New study on virus origins strengthens hypothesis of Wuhan market outbreak

Since the appearance of Sars-CoV-2 at the end of 2019, two theories have been in conflict. That of a leak from a city laboratory where related viruses were being studied, and that of an intermediary animal having infected the first patients.

Nearly five years after its emergence, the international scientific community has still not managed to determine with certainty the origin of Covid-19. However, it seems to have taken a new step forward, with a study published on Thursday, September 19, which provides new elements reinforcing the hypothesis of transmission to humans by infected animals in the Wuhan market.

Although the first cases were apparently detected in this Chinese city at the end of 2019, two theories are in conflict: that of a leak from a laboratory where similar viruses were being studied and that of an intermediary animal having infected people who frequented a local market. This last hypothesis 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 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 animals [présents sur le marché] 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 and 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 of these species, identified as probable intermediate hosts of the virus between bats and humans, at the market has been disputed and until now only photographic evidence and the results of a study describing the animals sold in Wuhan were available. 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 were in these samples “more DNA from wild mammal species than human”. 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 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 in the exact same location as the detected animals.”

The authors of the study

quoted by AFP

Another element points to the market as the starting point for the spread of the virus. The study establishes that “most recent common ancestor (MRCA)” of Sars-CoV-2 found in 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 Florence Débarre.

This new study “provides very strong evidence that the wildlife stalls at the market (…) were a hotbed of the emergence of the Covid-19 pandemic”says James Wood, an epidemiologist at the University of Cambridge, to the Science Media Center. “This work is important”he believes, because despite the efforts “at a global scale to strengthen laboratory biosafety (…), little or nothing has been done to limit trade in live wildlife, biodiversity loss or land use changes, which are the real likely drivers of past and future pandemic outbreaks”.