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Before crossing the world, causing millions of deaths and disrupting life as we knew it, SARS-CoV-2 must have somehow passed from animal host to human – but how?
While the exact route taken by the virus is hotly debated, the data suggests that the original reservoir of the SARS-CoV-2 precursor virus was likely bats.
“It is reasonable to believe that SARS-CoV-2 ultimately comes from bats because so many related viruses circulate in bats”, in particular coronavirussaid Dr Amesh Adalja, infectious disease specialist and principal investigator at the Johns Hopkins Center for Health Security in Baltimore. The related coronaviruses that previously caused epidemics, MERS-CoV and SARS-CoV, both evolved in bats and spread to humans via an intermediate species (camels in the case of MERS and civets in the case of MERS and civets in the case of MERS). the case of SARS).
Related: Quick guide: COVID-19 vaccines used and how they work
At the start of the pandemic, researchers at the Wuhan Institute of Virology released the genome of another strain of coronavirus (RaTG13) previously found in horseshoe bats (Horseshoe Bat affinis), according to nature. The genome of RaTG13 is 96% identical to that of SARS-CoV-2 and remains the most closely related coronavirus found to date.
But a 4% difference in the genome means it’s been around 50 years since they shared a common ancestor, which, in turn, suggests there may still be an intermediate species involved, according to Nature. .
Some of these differences are located at key points in the genome. For example, the genes that code for the virus spike protein, which the virus uses to bind to human cells – specifically the receptor binding domain, the location where the virus attaches to human cells – differ between RaTG13 and SARS-CoV-2, Adalja told Live Science. These critical differences explain “why SARS-CoV-2 is the virus that entered humans and caused the pandemic, and not RaTG13, said Adalja
The next bat coronavirus (RmYN02) has a genome 93.3% similar to that of SARS-CoV-2, according to a study published in June 2020 in the journal Current biology. Related coronaviruses have also been found in Shamel’s horseshoe bats (Horseshoe Bat honteli) which were sampled in Cambodia in 2010 and were recently analyzed, according to a World Health Organization (WHO) report on the origins of the coronavirus which was published in February 2021. The genomes of these coronaviruses (RshSTT200 and RshSTT182) were 92.6% similar to that of SARS-CoV-2.
“The results suggest that the geographic distribution of viruses linked to SARS-CoV-2 is much wider than expected,” according to the WHO report.
To look closer
Scientists don’t yet know where, how, or when SARS-CoV-2 made the changes needed to infect human cells. This process could have happened in bats, or the virus could have passed from bats to another species, such as pangolins, and evolve further there.
A study suggests that more than a century ago, a coronavirus line circulating in bats gave rise to SARS-CoV-2, RaTG13 and a Pangolin coronavirus known as Pangolin-2019, Previously reported live science. The ancestor of the Pangolin-2019 virus likely diverged from the other two at that time; then in the 1960s or 1970s, this line split again in two, creating the ancestor of RaTG13 and the ancestor of SARS-CoV-2.
Another more recent study published in the journal Cell discovered that a single mutation may have given the coronavirus the ability to infect human cells, but it is not known when or in which animal the virus would have acquired this mutation, Science News reported.
Scientists have discovered that pangolin coronaviruses have between 85.5% and 92.4% genomic similarity with SARS-CoV-2, according to another study published in Nature in March 2020. This raises the possibility that pangolins may have been the intermediate host for SARS-CoV-2.
“Evidence from targeted surveys and studies has so far found the most closely related viruses in bats and pangolins, suggesting they may be the reservoir for SARS-CoV-2,” according to the WHO report. But “the viruses identified so far neither in bats nor in pangolins are sufficiently similar to SARS-CoV-2 to serve as the direct ancestor of SARS-CoV-2”.
Mink and cats are also very susceptible to infection with SARS-CoV-2, suggesting that these animals may also serve as “potential reservoirs,” according to the WHO report.
But investigations to look for such viruses in potential reservoir species are not carried out systematically, and “potential reservoir hosts are massively under-sampled,” according to the report. In other words, the potential overflow host could be right under our noses, and we just haven’t tested enough to find it yet.
“It is possible that there is an animal that was not thought of” that served as an intermediate host for the new coronavirus, Adalja said.
It is not easy to disentangle the origin of SARS-CoV-2, or any virus that spreads to humans. “When this virus spread to humans, it did not announce it to the world,” Adalja said. The virus is believed to have first circulated in China in the fall of 2019, which also marks the start of the flu season. It is likely that the first cases of COVID-19 were assumed to be cases of the flu, which means COVID-19 was not diagnosed until the coronavirus became more prevalent, he said. In addition, “the Chinese government was not transparent about these first days of the pandemic and did not allow access to a lot of ongoing research,” he added.
“Because no one has identified a virus 100% identical to SARS-CoV-2 in any animal, researchers still have room to ask questions about other possibilities,” Arinjay Banerjee, virologist at Vaccine and Infectious Disease Organization in Saskatchewan, Canada, Told The Associated Press. One of those possibilities is the “lab leak theory”, which suggests that the virus did not pass from animals to humans around the world but accidentally spread from a sample to workers in a lab.
“I think it’s a possibility,” Adalja said. “We know there were labs that were working with very similar coronaviruses, including RaTG13, and we know biosafety issues exist in all of the labs.” This possibility should be thoroughly investigated and the Chinese government should be transparent, he added.
Yet with the current data, most experts support the hypothesis that SARS-CoV-2 has spread from animals around the world, he said. It will be important to determine the origin in order to be better prepared for the next pandemic, he said. For example, if the virus jumped onto humans from an intermediate animal, knowing which animal can help us reduce human interactions with it, he added.
“This is not the last coronavirus emergency we are going to face,” Adalja said. “Understanding and disentangling the early days of the SARS-CoV-2 pandemic is going to be really important in helping us become more resilient to the next pandemic and be much better prepared for emerging infectious diseases and zoonotic infections in the future. “
Originally posted on Live Science.
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