Scientist Looks Anew at Raccoon Dog Data, Stressing the Unknowns
A new study of genetic data from a market in Wuhan, China, says the data don’t support the case that the pandemic started with illegally traded animals, and how other scientists are critical of the puzzle. He touched on a renewed discussion about the samples he sees as a piece of coronavirus reaching humans.
of new researchWe investigated the relative amounts of animal and viral material in swabs collected from surfaces on the market in early 2020.
But several outside experts say the analysis, posted online this week by the study’s author, Jesse Bloom, a virologist at Fred Hutchinson Cancer Center, reveals many unknowns about how the data is filtered. Said it may have been influenced by variables and decisions.
For these reasons, this finding did not greatly sway their impression of previous studies. It was consistent with the possibility that it happened, but did not prove it happened.
Frederick Bushman, a microbiologist at the University of Pennsylvania who specializes in analyzing samples like those from Wuhan, said, “I think it’s quite possible that they picked up an infected raccoon dog, but that’s what caused it. It doesn’t prove that it happened,” he said. I was not involved in market research. “I don’t think Bloom’s paper changed my mind that much.”
Chinese researcher wrote about market data Gene sequences became available last year, allowing an international team of scientists to study them.that team wrote in the report Last month, based on data, it was not possible to conclusively identify the animal that transmitted the virus to humans.
But they say that animals thought to be susceptible to the virus, such as raccoons and palm civets, small Asian mammals implicated in the SARS outbreak 20 years ago, were on the market in late 2019. The data confirmed that the earliest Covid-19 patients were also working and shopping at markets.
The market was one of four reportedly selling live animals of the kind that could spread the virus in Wuhan, so scientists believe that so many early They said it was unlikely that the patients were brought to market purely by chance. rice field.
In this week’s study, a different approach was taken to analyzing gene sequences.
Dr. Bloom investigated whether the amount of genetic material from the virus correlated with the amount of genetic material from the susceptible animal species in the sample. If the species on the market are predominantly responsible for virus shedding, he said in interviews that there is a clear link between the amount of genetic material from the virus and the amount from that species. He said he expected it.
However, no such clear correlation was found in this study. Instead, a variety of fish sold at the market had the strongest correlation, indicating that they may not have been infected, indicating that infected people likely deposited viral material where the fish were. I’m here.
Dr. Bloom said the findings suggested the virus, also known as SARS-CoV-2, had spread widely throughout the market by the time swabs were collected in early 2020.
“Similarly, the fact that largemouth bass and catfish samples are heavily contaminated with SARS-CoV-2 shouldn’t be read too much. With the CoV-2 reading,” said Dr. Bloom.
But outside experts said various characteristics of the samples could undermine efforts to link animal and viral material. He considered conducting an analysis, but said it would risk giving misleading results. I was.
The virus’ genetic material is rapidly degraded, said Christopher Mason, an expert in environmental sampling at Weill Cornell Medicine. Importantly, viral material can decay at a different rate than material from animals, making it difficult to compare them in samples collected over the weeks following market closures.
The fish most closely associated with the virus may simply have been frozen or refrigerated, slowing the decay of viral material in those samples, says KU Leuven of Belgium. said evolutionary biologist Tom Wesselers.
The latest analysis confirms that “looking at this kind of correlation tells us very little about which host species might have been the plausible cause of the pandemic,” says Wenseleers. says Dr. This leaves scientists in the same situation as before, with market data not providing conclusive evidence for any particular origin scenario, he said.
The new study also looked closely at cotton swabs from market carts where an international team found traces of the virus along with the raccoon dog’s genetic signature, but no detectable genetic material from humans.
Dr. Bloom wrote that the swabs contained only trace amounts of viral material, and it was not clear why the Chinese researchers classified the swabs as positive for the novel coronavirus. According to his research, only swabs had significant amounts of raccoon dog genetic material, including traces of the virus.
But some scientists said Dr. Bloom’s analysis ran the risk of rejecting other Covid-positive swabs because the standard for the amount of animal genetic material in the samples was too high.
Dr. Bushman of the University of Pennsylvania said the threshold used in the analysis was “aggressive” and that it is best to compare results obtained from a series of different cutoffs.
An international team of scientists used a more sensitive threshold to screen multiple Covid-positive samples containing genetic material from raccoon dogs and samples with genetic signatures from different animals thought to be susceptible to the virus. identified.
Alexander Kritz-Christof, a former Johns Hopkins University computational biologist who led the international team’s analysis, also asked whether the Chinese researchers were correct in describing the swabs from the cart as positive for the virus. said to have investigated.
He noted that many other swabs from the same stall were clearly positive for the virus. said that it showed that most of the truly negative swabs contained no traces of the virus at all.
“This is an environmental sampling of a virus that is a tiny needle in a haystack,” Dr. Kritz-Christoff said.