‘SPILLBACK’: Animals that infect humans are scary, it’s worse when we infect them back
A spillback can ignite epidemics in wild species, including endangered ones, ravaging whole ecosystems. It can establish new wildlife reservoirs that shift the pathogens’ evolutionary trajectory, unleashing novel variants.
SONIA SHAH: Having long been targeted by anti-fur activists, mink farms don’t announce themselves with signposts or list their names and addresses in directories. When I visited in July, the $20 billion global mink industry was under scrutiny for a different reason: Mink farmers had been battered by the coronavirus, which first erupted among captive mink in Europe in late April 2020 and on United States farms four months later.
By June 2021, scientists estimated, the virus had infected as many as seven million mink on more than 400 farms in Europe and North America, killing more than 700,000 of the animals, a death toll orders of magnitude greater than that borne by any other nonhuman species. By the summer of 2021, coronavirus had infected thousands of mink on a dozen farms in Utah. Four farms in the state were still under quarantine.
Inside the shed, the still air was dense with flies. On either side, rows of wire cages stacked waist high contained the intertwined bodies of mink. Most were silently prostrate on their backs, their paws limp in the air, passed out in the nearly 100-degree heat. Mink waste piled up under their cages in low, long ridges…
When the novel coronavirus first erupted on two mink farms in the Netherlands, the world’s fourth-largest producer of mink pelts, in late April 2020, the Dutch government shut down streets around the farms, conducted mandatory screenings of all mink farms, quarantined infected farms and instructed farmworkers to don personal protective equipment. It didn’t work. By early May, two more mink farms reported outbreaks.
By the end of the month, the Dutch government started gassing all the mink on affected farms, many of them kits just a few weeks old. They screened any mink who died on a mink farm for coronavirus. They banned transport of mink and of mink manure. That didn’t work, either. By the end of July, investigators detected the coronavirus on 27 mink farms in the Netherlands…
The mink outbreak that began in August 2020 in the United States eventually engulfed 18 farms in four states. Meanwhile, the seepage of farmed mink into the broader environment continued as usual. Activists “liberated” captive mink as they have for years, sneaking onto mink farms in Utah and Idaho and unlatching the cages of 2,000 mink, who fled into the moonlight… The Covid outbreak on United States mink farms eventually subsided at the end of the year after farmers slaughtered 80 to 90 percent of their animals to sell their pelts. But so long as the novel coronavirus lurks among us, susceptible species beckon…
The Covid-19 pandemic has familiarized the world with the word “spillover,” which means when microbes in the bodies of animals spread into those of humans. Less discussed is spillover’s mirror image, “spillback,” also known as “reverse zoonosis,” by which microbes move from humans into nonhuman animals. Not every pandemic-causing pathogen can spill back into nonhuman species: Some become so genetically partial to Homo sapiens that they can no longer make the crossing, while others may never get the chance.
But those that can spill over and back expand their reign in the natural world, with unexpected results for both human and nonhuman animals. A spillback can ignite epidemics in wild species, including endangered ones, ravaging whole ecosystems. It can establish new wildlife reservoirs that shift the pathogens’ evolutionary trajectory, unleashing novel variants that can fuel new, dangerous waves of disease in humans.
Some scientists suspect, for example, that before erupting in humankind, Omicron may have brewed in a nonhuman animal as a result of a spillback. Its unusually large number of mutations compared with the original variant — around 50, including more than 30 embedded in its spike protein, nearly three times as many as the Delta variant — suggest a recent past inside an unusual host that forced it to evolve novel adaptations to survive. Which species that unusual host hailed from remains obscure. Seven of Omicron’s mutations are linked to adaptation in rodents.
Any likely contender would have to be a species able to contract the coronavirus from humans and also to pass it along to both humans and nonhuman animals. So far, other than the still-shadowy creature that likely ferried the coronavirus from bats to humans in the first place, the only nonhuman species known to have accomplished that feat is Neovison vison, the American mink. There’s no evidence that mink played any role in incubating the Omicron variant, but their biology and living conditions render them ideal hosts for incubating others…
Each spillback expands ecological opportunities for the next. An infected wild mink means the pathogen’s opportunities to colonize novel species are no longer limited to intimate encounters between nonhuman animals and infected hunters, farmers or other humans who regularly come into close contact with wild and captive animals…
A typical depiction of coronavirus in humans and animals, in which the figure of the human lies at the center with an array of microbially infested animals encroaching upon her, erases such interspecies transmissions. But as spillbacks show, the directionality of viral spread is not one-way — nor is humanity its central target… By providing more opportunities for replication and evolution, each new spillback species increases the likelihood of new variants that could circumvent our fortifications entirely, or in entirely new ways.
But the heightened probability is also qualitative, embedded in the nature of our kinship with nonhuman species that are biologically similar enough to share pathogens but with social behaviors and immune responses alien to our own. Pathogens that rely on social contact often evolve toward lower virulence as a trade-off for greater transmissibility, but spillback allows them to escape that virtuous circle, with potentially devastating consequences. SOURCE…
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