Scientists around the world, who are engaged in coronavirus research, have astonished about one thing: How did the virus get into the human host?
In order to colonise and multiply inside an unsuitable host, an organism must modify its genetic makeup. The mutation that helped coronavirus survive in human body would have changed its DNA, the hereditary material of life.
Understanding where SARS-CoV-2—the virus that caused the COVID-19 pandemic—came from and how it spreads is important for its control and development of drugs.
Researchers who studied the outbreak of Severe Acute Respiratory Syndrome (SARS) in 2003 had concluded that coronaviruses originated from bats. Now most experts agree that bats are a natural reservoir of SARS-CoV-2, but an intermediate host was needed for its ‘jump’ from bats to humans.
SARS-CoV-2 virus was first reported in Wuhan, the capital of the central Hubei province of China in December 2019. If bats were not available in the wet markets, how did the coronavirus get transmitted to people in Wuhan? Scientists who were looking at this missing link initially thought it was snake, which are sold in Chinese markets.
A new study published in the Journal of Proteome Research of the American Chemical Society suggests that scaly, anteater-like animals called pangolins are the missing link for SARS-CoV-2 transmission between bats and humans.
Pangolins, which are endangered species, are a great delicacy in China and their scales are valued for their medicinal virtues. Malayan Pangolins are among the most trafficked animals and they are illegally traded in the wet markets of Wuhan. This suggests pangolin as the most likely intermediate host for the new coronavirus before its transmission to humans.
The new study uncovered an error in the earlier analysis that suggested snakes as an intermediate host of the new virus. As coronaviruses are known to infect mammals and birds, it was a wrong to suggest snakes as this host.
The genome sequencing provided more clues to the virus' origins. The sequences of 2019-nCoV samples taken from patients in China were almost identical, which proved the ‘jump’ into human host was only recent.
The new study also exposed the false conclusion that there are ‘uncanny similarities’ between a key coronavirus protein and HIV-1 protein. Using new and larger data sets, scientists compared the sequence of the spike protein—a key protein responsible for getting the virus into mammalian cells—of the new coronavirus and HIV-1 protein.
They found that, in contrast to the claim that four regions of the spike protein were uniquely shared between SARS-CoV-2 and HIV-1, the four sequence segments could be found in other viruses, including bat coronavirus.
After uncovering an error in the previous analysis that said snakes as an intermediate host, the team searched DNA and protein sequences isolated from pangolin tissues for ones similar to SARS-CoV-2. The researchers identified protein sequences in sick pangolins’ lungs that were 91 percent identical to the human virus' proteins.
Furthermore, the study compared receptor-binding domain (RBD) of the spike protein from these animals with that of SARS-CoV-2 in human. While there were 19 amino acid differences between the human and bat viral proteins, the study showed only five differences between pangolin coronavirus and SARS-CoV-2.
However, researchers have not ruled out the possibility of additional intermediate hosts between bat and human.