Evidence points to Wuhan market as source of covid-19 outbreak

An analysis of genetic samples taken from the Huanan seafood market in Wuhan, China, has identified a shortlist of wild animals being sold there that were the most likely source of the virus that sparked the covid-19 pandemic.

While bats are thought to have been the original carrier of the SARS-CoV-2 virus, it has been previously proposed that an intermediate wild species became infected and brought it to the market, where an outbreak in humans began. Stalls there sold live animals as well as seafood.

An alternative suggestion is that the virus escaped from the Wuhan Institute of Virology, where staff were known to be studying bat coronaviruses, but not SARS-CoV-2.

However, a new study by an international team concludes it is more likely that the virus emerged from wild animals sold at the market and not from a lab escape. The researchers re-analysed data from 800 samples collected at the Huanan market by the Chinese Center for Disease Control and Prevention beginning on 1 January 2020, and also studied viral genomes from the earliest covid-19 cases.

Team member Ed Holmes at the University of Sydney says Chinese investigators swabbed surfaces, freezers, drains and cages to test for the presence of SARS-CoV-2.

“We analysed these data to determine which species were present and where they were found in the market, particularly in relation to where SARS-CoV-2 was found,” says Holmes.

Contrary to some statements made by Chinese investigators, the team found evidence for a variety of wildlife being sold at the market that could have been an intermediate host for the virus, including common raccoon dogs (Nyctereutes procyonoides), masked palm civets (Paguma larvata) and hoary bamboo rats (Rhizomys pruinosus).

Importantly, traces of these animals were found in exactly the same stalls as SARS-CoV-2, says Holmes. “This suggests – but does not prove – that the animals were infected. Hence, it is very likely that SARS-CoV-2 emerged in a live animal market.”

“All the scientific data point one way — to SARS-CoV-2’s natural zoonotic origin in the Huanan market, Wuhan,” says Holmes.

The genetic studies of the virus in the earliest covid-19 cases revealed that few, if any, people were infected prior to the market outbreak, the team reports. However, it couldn’t rule out the possibility that the virus was brought to the market by an infected person handling animals.

Team member Zach Hensel at NOVA University Lisbon, Portugal, says the study highlights the preventable risks posed by the human-wildlife interface in Huanan market, and the need to mitigate these risks in similar locations.

“Even though humans were, of course, everywhere in Huanan market and live mammals were concentrated in a small number of stalls, human viruses, other than SARS2, were rare and several animal viruses were quite abundant,” says Hensel.

“This included one influenza virus with zoonotic spillover potential and an animal virus closely related to others that had caused devastating animal outbreaks.”

Hensel says simple personal protective equipment that had been recommended for such settings prior to the outbreak of covid-19 could have prevented the entire pandemic.

Robert Edwards at Flinders University in Adelaide, Australia, who wasn’t involved in the research, says a “dream team of evolutionary biologists” conducted the latest study.

“There is little doubt about the origins of SARS-CoV-2 coming from the wet market now,” says Edwards. “The authors discuss humans causing the infection in the market, but any other origin story has to explain how it was only the market that was the source of so many outbreaks.”