This time five years ago, the virus that causes COVID-19 was spreading around the globe unchecked. One of the biggest questions that remains is: where did it come from?
Today, mounting evidence from more than a dozen studies point to a person, or people, catching the virus from a wild animal or animals at the Huanan Seafood Market in Wuhan, China, the city at the epicentre of the outbreak. And the animal at the top of the list is the raccoon dog (Nyctereutes procyonoides).
“There is a large focus on raccoon dogs,” says Kristian Andersen, an evolutionary biologist at Scripps Research in La Jolla, California.
Some scientists, including virologist Edward Holmes at the University of Sydney in Australia, have suspected raccoon dogs all along. On 21 January 2020, he sent an e-mail to Andersen and another colleague, with the subject ‘Outbreak poker’. In jest, he proposed a wager on the animal that might have carried the virus to people. “I’m betting raccoon dog,” he said. Holmes had seen raccoon dogs at the Huanan market when he travelled to Wuhan in 2014.
But part of the reason that raccoon dogs top the list of suspects is because they have been studied more than other animals, including ones also present at the market, says Michael Worobey, an evolutionary biologist at the University of Arizona in Tucson. There are yet more possible candidates, he says.
Marion Koopmans, a virologist at Erasmus MC in Rotterdam, the Netherlands, agrees. “We have to be modest about our ability to predict which animal species” sparked the COVID-19 pandemic, she says.
The origin of the pandemic is still deeply politicized, and the lack of clear answers hasn’t helped. The virus probably originated in bats living in southern China. From there, many scientists think it infected an intermediate animal that passed it to people. The virus could also have passed directly from bats, although that is considered less likely given their habitat is far from Wuhan. And some still suggest that the virus could have escaped, or been deliberately released, from the Wuhan Institute of Virology, which was known to be doing research on coronaviruses.
Viral host
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One of the reasons raccoon dogs were suggested as a prime candidate early on is because they were probably involved in passing another, related, virus to people. In 2003, researchers isolated close matches of the virus that causes severe acute respiratory syndrome (SARS) in several civets and a raccoon dog at a live-animal market in Guangdong, China.
This finding prompted researchers in Germany to investigate these animals’ susceptibility to SARS-CoV-21.
They found that raccoon dogs can be infected by SARS-CoV-2, and — despite not getting that sick themselves — can pass on the infection to other animals.
Studies by Holmes and his colleagues have also shown that farmed and wild raccoon dogs in China are often infected with many viruses that can jump between species. “Raccoon dogs are very common viral hosts,” says Holmes.
Right time, right place
Many of the first cases of COVID-19 involved the Huanan market, suggesting it was the location of the viral spillover. SARS-CoV-2 sequences from the first infected people, in late December 2019 and early January 2020, along with geolocation and epidemiological data, support this2.
During the outbreak, the market was shut down by the authorities, but researchers know that raccoon dogs were being sold there, for their fur and as food. In June 2021, a study described the results of monthly surveys of live wild animals sold across four markets in Wuhan between May 2017 and November 2019, including seven stalls at Huanan3. Every month, an average of 38 raccoon dogs were sold at these markets. The most-sold species was the Amur hedgehog (Erinaceus amurensis) at an average 332 individuals a month. Masked palm civet (Paguma larvata), hog badgers (Arctonyx albogularis), Chinese bamboo rats (Rhizomys sinensis), and Malayan porcupines (Hystrix brachyura) were also regularly sold.
December 2019 sales records from the Huanan market also list trading of live animals or products from bamboo rats, porcupines and hedgehogs, among others.
Further evidence to support the raccoon-dog theory came in 2023. Chinese researchers published genomic data of swabs taken at the Huanan market in January 2020, after it was shut down, including of stalls, rubbish bins and sewage4. Studies found mitochondrial DNA of raccoon dogs in several swabs, including those that also tested positive for SARS-CoV-2. Raccoon dogs and hoary bamboo rats (Rhizomys pruinosus) were the most common mammalian wildlife species detected in the mitochondrial DNA; material from civets and hog badgers was also found but not in many samples5. The findings don’t prove that the animals were infected with SARS-CoV-2, but had they been infected, this is the type of evidence you would expect to find, says Andersen.
Unpublished work by Angela Rasmussen, a virologist at the University of Saskatchewan in Saskatoon, Canada, suggests that some of the raccoon dogs and greater hog badgers (Arctonyx collaris) at the market were probably sick, although it is not clear what virus they were infected with.
Other animals at the market, which could also have been infected with the virus and possibly passed it to people, include bamboo rats, Malayan porcupines, and Amur hedgehogs. But researchers don’t know how susceptible those species are to SARS-CoV-2 infection and spread.
Cellular studies suggest that Himalayan palm civets can be infected with the virus, but whether they can pass the virus onto other animals hasn’t been studied, partly because such studies are expensive and weren’t a priority early in the pandemic, says Koopmans.