Within the area of some weeks, now we have all realized lots about COVID-19 and the virus that causes it: SARS-CoV-2. However there have additionally been a variety of rumours.
And whereas the variety of scientific articles on this virus is rising, there are nonetheless many gray areas as to its origins.
Wherein animal species did it happen? A bat, a pangolin or one other wild species? The place does it come from? From a cave or a forest within the Chinese language province of Hubei, or elsewhere?
In December 2019, 27 of the primary 41 individuals hospitalised (66 p.c) handed by means of a market situated within the coronary heart of Wuhan metropolis in Hubei province. However, in accordance with a examine performed at Wuhan Hospital, the very first human case recognized didn’t frequent this market.
As an alternative, a molecular relationship estimate based mostly on the SARS-CoV-2 genomic sequences signifies an origin in November. This raises questions in regards to the hyperlink between this COVID-19 epidemic and wildlife.
The SARS-CoV-2 genome was quickly sequenced by Chinese language researchers. It’s an RNA molecule of about 30,000 bases containing 15 genes, together with the S gene which codes for a protein situated on the floor of the viral envelope (for comparability, our genome is within the type of a double helix of DNA about three billion bases in dimension and accommodates about 30,000 genes).
Comparative genomic analyses have proven that SARS-CoV-2 belongs to the group of Betacoronaviruses and that it is extremely near SARS-CoV, answerable for an epidemic of acute pneumonia which appeared in November 2002 within the Chinese language province of Guangdong after which unfold to 29 international locations in 2003.
A complete of eight,098 instances had been recorded, together with 774 deaths. It’s identified that bats of the genus Rhinolophus (doubtlessly a number of cave species) had been the reservoir of this virus and small carnivore, the palm civet (Paguma larvata), could have served as an intermediate host between bats and the primary human instances.
Since then, many Betacoronaviruses have been found, primarily in bats, but in addition in people. For instance, RaTG13, remoted from a bat of the species Rhinolophus affinis collected in China’s Yunan Province, has not too long ago been described as similar to SARS-CoV-2, with genome sequences similar to 96 p.c.
These outcomes point out that bats, and particularly species of the genus Rhinolophus, represent the reservoir of the SARS-CoV and SARS-CoV-2 viruses.
However how do you outline a reservoir? A reservoir is one or a number of animal species that aren’t or not very delicate to the virus, which is able to naturally host one or a number of viruses.
The absence of signs of the illness is defined by the effectiveness of their immune system, which permits them to battle towards an excessive amount of viral proliferation.
On 7 February, 2020, we realized virus even nearer to SARS-CoV-2 had been found in pangolin. With 99 p.c of genomic concordance reported, this prompt a extra probably reservoir than bats.
Nevertheless, a current examine beneath assessment exhibits that the genome of the coronavirus remoted from the Malaysian pangolin (Manis javanica) is much less much like SARS-Cov-2, with solely 90 p.c of genomic concordance. This is able to point out that the virus remoted within the pangolin isn’t answerable for the COVID-19 epidemic presently raging.
Nevertheless, the coronavirus remoted from pangolin is analogous at 99 p.c in a particular area of the S protein, which corresponds to the 74 amino acids concerned within the ACE (Angiotensin Changing Enzyme 2) receptor binding area, the one that permits the virus to enter human cells to contaminate them.
Against this, the virus RaTG13 remoted from bat R. affinis is very divergent on this particular area (solely 77 p.c of similarity). Because of this the coronavirus remoted from pangolin is able to coming into human cells whereas the one remoted from bat R. affinis isn’t.
As well as, these genomic comparisons counsel that the SARS-Cov-2 virus is the results of a recombination between two completely different viruses, one near RaTG13 and the opposite nearer to the pangolin virus. In different phrases, it’s a chimera between two pre-existing viruses.
This recombination mechanism had already been described in coronaviruses, particularly to elucidate the origin of SARS-CoV. It is very important know that recombination ends in a brand new virus doubtlessly able to infecting a brand new host species.
For recombination to happen, the 2 divergent viruses should have contaminated the identical organism concurrently.
Two questions stay unanswered: during which organism did this recombination happen? (a bat, a pangolin or one other species?) And above all, beneath what situations did this recombination happen?
Alexandre Hassanin, Maître de Conférences (HDR) à Sorbonne Université, ISYEB – Institut de Systématique, Evolution, Biodiversité (CNRS, MNHN, SU, EPHE, UA), Muséum nationwide d’histoire naturelle (MNHN).
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