New Studies Contend: Wuhan Market Animals, Not Laboratory, First Infected Humans with SARS-COV2
Wild animal carcasses in the Huanan market in Wuhan on display just after slaughter.

Three pre-print papers published over the past few days have strengthened the case for the theory that SARS-COV2 first spread among people via infected animals sold and slaughtered at the Huanan wildlife market in Wuhan – rather than from the Wuhan Institute of Virology, the laboratory studying coronaviruses in bats.

The two theories have been hotly, and often bitterly, disputed by scientists around the world for over a year.

Significantly, one of the pre-print papers was published by a large group of Chinese researchers based at the China Center for Disease Prevention and Control.

It offers, after two years of silence, evidence that the first strain of the SARS-CoV2 virus to be identified circulating among people in Wuhan, dubbed SARS-CoV2 Lineage A, was also circulating  in the Huanan market in the early days of 2020 alongside its sequel, Lineage B.

That provides a critical missing link, insofar as other studies had previously only succeeded in identifying Lineage B in environmental samples taken from the Huanan market  – whereas Lineage A was the first to spread among people in the city of 10 million.

China study corroborates findings of University of Arizona researchers

Equally significant, the findings of the China researchers also corroborate the conclusions of two studies led by Michael Worobey, Head of Ecology and Evolutionary Biology at the University of Arizona, and colleagues.

The latest paper, published on 26 February, concludes that critical events of virus transmission from animals to humans happened in two different events at the market, possibly a week apart, and involving strains of the virus dubbed SARS-COV2 A and SARS-COV2 B – the main cases circulating in China during the early days of the outbreak.

While all of the samples reviewed in both the Chinese study, as well as the one by Worobey, were taken from environmental surfaces – not the animals themselves – they are most evident in the areas of the market where wild animals were kept captive and slaughtered – including items like cages where the animals were held.

The newly-published papers recently still fail to identify a single species as that elusive ‘Animal X’ – the so-called “intermediate host” that transmitted the virus originating harbored by  bats to humans. But the studies still offer the most conclusive evidence, to date, that animals in the Huanan market indeed may have been the first to infect people in the city of 10 million people with SARS-CoV2.

Where is ‘Animal X’? Summary of SARS-CoV2 Origins Report

Worobey and colleagues examined over 700 complete genomes of SARS-CoV-2 that could be mapped from the environmental samples in the market, taken between December 2019 and up until mid-February 2020.  Around one-third were lineage A and two-thirds were lineage B.

“We find that there were very likely at least two origins of SARS-CoV-2 – one for lineage A and one for lineage B.  The patterns in the phylogeny are the giveaway,” according to Worobey.

The study adds that “multiple lines of evidence” from the environmental samples all point to wild animals – even if animal samples, per se, were not available for the study. Those include:  a high concentration of SARS-CoV2 positive samples taken from surfaces in the southwestern corner of the market where wild mammals were sold and slaughtered, and the highest concentration of early SARS-CoV2 cases among vendors in the areas where live mammals were sold.  And while no single animal was identified as the main cause of transmission, the study also singles out a particular cluster of positive virus samples in the area where racoon dogs were illegally sold, as well as a cage where the dogs were housed.

In a detailed series of tweets, Worobey zeroes in on the racoon dogs further saying: “One striking (to us at least) finding: one stall had 5 environmental positive samples for very animal-centric surfaces, including a “metal cage in a back room”. …one of the stalls we know was selling live mammals illegally in late 2019. But, there’s more…

“It happened to be a stall that one of us, @edtwardcholmes, had visited 5 years before the pandemic, and where he had taken a photo of this racoon dog” – an animal susceptible to the SARS-CoV2 virus.

Market is epicentre

The other paper makes a detailed examination of the spatial evidence on the proximity of the market to the first clusters of human cases in the Wuhan community. It refers to maps from the World Health Organization (WHO) report on the origins of SARS-CoV-2, which enabled researchers to plot the density of the first COVID-19 cases in Wuhan in December 2019 – even before the outbreak was publicly reported.

According to Worobey, “We found that cases in December were both nearer to, and more centered on, the Huanan market than could be expected given either the population density distribution of Wuhan, or the spatial distribution of COVID cases later in the epidemic.”

Based on these maps, “Huanan market sits right in the highest density region,” he adds. 

“This is a clear indication that community transmission started at the market,” added Worobey in his lengthy Twitter thread explaining the findings of the two studies

In addition, the mapping showed that both cases of people infected with both SARS-CoV-2 lineage A and lineage B had a strong association with the market.

Chinese paper also points to the market

In the case of the other pre-print published by the group of China CDC researchers led by George Gao, Gao and colleagues examined 1380 samples collected from both the environment and animals at the market in early 2020. Of these, 73 environmental samples tested positive for SARS-CoV-2 and three live viruses were successfully isolated, they reported.

The viruses “shared nucleotide identity of 99.980% to 99.993% with the human isolate”, they reported.

Here too, no SARS-CoV2 virus was detected in the animal swabs covering 18 species of animals on sale in the market – despite the fact that such samples were taken and study.

But the paper still concludes that there is “convincing evidence of the prevalence of SARS-CoV-2 in the Huanan Seafood Market during the early stage of COVID-19 outbreak”.

Although that conclusion does not go as far those of Worobey and his colleagues, it is significant insofar as the study’s authors are affiliated with China CDC.

This also suggests that Chinese authorities may be finally coming to terms with the overwhelming evidence about the Chinese origins of the virus outbreak in humans, which some reports earlier had tried to attribute to factors such as the import of frozen foods, or an imported outbreak from a foreign military base.  And in light of that, scientists are being allowed to release some long-sought evidence about the presence of the virus in the Wuhan market during the early days of the outbreak.

-Elaine Ruth Fletcher contributed to this story.

Image Credits: Arend Kuester/Flickr.

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