Deep in the limestone caves of northern Laos, scientists have discovered bats that harbor coronaviruses with startling similarities to SARS-CoV-2 — that's the coronavirus that's responsible for much of the world's worries over the past 21-or-so months. Much of COVID-19's origin story remains unknown, but this discovery is another clue pushing scientists towards understanding how SARS-CoV-2 first emerged.
The study, which is currently under peer-review and being considered for publication in a Nature Portfolio Journal, has been posted on the preprint server Research Square.
Scientists at the Pasteur Institute and the University of Laos took samples from 645 bats, belonging to 46 different species, from the wild in the limestone karstic terrain in North Laos. Among the 25 different coronaviruses they identified, at least three — ironically named BANAL-52, BANAL-103, and BANAL-236 — caught the researchers’ attention. Found in Rhinolophus affinis horseshoe bats, the study authors describe these three coronaviruses as “the closest ancestors of SARS-CoV-2 known to date.”
All three coronaviruses have a receptor-binding domain — the key part of a virus that allows viral entry into host cells through ACE2 on the surface of human cells — that’s extremely similar to that of SARS-CoV-2. The team demonstrated that the receptor-binding domains of these new-found viruses could attach to the ACE2 receptor on human cells just as efficiently as some early variants of SARS-CoV-2. This means that the viruses could potentially represent a risk for human health. Plenty of SARS-CoV-2 relatives have previously been identified, notably among wild bats in China and Southeast Asia, but none have featured this vital piece of the puzzle.
Crucially, the three bat viruses don't harbor a furin cleavage site in the spike, which plays a vital role in mediating viral entry into respiratory epithelial cells.
Since these are naturally occurring viruses found in wild bats, it could be used as evidence for the theory that COVID-19 is of zoonotic origin, meaning it jumped from an animal into a human. Nevertheless, this latest discovery is unlikely to convince ardent proponents of the fringe "lab-leak hypothesis". The minority of researchers who contend that SARS-CoV-2 was tweaked by scientists before it escaped into the outside world, like a Frankenstein experiment gone terribly wrong, often point towards the furin cleavage site of SARS-CoV-2 as "smoking gun" evidence that the virus was genetically engineered in a lab. Plenty of other coronaviruses do possess furin cleavage sites, but some argue that the ones belonging to SARS-CoV-2 contain some unusual qualities.
The three close relatives in this study do not contain a furin cleavage site, which leaves this point of detail hanging unanswered. With that small caveat in mind, the researchers who worked on this recent project argue that their discovery strongly hints that SARS-CoV-2 has a natural origin.
"The existence of these viruses discovered in the bat animal reservoir backs up the theory that SARS-CoV-2 may originate from bats living in the vast karst highlands in the Indochina peninsula, which stretches across Laos, Vietnam and China. Our results suggest that other related viruses could represent a risk for human health," Marc Eloit, Head of the Pathogen Discovery laboratory at the Institut Pasteur and a Professor of Virology at the Alfort National Veterinary School, said in a statement.