Research so far indicates that the pangolin, a scaly mammal that is illegally traded, was most likely an intermediary between bats and humans, but not the only one.
Ever since the novel coronavirus started spreading Covid-19 far and wide, researchers have been scrambling to figure out how this virus was transmitted to humans.
The first known case in a human stemmed from a wet market in Wuhan, China, where not only was fresh meat available for sale, but there was also trade in legal and illegal wild animals.
Genetic analysis of the virus, also known as the SARS-CoV-2 due to its similarity to the original Severe Acute Respiratory Syndrome (SARS) virus, has revealed that the virus originated in bats.
But how it came from bats to humans, whether directly or through an intermediate animal, is yet to be determined.
And understanding this is key to figuring out how to control the disease and prevent it.
Initial reports suggested that the virus might have come from bats through snakes, but this was quickly replaced by pangolins.
Pangolins are endangered, anteater-like mammals that are trafficked illegally in China due to their use in pseudoscientific traditional medicine.
The pangolin theory took hold for a while, but soon, studies came out showing that the similarities between the novel coronavirus and the viruses found in pangolins’ bodies were not enough to be conclusive.
And this week, yet another study came out with even more worrying information about pangolin coronaviruses.
What are these researchers working on exactly to determine this intermediate host?
Virus entry mechanism
The virus enters the body by binding to something called the ACE2 receptor.
ACE stands for Angiotensin converting enzyme, and ACE2 is an enzyme that is attached to the cell membranes in our lungs, kidneys, heart, intestines, and arteries.
ACE2 is capable of lowering blood pressure and plays a protective role in cardiovascular health.
Previous studies have noted that the SARS-CoV-2 seems to be optimised for binding to ACE2 through a ‘spike protein’.
This protein helps with the entry in a manner similar to SARS — both viruses have the same protein.
The SARS and SARS-2 viruses share almost 80 per cent of their genetic material.
The new coronavirus has four kinds of structural proteins: The spike (S) protein, the nucleocapsid (N) protein, the membrane (M) protein, and the envelope (E) protein.
The S protein helps the virus’s membrane and the cells to fuse together.
This offers promise, as therapies to treat Covid-19 could actually use ACE2 antibodies to block the novel coronavirus entry.
Such studies are currently being conducted in many parts of the world.
But these four proteins have been found in many viruses, including HIV and others found in bats.
At the point where the S protein enters a cell, there are amino acids present that are essential to the binding process.
The residues of these amino acids can be found in infected cells.
Scientist have found that when they compared the new virus to the viruses found in a pangolin, there were only five differences in amino acids.
For context, when the same coronavirus is studied in humans and in bats, there are 19 differences.
This was the main evidence that pointed to pangolins as the potential intermediate animal.
Study of pangolins
Pangolins are the only known mammals with scales. There are eight species, and they are spread throughout Asia and Africa.
They are also the most trafficked mammal in the world, with all eight species under threat from poaching and illegal trade.
They are hunted for both food as well as for their scales, which are crushed and used in Chinese medicine.
The practice of eating exotic food like pangolins is confined to the upper classes of Chinese society.
It was very likely that pangolins were in the wet market that has since been shut down to prevent the spread of the disease.
The shutdown of the market had the one unfortunate effect — that researchers cannot actually trace the pangolin that gave it to a human and find out where it got the virus from.
“Indeed, a number of mammalian species were available for purchase in the Huanan seafood market prior to the outbreak. Unfortunately, because the market was cleared soon after the outbreak began, determining the source virus in the animal population from the market is challenging,” researchers write in a new paper.
The researchers analysed samples collected from 18 Malayan pangolins from August 2017-2018, which were confiscated at customs. These samples were of lungs, intestine and blood.