Wet market perfect for outbreak: investigator

5 minute read

Professor Dominic Dwyer, who went to Wuhan with the WHO team, says conditions were ideal for disease but the source of covid-19 is still unknown.

The sole Australian member of the WHO team that travelled to Wuhan, China earlier this year to investigate the origins of the covid-19 pandemic, Professor Dominic Dwyer, has shared his thoughts at the Australian Rheumatology Association conference dinner.

Professor Dwyer is fairly convinced that the Huanan seafood market played a leading role in getting the pandemic started, even though the “smoking gun” (the exact origin of infection) is yet to be found.

“You could not script a better place to have an outbreak of a disease” than the Wuhan wet markets, said Professor Dwyer, who is a microbiologist and infectious diseases expert with NSW Health Pathology.

When the WHO’s team of international investigators visited the Wuhan wet markets (which have now been shut down), they saw a venue that put humans and animals in very close proximity.  

In the past, different animal species had been housed in adjacent cages, and human sleeping quarters were close by. The market was cramped with poor ventilation and drainage.

Records from December 2019 show that poultry, rabbits, ferret-badgers, giant salamanders, two kinds of crocodile and snakes were traded at the markets.

At the time of the covid-19 outbreak, the market had around 10,000 people visiting per day.

Chinese officials tested the markets extensively, including 1000 samples from doors, bins, toilets, stalls and stray cats and mice, and samples from 188 animals (across 18 species) sold at the markets.

Everything tested negative for SARS-CoV-2. However, not everything sold at the market was tested and contact tracing pinpointed the wet markets as the source of a cluster in the local population.

The first case of covid-19 was diagnosed in December 2019. By January 2020, 27 (66%) of 41 patients with covid-19 had been exposed to the Huanan seafood market.

Of the 174 covid-19 cases diagnosed in the very early stages of the pandemic, some (but not all) had been exposed to the wet market.

Whole genome sequencing of SARS-CoV-2 circulating in December 2019 suggested the virus was linked to the Wuhan wet markets, but there were other viral sequences as well.

It was likely that SARS-CoV-2 was circulating in the human population of Wuhan as early as November, Professor Dwyer said.

So, where did the virus come from? That remains uncertain. It could be that the virus jumped from wildlife (bats) to an intermediary species and was then brought to the wet markets.

The WHO investigator team of 34 scientists hasn’t ruled out frozen or refrigerated food as a source of transmission either.

One theory is that the virus emerged elsewhere in farming, processing, transporting, refrigerating, or frozen food. Cold chain products at the Wuhan market were not tested for the virus.

Bats remain a likely source of SARS-CoV-2, said Professor Dwyer. Bats make up 20% of mammalian species. Around 10% of bats carry coronaviruses and some strains are similar to SARS-CoV-2.

If this turns out to be true, the bat that is carrying SARS-CoV-2 might not even be in China, said Professor Dwyer. Bats with coronaviruses are found in southeast China and Indonesia but also across Europe.

As to the question of whether SARS-CoV-2 was cooked up in a lab as part of a military conspiracy, Professor Dwyer said there were “other much easier explanations”.

“If you’re going to pick a bioweapon, SARS-CoV-2 is a pretty useless one,” he said.

“It kills older people. It doesn’t kill healthy people between 20 and 40 who are going to be in the military forces. Most people actually don’t get sick. So, if you’re going to choose a weapon, it’s actually not a very good one.”

Professor Dwyer’s visit to China also gave him an insight into some of the cultural and political circumstances that influenced the management of the early days of the pandemic.

One example of this was the Chinese CDC waiting about a week to publicly release the full genome sequence for covid-19. (Normally, genome sequences were published online immediately so that tests could be quickly developed, said Professor Dwyer.)

An evolutionary virologist at the University of Sydney, Professor Edward Holmes, published (with Chinese colleagues) the full SARS-CoV-2 genome on 10 January 2020.

He was in possession of the full genome sequence for precisely 52 minutes before he posted it online, the BBC reported.

The short delay was due to Professor Holmes phoning his colleague, Professor Yong-Zhen Zhang at the Chinese CDC, to ask for permission to publish.

“He told me he needed to think about it – there was some pressure not to release too much information about the outbreak,” Professor Holmes said, according to the BBC. “He called me back about a minute later and said, ‘OK, let’s do it.'”

The WHO investigators spoke to Chinese journalists to see what, if any, investigative journalism was done around the time of the initial outbreaks.

“They said, ‘Oh, we reported data based on what the government said’,” said Professor Dwyer.

“The transparency of information is really important for origins investigations,” he said.

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