NEW YORK (BLOOMBERG) - Wildlife species sold in wet markets in China were linked to the emergence of Covid-19 and the severe acute respiratory syndrome, or Sars.
Now, a comprehensive survey of viral pathogens has found that they harbour a range of diseases threatening humans and other animals.
A study of more than a dozen species of game animals traded, sold and commonly consumed as exotic food in China identified 71 mammalian viruses, including 18 deemed "potentially high risk" to people and domestic animals.
Civets, the cat-like carnivores implicated in the spread of Sars in markets in southern China almost 20 years ago, carried the most worrisome microbes, according to the research released on Friday (Nov 12).
Although the authors in China, the United States, Belgium and Australia did not find anything resembling Sars-CoV-2, the coronavirus responsible for the Covid-19 pandemic, they showed that strains carried by bats are transmitted across the species barrier to infect other animals in spillover events that risk seeding dangerous outbreaks.
They also found that game animals were infected with viruses previously thought to exist only in people.
"This study highlights exactly why the wildlife trade and live animal markets are a pandemic accident waiting to happen," said co-author Edward Holmes, an evolutionary biologist at the University of Sydney, in an e-mail.
"This paper also shows that humans regularly transmit their viruses to other animals. There's clearly two-way virus traffic."
The research, supported by the National Key Research and Development Programme of China and others, shows the commitment to searching for future pandemic threats.
The 40-page manuscript was released on bioRxiv, an open access pre-print repository, ahead of peer review and publication.
The comprehensive investigation of the diversity and abundance of vertebrate-associated viruses in game animals in China is the first to assess which species have the greatest potential for carrying viruses that could set off an outbreak.
Scientists have yet to determine the origins of Sars-CoV-2.
Debate about its genesis has coalesced around two competing ideas: a laboratory escape or a spillover from animals.
Studies identifying closely related coronaviruses, including in bats dwelling in limestone caves in northern Laos and in Cambodia, support the latter hypothesis, especially since live animals susceptible to the infection were known to be sold in markets in Wuhan, the central Chinese city where the earliest Covid-19 cases were detected.
Wildlife species captured in their natural habitats or raised on farms are traded both legally and illegally around the world to meet demand for food, fur, traditional medicines, exotic pets and zoo exhibits. The market in China was worth an estimate 520 billion yuan (S$110.3 billion) in 2016.
China banned the trade after Covid-19 emerged, and the government has subsequently prohibited human consumption of terrestrial wild animals.
The move in early 2020 recognised that the hygiene-poor conditions and close contact between animals and humans, as well as the wide mix of species within live animal markets and the restaurants they serve, create an ideal breeding ground for emerging infectious diseases.
Nanjing Agricultural University veterinary medicine professor Shuo Su and colleagues examined 1,725 game animals from 16 species commonly hunted or consumed for food across 19 provinces in China.
Among dozens of mammalian viruses identified over the past five years, 45 had not previously been described.
"With the exception of pangolins, there has been little investigation of game animals, even though they have close contact with humans and domestic animals and, hence, provide a link to other wildlife species," the authors said.
Many of the species investigated were displayed on a price list at the Huanan Seafood Wholesale Market in Wuhan, the epicentre of the initial Covid-19 outbreak.
Among notable virus findings, the scientists identified for the first time the presence of hepatitis E virus and the H9N2 strain of influenza in badgers and civets.
That particular avian flu virus is now the most prevalent strain in chickens and ducks, and has led to numerous infections in people in China, the authors said.
It caused obvious respiratory symptoms in Asian badgers, though an infected civet appeared healthy, they said.
The scientists also found a moderate-to-high number of pathogens thought to be human-specific in pangolins, civets and bamboo rats. They included norovirus, a notorious source of vomiting and diarrhoea on cruise ships, and influenza B.
Additionally, the authors identified cross-species transmission of several animal viruses, including a bat-associated coronavirus in a civet, a bird-associated coronavirus in porcupines and a pig-associated swine pneumovirus in pangolins.
The transmission of a bat coronavirus to a civet is especially troubling, University of Sydney's Professor Holmes said.
"A further species jump from civets into humans could easily start a major outbreak," he said, adding that a similar pattern of cross-species transmission is very likely to have caused the emergence of Sars-CoV-2.
"The animals sold as game in live animal markets carry a wide range of viral pathogens," said Prof Holmes, who was awarded the Australian Prime Minister's Prize for Science this month. "The right virus in the right animal at the right time could easily trigger a global pandemic."