Alexandre Hassanin is a lecturer at Sorbonne University and a researcher in evolutionary biology at the National Museum of Natural History. It details research on the animal origin of the Sars-Cov-2.
What do we know about the origin of the SARS-Cov-2 virus, which is responsible for the covid-19 epidemic?
“Several hypotheses have been put forward to explain its origin and its transmission to humans, without it being possible at present to be certain. This virus belongs to the group of Betacoronaviruses, within which it is close to SARS-Cov, a virus that caused the SARS epidemic in 2003, which also started in China, and it is likely that the reservoir for this new virus is also a bat of the genus Rhinolophus, because these animals live in colonies in caves and a large number of sequences close to human viruses have been discovered in these bats. “
What is a tank?
“We call a reservoir a species little or not sensitive to the virus, whose representatives do not develop symptoms of the disease. As a result, the virus is free to evolve in the populations of this reservoir species. It can then according to circumstances jump to one or more so-called sensitive hosts, the representatives of which will develop symptoms of the disease. In the 2003 SARS epidemic, the virus migrated from the tank bat Rhinolophus sinicus to humans, probably passing through an intermediate host, the palm leaf civet mask (Paguma larvata). “
Was there direct transmission of bats to humans in the case of SARS-Cov-2?
“This first hypothesis of direct transmission from a bat to humans is supported by the discovery of a coronavirus isolated from a bat of the species Rhinolophus affinis collected in Yunan province in 2012, whose genome has 96% identity with that of the human SARS-Cov-2 virus.
However, the sequences of the protein S gene, expressed at the level of the viral membrane, are very different between the bat and human viruses. However, a short region of this protein S is the key that allows the virus to enter human cells and thus infect them. This makes the hypothesis of direct transmission to humans more fragile, even if the discovery of new viral strains of bats, this time presenting an S protein closer to SARS-Cov-2, could quickly rehabilitate this hypothesis.”
What about pangolin transmission?
“This second hypothesis is based on the sequencing of several coronaviruses in the Malaysian pangolin (Manis javanica), which have between 90 and 91% identity with SARS-Cov-2. It is less than the bat, but in some of these genomes the key to protein S is very close to that of SARS-Cov-2. This suggests that a bat may have infected a pangolin in the past, which would then in turn have infected the The problem with this assumption is that the Malaysian pangolin is not supposed to be present in the forests near the city of Wuhan. It is present more in the South of China or in the other bordering countries, such as Myanmar, Thailand, Laos and Vietnam. This means that either the human index case (the first infected person) traveled from the South to Wuhan, or sick pangolins were transported from the South to Wuhan. “
Does that mean they are animals trafficked from wildlife?
“Pangolins are endangered species because they are trafficked for their scales, to which people in Asia attribute aphrodisiac virtues.
All pangolins tested for the presence of coronavirus by Chinese researchers were seized by customs and most were sick. Knowing that these animals are solitary, we can think that they contracted the virus during their detention in a cage near their congeners and / or the tank bat. “
Where does this controversy about the manufacture of the virus in the laboratory come from?
“It is based on several elements, but seems not very credible. Some people put end to end several disparate elements to make a hypothesis. In 2015, a team of American and Chinese researchers had created a chimera virus in the laboratory to verify that SARS -Cov could be transmitted directly from the bat to humans. This team included two researchers from a P4 laboratory in Wuhan (located near the epicenter market of the epidemic in December 2019). who published the genome of the bat virus Rhinolophus affinis, which currently remains the closest sequence to SARS-Cov-2, hence the theory that these same researchers created a new virus from scratch.
When you build a chimera virus in the laboratory, you usually use two known viruses, that is, already sequenced. In the case of SARS-Cov-2, the nucleotide sequence of its genome is not found in part in any of the genomes listed in the databases. This hypothesis of a virus created in the laboratory is therefore not supported by sequence analysis. “
What can research bring in the field?
“To better understand the respective roles of bats and pangolins in transmission to humans, it will be necessary to study the diversity and prevalence of closely related SARS-Cov-2 viruses in bats and pangolins collected in their natural environment.
Such a study will obviously have no interest in finding a cure or a cure for covid-19 disease, but it would, on the one hand, put an end to rumors suggesting a laboratory origin of the virus, and on the other hand, to better understand the evolutionary dynamics of this group of viruses. “