Scientists tracing the origins of the Covid-19 pandemic believe they’ve identified a possible transmission source: China’s thriving wildlife trade.
The highly anticipated findings from experts convened by the World Health Organization and the Chinese government are expected to show parallels to the spawning in 2002 of severe acute respiratory syndrome, or SARS, a bat-borne coronavirus spread by civets that killed 800 people. The path trod by SARS-CoV-2--as the new coronavirus is known--before it emerged in central China in December 2019 remains a mystery, though it’s one researchers say can be solved.
In Wuhan, where the first cluster of cases occurred, scientists involved in the hunt identified four hypotheses to explain the virus’s genesis, including two that stoked controversy even as they were deemed unlikely. The idea that the virus was introduced via contaminated food or packaging is one embraced in Beijing, while the Trump administration said it may have been the result of a laboratory accident. But the most plausible theory, say experts involved in the mission, concerns China’s wildlife trade for food, furs and traditional medicine, a business worth about 520 billion yuan ($80 billion) in 2016.
Live animals susceptible to coronavirus infection were present at the Huanan food market in downtown Wuhan, the city where the first major Covid-19 outbreak was detected. It’s possible they acted as conduits for the virus, carrying it from bats -- likely the primary source -- to humans, says Peter Daszak, a zoologist who was part of the joint research effort, which saw international experts visit Wuhan earlier this year after months of stonewalling by the Chinese government.
“The main conclusion from this stage of the work -- and it’s not over yet of course -- is that the exact same pathway by which SARS emerged was alive and well for the emergence of Covid,” said Daszak, who is also president of EcoHealth Alliance, a New York-based nonprofit that works to prevent viral outbreaks around the world.
The scientists’ report, slated for release this week after delays due to political wrangling, is likely to be far from conclusive. More studies are planned, including outside China, with deciphering Covid-19’s creation story vital to understanding how best to thwart its resurgence, and to help avert similar catastrophes in the future.
While the hunt for the virus’s origin has become political football for the world’s superpowers, Daszak says he thinks the scientific process will prevail. Significant data on where SARS-CoV-2 came from and how it emerged will be uncovered over the next few years, he said during a March 10 webinar organized by Chatham House.
Farmed and wild-caught civets, a small, nocturnal mammal consumed in China, were blamed for spreading the SARS virus in a market in the southern province of Guangdong in 2003. Scientists later found the infection originated in horseshoe bats, a natural reservoir of coronaviruses.
The two species likely collided in markets where live animals are caged in crowded conditions, potentially allowing the bat-borne virus to adapt and amplify before it spilled over to humans, initially among workers and those handling the animals.
Scientists working on the origin hunt say a similar scenario may have played out with Covid-19. A study of the first 99 patients treated at an infectious diseases hospital in Wuhan found half were linked to the Huanan seafood market, which also reportedly sold live animals, some illegally captured in the wild and slaughtered in front of customers.
It’s possible the virus was introduced through an infected animal that was sold at the Huanan market or somewhere else in Wuhan, said Dominic Dwyer, a microbiologist in Sydney who was part of the WHO-convened team that traveled to the Chinese city in February.
Still, questions remain about the market’s ultimate role.
Testing after it was shut down in December 2019 failed to turn up any infected animals. Contaminated surfaces were widespread, compatible with the virus being introduced via infected people or tainted animal products. Compounding the confusion, the first known Covid-19 patient developed symptoms four days before the earliest cases tied to the market.
An analysis of SARS-CoV-2 samples collected in mid-December found subtle genetic differences between them. The variation indicates the virus may have circulated surreptitiously for weeks in the community before doctors were alerted to it via a handful of severely ill patients with a mysterious viral pneumonia.
The original spill over of the coronavirus to a human was probably followed by rapid adaptation of the virus, said Joel O. Wertheim, an associate professor of medicine at the University of California, San Diego. It’s possible the virus was transmitted multiple times and went extinct when infected individuals didn’t transmit the virus to anyone, Wertheim and colleagues said in a paper published March 18 in the journal Science. Eventually, the virus infected someone who passed it to several people, who also passed it on to others, possibly in a super-spreading event.
The Huanan market may have been where that occurred, Wertheim said in an interview. “The market may have been key to the virus ensconcing itself in humans.”
Current evidence suggests the market is where SARS-CoV-2 was amplified, and not necessarily its birthplace, Dwyer said.
“When you visit the market, you realize that it’s a perfect place for an outbreak to occur because it’s crowded, lots of stalls, lots of animal products, and ventilation and drainage are a bit suboptimal,” he said in an interview. “It’s not surprising we had an explosion through there.”
The WHO research team found evidence that wildlife farms in southern China were supplying vendors at the Huanan market, Daszak told the U.S.’ National Public Radio. It also found a route from southern provinces such as Yunnan -- where the closest known coronavirus to SARS-CoV-2 was found in horseshoe bats in 2013 -- to Wuhan, he said on the Chatham House webinar.
“It provides a link and a pathway by which a virus could convincingly spill over from wildlife into either people or animals farmed in the region, and then shipped into a market by some means,” Daszak said. “That’s a really important clue. Those beginnings of an understanding of a pathway need to be followed up pretty rapidly.”
For decades, the government of China promoted the farming of wildlife to bolster rural incomes. The practice provided an especially valuable alternative source of meat after African swine fever emerged in 2018. The deadly outbreak resulted in an unprecedented shortage of pork, researchers at the South China Agricultural University and University of Glasgow said in a study that was released in February without a formal peer-review. China typically consumes half the world’s pigmeat.
While the pork shortage bolstered wildlife consumption, eating birds, snakes, bamboo rats, squirrels, porcupines and other non-domesticated animals was already popular, especially in southern provinces, the researchers said.
They cited a 2004 survey by the China Wildlife Conservation Association that found 46% of urban residents had consumed wildlife and 2.7% were regular consumers. A January 2017 survey found 52% of markets they investigated were trading wildlife, while 40% of restaurants were providing wild animal dishes.
Much of the trade supposedly ended just over a year ago. After President Xi Jinping warned that the consumption of wildlife posed an immense risk to public health, the Standing Committee of the National People’s Congress decided on Feb. 24, 2020, to expand the scope of China’s Wildlife Protection Law to ban the consumption of almost all wild animals.
Amid international criticism of its handling of the early days of the pandemic, China’s official rhetoric is focused on creating doubt that the pathogen originated within its borders. But China targeted the wildlife trade a year ago for a reason, Daszak told NPR.
“The reason was, back in February 2020, they believed this was the most likely pathway” for the coronavirus to reach Wuhan, he said. “And when the WHO report comes out...we believe it’s the most likely pathway too.”