A year and eight months after the first Covid-19 cases in the city of Wuhan, China, it is still a mystery where the Sars-CoV-2 coronavirus, the agent responsible for the disease, came from.
In the last week, however, three independent events have rekindled the debate over the origin of the virus.
First, last Tuesday (24), US President Joe Biden received the final version of the report from the intelligence agencies, commissioned three months earlier by him. The text states that it was not possible to confirm or deny the so-called laboratory escape theory, that is, that the virus was studied in a laboratory, from where it escaped and started to infect human beings.
Although there is still some suspicion of the government itself about the result, the American intelligence agencies consider that the hypothesis of animal origin of the virus is the most probable.
Last Wednesday (25), a team of experts from the WHO (World Health Organization) quoted to carry out searches in China regarding the origin of Covid argued that it is becoming increasingly difficult to gather evidence about where the pathogen appeared. The report released by the WHO in March of this year was also inconclusive, although it also placed the hypothesis of animal origin as more likely.
And finally, a group of researchers published last Friday (27), in the scientific journal Science, an article that provides a good summary of the situation and presents arguments in favor of a third transmission route: the consumption chain, in which wild caught animals can potentially infect others raised on farms.
Scientists argue that, in addition to a natural passage from a host to humans — the so-called “spillover”, or the “jump” between species, something already known for other coronaviruses — it is necessary to consider the hypothesis of an indirect contagion through an animal raised and traded for both meat and fur.
The hypothesis in favor of the natural origin of Sars-CoV-2 already brought evidence from several other studies carried out in the past, including the largest evolutionary study of coronaviruses, with more than 2,000 unique genomes, showing an origin from bats of the virus to the humans.
However, the fact that the natural host of a species close to Sars-CoV-2 — the coronavirus isolated from RaTG13 bats, previously considered the most recent common ancestor — has never been found, shares only 96.3% of the genome with the Sars-CoV-2, which represents a big evolutionary leap— still intrigues experts.
How, then, would a virus of the Sars-CoV type have arisen in Wuhan, about 1,500 kilometers from Yunnan, where RaTG13 was found, almost a decade later, evolved and jumped into humans?
The focus, the researchers say, should not be on the Yunnan bat population, but rather the distribution of species believed to be potential reservoirs for viruses across China, from east to west, and in much of Southeast Asia and beyond. Japan.
like bats of the genus Rhinolophus, also known as horseshoe bats, and from where the RaTG13 samples were obtained, have a wide distribution in Asia, distance would not be a problem.
Recently, a study published in the journal Cell identified, from more than 400 bat samples Rhinolophus, 24 complete genomes of coronaviruses, of which seven were unknown and four were of the Sars-CoV type.
One of these coronavirus species found in bats, RpYN06, is today the most closely related to Sars-CoV-2, with which it shares an extensive portion of its viral genome.
The theory defended by scientists is that a leap for humans could have occurred through the exchange of viruses between wild animals sold in markets and those raised on farms, since both are traded, transported and confined to the same spaces in most markets. Asians. However, there is still no clear definition of how this passage would have occurred.
The contamination of meat and other items of animal origin has already been investigated by the WHO, which tested about 80 thousand animals and animal products for the coronavirus in its investigation. But even a sample of this size represents a “needle in a haystack” if one does not consider the entire universe of possibilities as yet unknown.
“The scale in large quantities of meat transport, including animal carcasses susceptible to Sars-CoV-2 transmission, may have played a role in the emergence of coronavirus in Wuhan,” the authors state, and thus come from a much further away, not just from the city that was the epicenter of Covid-19.
The authors’ argument is that the great shortage of pork in 2019 due to the African swine fever virus in China would have shifted consumption to other meats, including wild species, which makes the contact of these animals with farmed animals much closer.
Two other studies released this year already pointed to the risks of the mode of transport and production associated with the sale (illegal or not) of animal-derived products. One of them brought a risk classification for new zoonosis events according to the type of market, dividing them into for consumption or not, with perishable goods (fresh meat, including live animals) or non-perishable.
The conclusion of this study is that there is a greater risk to health and also to biodiversity in the so-called wet markets, such as the Huanan market.
The theme is complex, since it consists of carrying out exhaustive genomic monitoring in all populations of animals that may be natural reservoirs of these viruses —which includes species as distinct as chickens and minks—, and it is still unknown whether the passage of the coronavirus from bats to humans had or did not have an intermediate host.
For science, the more time passes, the more difficult it becomes to gather evidence that can unravel this mystery. That’s because biological samples, like blood samples from early patients, begin to degrade. Genetic traits of the virus and its ancestors in likely places of origin or indirect contagion, such as the animal market in Huanan, are also lost.