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In public opinion, the coronavirus origin story seems to be well-settled: in late 2019, someone at the now-famous Huanan seafood market in Wuhan was infected with a virus from an animal.
The rest is part of a terrible story still in the works, with Covid-19 spreading from that first group in the Chinese province capital of Hubei to a pandemic that has killed some 80,000 people so far.
Stock images of pangolins, a scaly mammal that looks like an anteater, have appeared in newsletters, suggesting that this animal was the site of preparation for the virus before it spread to humans.
But there is uncertainty about various aspects of the Covid-19 origin story that scientists are trying to decipher, including the species that passed it on to a human. They go to great lengths because knowing how a pandemic begins is the key to stopping the next one.
Professor Stephen Turner, head of the microbiology department at Melbourne’s Monash University, says the virus most likely originated from bats.
But that’s where his certainty ends, he says.
On the hypothesis that the virus emerged in the Wuhan live animal market from an interaction between an animal and a human, Turner says, “I don’t think it’s conclusive in any way.”
“Part of the problem is that information is as good as surveillance,” he says, adding that viruses of this type circulate all the time in the animal kingdom.
The fact that the virus has infected a tiger at a New York zoo shows how viruses can move between species, he says. “Understanding the breadth of species that this virus can infect is important, as it helps us narrow down where it might have come from.”
Scientists say the virus most likely came from bats, but it first passed through an intermediary animal in the same way that another coronavirus, the 2002 Sars outbreak, went from horseshoe bats to cat-like civets before infect humans.
An animal implicated as an intermediate host between bats and humans is the pangolin. The International Union for Conservation of Nature says they are “the most illegally traded mammals in the world” and are prized for their meat and the medicinal properties of their scales.
As reported in Nature, pangolins are not listed in the inventory of items sold in Wuhan, although this omission could be deliberate as it is illegal to sell them.
“It is not clear if the poor pangolin was the species it jumped into,” says Turner. “Is it mixed in something else, mixed in a poor pangolin, or has it skipped people and evolved into people.”
Professor Edward Holmes of the University of Sydney co-authored a Nature study that examined the probable origins of the virus by looking at its genome. In social networks, he stressed that the identity of the species that served as an intermediate host for the virus isstill uncertain“
A statistical study looked at a characteristic of the virus that evolved to allow it to adhere to human cells. Pangolins were able to develop this characteristic, but also cats, buffaloes, cows, goats, sheep and pigeons.
Another study claimed to have ruled out pangolins as intermediaries entirely, because samples of similar viruses taken from pangolins lacked an amino acid chain seen in the virus now circulating in humans.
The study Holmes worked on suggested that the scenario in which a human in the Wuhan marketplace interacted with an animal carrying the virus was just a potential version of Covid-19’s origin story. Another was the possibility that a descendant of the virus would jump to humans and then adapt as it passed from human to human.
“Once acquired, these adaptations will allow the pandemic to take off and produce a group of cases large enough to activate the surveillance system that detected it,” the study said.
Analysis of the first 41 Covid-19 patients in The Lancet medical journal found that 27 of them had direct exposure to the Wuhan market. But the same analysis found that the first known case of the disease did not.
This could be another reason to doubt the established history.
Professor Stanley Perlman, a leading University of Iowa immunologist and expert on previous animal-originated coronavirus outbreaks, says the idea that the link to the Wuhan market is a coincidence “cannot be ruled out,” but That possibility “seems less likely” because the genetic material of the virus had been found in the market environment.
Perlman told Guardian Australia that he believes there was an intermediary animal, but adds that while pangolins are potential candidates, “they have not been shown to be the key intermediaries.”
“I suspect that any evolution [of the virus] occurred in the intermediate animal if there was one. There have been no substantial changes in the virus in the three months of the pandemic, indicating that the virus is well adapted to humans. “
So-called wet markets, where live animals are traded, have been implicated in previous outbreaks of coronaviruses, in particular Sars.
Dr. Michelle Baker, a CSIRO immunologist who studies viruses in bats, says that part of the research into the origins of Covid-19 has been based on what was known from the past.
But “we really don’t know” how accurate the origin story is, she says: “There is some kind of connection [to the Wuhan market] and there were people exposed to the market who were infected. “
Baker says what is “very likely” is that the virus originated from a bat. “It is a probable scenario, but we will never know. The market cleared up fairly quickly. We can only speculate.
“These wet markets have been identified as a problem because there are species that interact,” she says. “It is an opportunity to highlight their dangers and an opportunity to suppress them.”
Turner adds: “We have found the ancestors of the virus, but having a broader understanding of the coronavirus in other species could give us a clue to the evolution of this thing and how it sprang up.”
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Due to the unprecedented and continuing nature of the coronavirus outbreak, this article is periodically updated to ensure it reflects the current situation as of the date of publication. Significant corrections made to this or previous versions of the article will continue to be noted in accordance with Guardian’s editorial policy.
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