Pangolins that were smuggled into China carry coronaviruses that are closely related to the one behind the COVID-19 pandemic, according to a study which sheds more light on the origins of the deadly virus.
However, the study, published in the journal Nature, said the degree of similarity between the virus in the small anteater mammals, and the one causing the pandemic is not sufficient to suggest that the animals are the intermediate hosts behind the current outbreak.
According to the researchers, including those from The University of Hong Kong, the findings suggest that pangolins are a second mammalian host of coronaviruses.
They said the sale of pangolins in wildlife markets should be strictly prohibited to minimise the risk of future virus transmission to humans.
While evidence suggests that bats may be the reservoir for the pandemic causing virus, SARS-CoV-2, the researchers said the identity of intermediate host animals -- that could have facilitated its transfer to humans -- remains unknown.
A seafood market linked to early cases of the recent outbreak of respiratory disease was cleared out shortly after the outbreak began, the scientists said, impeding the search for the animal species that is the source of the coronavirus.
One possible host, they said, are pangolins -- the most-commonly illegally trafficked mammal, that are used both as food and in traditional medicine.
In the study, Yi Guan and his colleagues analysed samples taken from 18 Malayan pangolins that were obtained from anti-smuggling operations in southern China between August 2017 and January 2018.
They detected SARS-CoV-2-related coronaviruses in 5 of these animals.
On further analysis, they reported the presence of similar coronaviruses in three out of 12 additional animals seized in a second province in 2018, and in an additional animal from a third province from which a sample was collected in 2019.
The viruses isolated from these samples have a sequence similarity of approximately 8592 per cent to SARS-CoV-2, the study noted.
One virus, the scientists said, shows strong similarity in the sequence of the receptor-binding domain -- a region that encodes the spike' of the virus that facilitates entry into host cells.
However, they said all of the pangolin coronaviruses identified to date lack a specific alteration in their sequences that is seen in human SARS-CoV-2.
They said this places uncertainty on their role in the transmission of the novel coronavirus into humans.
According to the researchers, pangolins are the only mammals other than bats that have been found to be infected with a SARS-CoV-2-related coronavirus.
Based on the findings, they said there is a potentially important role for pangolins in the ecology of coronaviruses.
However, the scientists said pangolins cannot be directly implicated in the transmission of SARS-CoV-2 to humans.
They said these mammals should be handled with caution, suggesting that further monitoring of pangolins is needed to understand their role in the emergence of coronaviruses with the potential to infect humans.
"The discovery of multiple lineages of pangolin coronavirus and their similarity to SARS-CoV-2 suggests that pangolins should be considered as possible hosts in the emergence of novel coronaviruses, and should be removed from wet markets to prevent zoonotic transmission," the researchers wrote in the study.
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