Pangolins: An intermediate coronavirus carrier?
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The spread of the novel SARS-CoV-2 coronavirus has been associated with the Huanan seafood market in Wuhan, China. Previously, the 2002 SARS outbreak had also implicated similar wet markets. The wet market in Wuhan was cleared shortly after the outbreak, making the identification of the origin of the virus all the more challenging. Though bats are thought to be the original source of this novel virus, the virus may have spread to humans through an intermediate animal carrier.
Pangolins are one of the most illegally trafficked mammals globally. They are used as a source of food and in traditional medicine. SARS-CoV-2 related coronaviruses have already been identified in bats but similar viruses have not yet been identified in other wildlife species.
To test whether coronaviruses are present in other mammals, researchers from the University of Hong Kong studied the RNA sequence of pangolins from Southern China. They used frozen tissue samples of 18 Malayan pangolins that were obtained from Guangxi customs officers during anti-smuggling operations in late 2017 and early 2018.
RNA sequencing indicated that coronaviruses were present in 6 out of 43 samples, but this did not provide the entire RNA sequence of the samples. Using experimental approaches based on known viral genomes, the researchers amplified the entire coronavirus genomes from all six samples. These coronaviruses were found to be closely related to SARS-CoV-2.
Based on these 6 genomic sequences, the scientists also tested archived pangolin tissue samples that were collected in Guangxi in mid-2018 and Guangzhou in early 2019.
From all these samples, they found that the pangolin coronaviruses have 85.5% to 92.4% sequence similarity to the novel SARS-CoV-2 and represent two genetically distinct coronavirus sub-lineages. Surprisingly, one set of pangolin coronaviruses was 97.4% similar to the human SARS-CoV-2 in the protein that binds to human cells through a receptor.
These results tell us that intermediate animal hosts like pangolins may be involved in the spread of viruses in humans. Pangolins and bats are the only known mammals to be infected with SARS-CoV-2 related coronaviruses. It is still unclear whether pangolins were infected by bats or whether there was another animal involved that is yet to be identified. Till we understand the true sequence of events in the spread of the virus, the exact role of pangolins will remain a mystery.
About the author: Aditi Karmarkar is an alumnus of the Tata Institute of Fundamental Research, Mumbai. She currently works as a medical writer for a pharmaceutical company and freelances as a science writer in her spare time