Nearly five years after its emergence, the international scientific community has still not managed to determine with certainty the origin of Covid-19.
Although the first cases were initially detected in Wuhan at the end of 2019, two theories are currently being considered: a leak from a laboratory in the city where similar viruses were being studied, or an intermediary animal that infected people who frequented a local market. The scientific community favours this last theory.
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 stalls in the market, including those selling wild animals, and from sewers.
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, notably belonging to species such as raccoon dogs or civets. And that these animals were present in the southwest corner of the market, which also happens to be an area in which many Sars-CoV-2 viruses, responsible for Covid-19, have been detected,” she explains to AFP.
The presence at the market of these species, identified as probable intermediate hosts of the virus between bats and humans, has been disputed and until now only photographic evidence and the results of a study describing the animals sold in Wuhan were available.
Infected cages
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 either that animals present at this stall shed the Sars-CoV-2 detected on the animal equipment, or that unreported early human cases of Covid-19 shed the virus at the exact same location as the detected animals,” the study authors explain. 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, i.e. 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.
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.
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