestock or more exotic captive creatures raised on one of China’s many wildlife farms. Perhaps the bats (or another vector) were smuggled across the southern border from a neighboring country, like Myanmar or Vietnam. Or maybe the virus was intermittently infecting animals and people in rural areas for years before finally finding a route to a major city. Regardless of SARS-CoV-2’s precise trajectory, experts agree that Covid-19 is a zoonosis, a disease that jumped from animals to humans.
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Between 60 and 75 percent of emerging infectious diseases in humans come from other animals. Many zoonoses — rabies, Lyme, anthrax, mad cow disease, SARS, Ebola, West Nile, Zika — loom large in public consciousness; others are less familiar: Q fever, orf, Rift Valley fever, Kyasanur Forest disease. More than a few, including influenza, AIDS and the bubonic plague, have caused some of the deadliest outbreaks in recorded history. Although zoonoses are ancient, thought to be referenced in Mesopotamian tablets and the Bible, their numbers have increased in the last few decades, along with the frequency of outbreaks.
Zoonotic pathogens do not typically seek us out nor do they stumble onto us by pure coincidence. When diseases move from animals to humans, and vice versa, it is usually because we have reconfigured our shared ecosystems in ways that make the transition much more likely. Deforestation, mining, intensive agriculture and urban sprawl destroy natural habitats, forcing wild creatures to venture into human communities. Excessive hunting, trade and consumption of wildlife significantly increase the probability of cross-species infection. Modern transportation can disperse dangerous microbes across the world in a matter of hours. “Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations,” David Quammen wrote in his 2012 book “Spillover,” “while human technology and behavior are spreading those pathogens ever more widely and quickly.”
More: https://www.nytimes.com/2020/06/17/magazine/animal-disease-covid.html
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