COVID virus spread too quickly for bats alone to be responsible

A virus doesn’t need wings to travel. It just needs a ride – and sometimes, that ride is a cage packed into the back of a truck. That’s what scientists are now suggesting about the ancestor of SARS-CoV-2, the virus that caused the COVID-19 pandemic.

New research suggests that the ancestor of this virus likely originated in Western China or Northern Laos. But it showed up nearly 2,700 kilometers away in Central China just a few years later – too far for its natural host, the horseshoe bat, to cover on its own.

Hidden journey of the COVID virus

The study comes from scientists at the University of California San Diego School of Medicine, working with collaborators from other institutions.

The team concluded that the virus was likely transported by other animals involved in the wildlife trade – similar to what happened during the SARS outbreak in 2002.

Horseshoe bats are known carriers of sarbecoviruses, the virus family that includes both SARS-CoV-1 and SARS-CoV-2.

These viruses don’t make bats sick, but they can jump to humans through a process called “zoonotic spillover.” It’s still debated how that spillover happens and which animals might be involved beyond bats.

Complex evolution of viruses

To trace the virus’s path, the researchers looked at viral genomes shared online and mapped out how these viruses evolved over time.

That task wasn’t easy. These viruses tend to recombine – shuffling their genetic material like a deck of cards when two viruses infect the same bat.

“When two different viruses infect the same bat, sometimes what comes out of that bat is an amalgam of different pieces of both viruses,” said Dr. Joel Wertheim from UC San Diego School of Medicine.

“Recombination complicates our understanding of the evolution of these viruses because it results in different parts of the genome having different evolutionary histories.”

Bats carrying the COVID virus

To work around the recombination issue, the scientists identified parts of the virus genomes that had not recombined. These stable sections allowed them to paint a clearer picture of the virus’s evolutionary path.

The analysis showed that sarbecoviruses, including those related to SARS-CoV-1 and SARS-CoV-2, have been circulating in Western China and Southeast Asia for a long time. As the bats moved across the landscape, the viruses moved with them – at roughly the same pace.

“Horseshoe bats have an estimated foraging area of around 2-3 kilometers and a dispersal capacity similar to the diffusion velocity we estimated for the sarbecoviruses related to SARS-CoV-2,” said Dr. Simon Dellicour from Université Libre de Bruxelles and KU Leuven.

But that wasn’t the case for the specific ancestors of SARS-CoV-1 and SARS-CoV-2. These viruses showed up far from their point of origin, and they did it fast – less than 10 years before the outbreaks.

Looking to the wildlife trade

Dr. Jonathan E. Pekar is a postdoctoral researcher at the University of Edinburgh and a graduate of the UC San Diego School of Medicine.

“We show that the original SARS-CoV-1 was circulating in Western China – just one to two years before the emergence of SARS in Guangdong Province, South Central China, and SARS-CoV-2 in Western China or Northern Laos – just five to seven years before the emergence of COVID-19 in Wuhan,” said Dr. Pekar.

That timeline makes it highly unlikely that bats alone moved the virus such a long distance. More likely, the viruses hitched a ride through the wildlife trade – moving with infected animals sold for food or fur.

Following the trail of infected animals

This theory isn’t new. Previous research suggested that palm civets or raccoon dogs helped carry SARS-CoV-1 from Yunnan Province to Guangdong Province. The latest study strengthens the idea that SARS-CoV-2 traveled in the same way.

“The viruses most closely related to the original SARS coronavirus were found in palm civets and raccoon dogs in southern China, hundreds of miles from the bat populations that were their original source,” said Dr. Michael Worobey from The University of Arizona.

“For more than two decades the scientific community has concluded that the live-wildlife trade was how those hundreds of miles were covered. We’re seeing exactly the same pattern with SARS-CoV-2.”

The findings challenge claims that SARS-CoV-2 might have come from a lab leak.

“At the outset of the COVID-19 pandemic, there was a concern that the distance between Wuhan and the bat virus reservoir was too extreme for a zoonotic origin,” said Dr. Wertheim. “This paper shows that it isn’t unusual and is, in fact, extremely similar to the emergence of SARS-CoV-1 in 2002.”

Understanding virus movement

The frequency of zoonotic spillover events is rising. Wildlife trade, urban growth, and habitat loss are bringing humans and animals into closer contact than ever before.

Scientists believe that keeping tabs on wild bat populations and tracking the viruses they carry could help us predict where the next pandemic might start.

Understanding how viruses move through the world – whether by wing or by wagon – could be a key step in keeping us safer in the years to come.

The full study was published in the journal Cell.

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