Wild animals sometimes found on the menu in Asian countries harbor a bewildering panoply of viruses, a new study has found—including many that can infect humans. Although none is closely related to the coronavirus that touched off the COVID-19 pandemic, the study sends a clear warning that other viral threats are lurking in the animal kingdom, scientists say.
Live-animal markets are known to have sparked outbreaks, such as severe acute respiratory syndrome (SARS) 2 decades ago. But the study underscores the extent of the threat, showing“there is an enormous amount of unsampled viral diversity” in the animals, says Harvard University evolutionary biologist William Hanage, who was not involved in the work. “We humans need to understand that for a virus, different mammal species can look pretty alike, provided their cells have appropriate receptors.” China has clamped down on the sale of the animals sampled in the study, but other countries in the region have not.
The researchers, led by veterinarian Su Shuo of Nanjing Agricultural University, took samples from nearly 2000 animals representing 18 different species at venues in China including fur farms, zoos, and natural habitats. Most were species that are traditionally eaten as delicacies in China, including civets, raccoon dogs, badgers, bamboo rats, and porcupines. Using a “metagenomics” technique, which probes samples for RNA transcripts that viruses make when they copy themselves, they identified 102 virus species from 13 different viral families in the animals’ noses, feces, and tissues. Sixty-five of the viruses had never been described before. The researchers deemed 21 as “high risk” to humans, because they had infected people in the past or simply had a history of readily jumping between species.
“Our results provide important insights to those game animals and their viruses that might lead to the next pandemic,” says Su, whose group published its work online yesterday in Cell.
Among the worrisome finds were several coronaviruses. For example, a hedgehog was infected with a virus resembling the one that causes Middle East respiratory syndrome in humans. Four canine coronaviruses found in raccoon dogs were about 94% similar to coronaviruses recently found in humans in Malaysia and Haiti. “These viruses can infect many animals,” Su says.
Some of the species sampled in the study could act as “intermediary” hosts that bat coronaviruses infect before they make the jump to humans. Indeed, a coronavirus close to one found in bats turned up in a civet. Most researchers think both SARS-CoV-2 and SARS-CoV-1—the cause of SARS—became human pathogens after passing through an intermediate host.
The researchers also detected several influenza viruses, another family that could trigger a new pandemic. In a finding “of considerable significance,” the authors write, civets and Asian badgers were found to carry H9N2, an influenza A virus that has become increasingly common in chickens and ducks. There have been fewer than 50 documented human cases of H9N2 infection, a February 2020 report noted, because the virus does not transmit efficiently between people. But researchers fear that by replicating in other mammals it has more opportunities both to infect humans and to adapt to them. The infected badgers had runny noses and presumably could transmit to humans through the respiratory route.
Other viruses detected in the study that can infect people include influenza B, Norwalk, human parainfluenza virus 2, rotaviruses, and orthoreoviruses.
Markets that sell live animals—often called “wet markets”—are ideal places for viruses to transmit to humans, both because of the density of animals and because the stress they suffer makes them prone to shedding viruses, says medical virologist Marietjie Venter of the University of Pretoria, Hatfield. The new findings “confirm that trade and consumption of these animals should be avoided and support the actions taken by China to ban the trade of many of these animals,” says Venter, who is a member of the World Health Organization’s Scientific Advisory Group for the Origins of Novel Pathogens.
After SARS, China made the sale of many of the animals sampled in the study illegal, but they were still readily available in Wuhan markets in 2019, just before the start of the pandemic, including at the Huanan Seafood Market, which had the earliest identified cluster of COVID-19 cases. Su says the government has cracked down hard on illegal sales since then. “With very strict legislation, as well as screening checks, it is now difficult to find wildlife” for sale, Su says. “What worries me is that it seems that in Southeast Asia, where the economy is lagging, this wild animal trade is continuing.”
Evolutionary biologist Edward Holmes at the University of Sydney, a co-author of the new study, “strongly suspects” SARS-CoV-2 jumped into humans at the Huanan market. As long as wild animals are sold, the risk of similar jumps remains high, he says. “It’s hard to think of a more effective way to ignite and fan the flames of an epidemic,” Holmes says. “We keep allowing these things to flourish and it’s only a matter of time before we get another outbreak and perhaps another pandemic.”