Why pangolins may not be the intermediate host

The origin debate over the novel coronavirus (SARS-CoV-2) has yet to be settled. Most of the evidence suggests that the virus jumped the species barrier directly or through an intermediate host from bats to humans. There is another opinion that the virus may have escaped or leaked from the laboratory of the Wuhan Institute of Virology.

Such lab leaks had indeed occurred in the past. In 2004, two researchers in a virology lab in Beijing working on the 2002 SARS virus were independently infected. The virus was transmitted to a total of seven people, but the outbreak was brought under control quickly enough. Some of scientists and others believe that the SARS-CoV-2 virus may have followed the same course but caused the pandemic.

No conclusive evidence

Though irrefutable evidence for a natural origin is lacking, as scientists have so far been unable to identify bats harboring viruses very similar to the novel coronavirus to establish that the virus had indeed jumped directly from bats to humans. Nor have they been able to definitively identify the intermediate host from which the virus jumped onto humans and started spreading among humans.

While the SARS-CoV-2 virus is quite similar to the RATG13 coronavirus found in horseshoe bats, the genomes of the two viruses are only 96% similar. So the virus, if it had jumped from bats to humans, has yet to be identified in bats. Pangolin has been suggested as a potential intermediate host that could have harbored the coronavirus before it made the big leap of spreading to humans. Many studies have found similarities between the coronavirus in pangolin and the SARS-CoV-2 virus in terms of genome sequences. A recently published study in the news iScience evaluated the biological characteristics of the pangolin coronavirus. The researchers from Beijing University of Chemical Technology studied the pathogenicity and transmissibility of pangolin coronavirus by infecting golden Syrian hamsters and compared it with hamsters infected with SARS-CoV-2.

They found that the pangolin coronavirus could not only effectively infect hamsters, but also caused similar reactions in tissues as the new coronavirus. Although both viruses appear to have the same affinity for the receptors, the pangolin coronavirus was able to replicate efficiently in the respiratory system and brain, much like the SARS-CoV-2. However, the scientists were unable to find contagious pangolin coronavirus in organs other than the respiratory system and brain, which is different from hamsters infected with the SARS-CoV-2 virus.

Hamsters infected with the pangolin coronavirus did not suffer substantial body loss, while hamsters infected with SARS-CoV-2 showed a slight decrease in body weight within the first five days of infection and then regained weight.

Pathogenesis difference

Alveolar wall thickening of lungs from hamsters infected with pangolin coronavirus was “widespread”, while alveolar wall thickening in hamsters infected with SARS-CoV-2 was severe. There were also other differences in pathogenesis. Overall, the pangolin coronavirus caused moderate disease in hamsters and was less virulent than SARS-CoV-2.

There were differences in the way the viruses replicated — SARS-CoV-2 replicated much higher in the hamsters than the pangolin coronavirus could. Viral shedding from hamsters infected with the pangolin coronavirus lasted for three days, while hamsters infected with SARS-CoV-2 shed virus for five days.

The main difference was in the transmission route. While the pangolin coronavirus did not spread via aerosols but only via contact transmission, SARS-CoV-2 showed “efficient contact transmissibility and efficient aerosol transmissibility with a transmission efficiency of 100%,” the authors write. One reason for the lack of aerosol transmission in the case of pangolin coronavirus could be because of the less viral aerosols produced by the infected hamsters. The second likely reason could be the larger size of the viral particles exhaled by hamsters infected with the pangolin coronavirus. In contrast, SARS-CoV-2 infected hamsters produced significantly higher amounts of viral aerosols, they found. Also, the amount of virus particles exhaled per minute by hamsters infected with SARS-CoV-2 was almost 2.5 times more than hamsters infected with the pangolin coronavirus.

Coronavirus spread

Based on the study, the researchers conclude that the infection characteristics of pangolin coronavirus and SARS-CoV-2 are similar, although the pathogenicity and transmissibility are much more in hamsters infected with the SARS-CoV-2 virus. Although the pangolin coronavirus spread from one hamster to another only through direct contact, not aerosolization, the “public health risk that the pangolin coronavirus are potential candidates for global spread” could not be ignored, they write. They also warn that “continued monitoring of the mutation and evolution of pangolin coronavirus should be implemented in the future and the illegal wildlife trade of pangolins should be effectively controlled.”

The study does not conclusively demonstrate that pangolins may have been the intermediate hosts. But the possibility of pangolin coronavirus crossing the species barrier and infecting humans in the future cannot be ruled out.

SOURCE – www.thehindu.com

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