Covid Spilled From Animals to Humans. Now It’s Spilling Back

A year and a few days after the World Health Organization declared Covid-19 a pandemic, there’s a palpable sense that the pendulum is swinging back: Vaccines have been approved, countries are receiving them through their own purchases or via the international collaboration called Covax , people are making plans to take up their lives again.sanitation workers cleaning stairs

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Not to be a downer, but: not so fast. A small cadre of scientists is warning that we have not paid sufficient attention to the possibility that SARS-CoV-2, the virus that causes Covid, may be with us long-term. That is not just because it might become an endemic disease , surging up periodically when population immunity dips low enough to let it gain a foothold. It’s also because we haven’t dealt adequately with the implications of the coronavirus being a zoonotic infection, one that leapt between species to cause illness in the human world.

To the degree we’ve approached that problem, it has been by investigating—via an official WHO-sponsored mission and also via conspiracy theorizing—how the coronavirus accomplished its spillover from an asymptomatic bat pathogen to a lethal human one. We haven’t yet tackled the dimensions of a second phenomenon, what researchers are calling spillback. That is the process by which the novel coronavirus jumps from humans into additional animal species, giving it new territory in which to survive and mutate, and maybe jump again. There are already signs that may be happening—and we have not yet begun to set up the systems that will tell us what the virus is doing in its new home.

“Covid-19 is first and foremost a medical public health crisis,” says Christine Kreuder Johnson, a veterinary epidemiologist and professor at the UC Davis School of Veterinary Medicine who directs the EpiCenter for Emerging Infectious Disease Intelligence, a project funded by the National Institutes of Health to detect wildlife-to-human spillovers. “But there are other professions that need to be engaged in this, from the veterinary and agriculture side as well as the environment, in terms of policy and surveillance and ongoing monitoring. We need to understand that this is going to be a long-term problem.”

We’ve always known that Covid-19 had an animal connection. The discovery that the coronavirus causing it was bat-associated came early in the pandemic, and scientists have subsequently theorized that a second, still-unknown species helped the virus make the evolutionary adaptations that allow it to infect humans.

All of that happened before Covid-19 began spreading among people in China in December 2019, breaking out into the world’s notice just before the turn of the year. But within a few months, as the coronavirus spread rapidly through the world’s human population, it leapt from people back into an animal species: minks, being raised in confinement on fur farms.
In April, workers on two fur farms in the Netherlands unknowingly passed the virus to minks being raised there. As it spread from farm to farm, health authorities decided drastic action was necessary, and hundreds of thousands of the animals were slaughtered to prevent the virus from spreading. But by July, SARS-CoV-2 was also in mink farms in Spain. By October, it had landed in Denmark, the largest producer of mink in the world outside China—and by November, the Danish government decided to kill every mink in the country, all 17 million, in order to forestall any evolution of the virus as it passed among them.

This went badly. The minks were asphyxiated and buried in giant trenches, and within a month, gases from decomposition started to push the decaying bodies out of the ground, leading to claims of mink zombies. (They were not zombies.) The new minister of agriculture—the previous one was forced to resign over the mink slaughter—vowed to have the dead minks dug up and incinerated instead.