the essential
A study published this week provides new information on the origin of Covid-19. It strengthens the hypothesis of transmission to humans by infected animals introduced to a market in Wuhan (China) at the end of 2019.
Nearly five years after its emergence, the international scientific community has still not managed to determine with certainty the origin of Covid-19.
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What are the main hypotheses about the origin of the virus?
Although the first cases were apparently detected in Wuhan at the end of 2019, two theories are in conflict: a leak from a laboratory in the city where similar viruses were being studied, or an intermediary animal having infected people who frequented a local market.
This last line of inquiry is favored by the scientific community. The study published Thursday in the journal Cell is based on the analysis of more than 800 samples collected in this market where different species of wild animals were sold. Collected in January 2020, after the market closed, they were taken from surfaces, from various market stalls, including those selling wild animals, and from sewers.
Why is the Wuhan market hypothesis favored?
With this type of data, made available to researchers by Chinese scientists, “we cannot say with certainty whether the animals (present on the market) were infected or not,” warns Florence Débarre, a CNRS researcher and co-author of the study.
But, “our study confirms that there were wild animals in this market at the end of 2019 […] present in the southwest corner of the market, which also happens to be an area where a lot of the SARS-CoV-2 virus, responsible for Covid-19, has been detected,” she explained to AFP.
What strain of the virus was found in the Wuhan market?
Another element points to the market as the starting point for the spread of the virus. The study establishes that the “most recent common ancestor (MRCA)” of SARS-CoV-2 found in the market samples, that is, the original strain, is “genetically identical” to the MRCA of the pandemic as a whole.
“This means that the early diversity of the virus is found in the market, as we would expect to see if this is the place of emergence,” explains Ms. Débarre.
Has the hypothesis of transmission by a pangolin been abandoned?
As part of the study, “animal carts, a cage, a garbage cart, and a hair and feather removal machine from a wildlife stand” tested positive for SARS-CoV-2. And there was “more DNA from wild mammal species than human” in these samples.
Wildlife DNA was found in positive samples from this stand, including species such as civet cats, bamboo rats and raccoon dogs.
“These data indicate that either the animals present at this stall shed SARS-CoV-2 detected on the animal equipment, or that unreported early human cases of COVID-19 shed the virus in the exact same location as the detected animals,” the authors of the study explain. In the latter, there is no longer any mention of pangolins.
How to limit the emergence of new viruses?
This new study “provides very strong evidence that wildlife stalls in the market […] were a hotbed of the Covid-19 pandemic,” said James Wood, an epidemiologist at the University of Cambridge, to the Science Media Center.
“This work is important,” he said, because despite efforts “on a global scale to strengthen laboratory biosafety […]little or nothing has been done to limit the live wildlife trade, biodiversity loss or land use changes, which are the likely real drivers of past and future pandemic outbreaks.”
