Study: Sars-CoV-2 probably originates from wild animals
Published: Friday, Sep 20th 2024, 13:00
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For years, the coronavirus pandemic had the world in its grip - and almost five years after the outbreak, the origin of the Sars-CoV-2 pathogen has still not been clarified beyond doubt. A study has now provided further evidence that it originally came from wild animals traded on the market in the Chinese metropolis of Wuhan and that it did not escape from a laboratory located in the city.
The international research team analyzed more than 800 samples taken by the Chinese Center for Disease Control and Prevention (CDC) in various areas of the Huanan Seafood Market in Wuhan from 1 January 2020. The first Covid-19 cases were recorded in the area surrounding the market, where wild animals were also sold, at the end of 2019. The market had been closed shortly before the samples were taken, meaning that the samples did not come directly from animals, but from the floors, surfaces of cages and stalls, as well as from sewage channels, among other things.
In particular, the samples from the market area where wild animals were traded contained genetic residues of Sars-CoV-2 in addition to the genetic material of animals. The animals included raccoon dogs, slinkies, bamboo rats and porcupines, as the team writes in the specialist journal "Cell". Tanuki in particular are known to carry coronaviruses.
"Many species were removed from the market before the Chinese CDC teams arrived, so we have no direct evidence that the animals were infected," says co-author Florence Débarre from Sorbonne University in Paris. However, the simultaneous presence of animal genetic material and Sars-Cov-2 residues in samples suggests that animals at this market were infected.
Samples contained both founding lines of Sars-CoV-2
This is also supported by the fact that the samples from the market contain the two early pathogen lines - known as A and B. The virus in some samples is very close to the reconstructable origin diversity of the virus, commented virologist Christian Drosten from Charité in Berlin, who was not involved in the study, on this result. "In addition, both founding lines of the pandemic were detected on the market." This can best be explained by the fact that the pathogen has jumped from animals to humans several times.
However, the study does not provide any proof that the pathogen originated from wild animals - this would have required direct samples from traded animals, and these do not appear to exist. "An accumulation of positive samples at a market stall that has sold animals can be explained by infected humans as well as infected animals," emphasizes virus evolution expert Richard Neher from the University of Basel, who was also not involved in the study. "Most of the viral RNA in the samples probably comes from infected humans. At the time the market was closed and the first samples were collected, more than a hundred people at the market were probably infected with Sars-CoV-2 and excreted much more virus than possibly infected animals." The first cases of Covid-19 probably occurred in November 2019.
Expert: "Spark on a powder keg"
Nevertheless, co-author Michael Worobey from the University of Arizona interprets the study results as strong evidence that the pandemic originally came from wild animals. The study is the final piece in a puzzle whose picture was already quite clear anyway, he emphasizes - and describes the most likely scenario. "Bringing wild animals with viruses into contact with humans in the middle of large cities with a high population density is one of the riskiest things you can do," he emphasizes. Not every virus has the potential to cause a pandemic, but it is potentially like "putting a spark to a powder keg".
"These new data are another strong indication that the Covid-19 pandemic started with Sars-CoV-2 infected animals that were caught south of Wuhan and traded and slaughtered at the Huanan animal market," says Friedemann Weber from the University of Giessen, who was also not involved in the study. "During transportation and caging at the market, the virus was able to spread among the animals and jump from them to the people at the highly frequented market. There is no such data for the alternative scenario of the virus escaping from a laboratory."
The study does not specify exactly which animal species are responsible for the virus spreading to humans. According to the study, this is particularly likely to be the case for raccoon dogs, crawling cats, bamboo rats and Malayan porcupines.
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