The term “pangolin bat” links two different mammals in discussions about the origins of zoonotic diseases. The connection between pangolins, solitary scaled mammals, and bats, known reservoirs of numerous viruses, gained global focus during the COVID-19 pandemic. Understanding this link requires exploring the biology of each animal and the circumstances that may have brought them into contact.
Pangolins: The World’s Most Trafficked Mammal
Pangolins are the only mammals wholly-covered in keratin scales, which they use for protection. Eight species are found across Asia and Africa, with diets consisting mainly of ants and termites. These nocturnal and solitary creatures are shy, rolling into a tight ball when threatened.
This defense mechanism is ineffective against their primary threat: humans. Pangolins are the world’s most trafficked mammals, driven by high demand in Asia for their meat and scales, which are used in traditional medicine. This has led to all eight species being listed as threatened with extinction.
The illegal trade places them in situations, such as wildlife markets, where they come into close contact with other species they would never encounter in the wild. This proximity is a factor in their connection to zoonotic disease discussions.
Bats: Natural Reservoirs of Viruses
Bats are natural reservoirs for many viruses that can cause severe illness in humans, including those for SARS and MERS. They can carry and transmit these pathogens without showing signs of illness. This is possible because their unique immune systems, influenced by the high metabolic demands of flight, can tolerate viral loads that would be lethal to other mammals.
This tolerance allows viruses to persist within bat populations. Their social behaviors, such as roosting in large colonies, further facilitate the spread of viruses among individuals. With over 1,200 species distributed worldwide, bats are a primary focus for scientists studying emerging infectious diseases.
The COVID-19 Connection: Investigating the Bat-Pangolin Hypothesis
During the investigation into the origins of SARS-CoV-2, the virus causing COVID-19, a hypothesis emerged involving both bats and pangolins. Scientists agree the virus likely originated in bats, as a coronavirus with 96% genetic similarity was found in horseshoe bats. However, an evolutionary gap suggested a possible intermediate host was needed for the virus to jump to humans.
Pangolins became a candidate for this intermediate role. The theory suggests the virus was transmitted from a bat to a pangolin, likely within a wildlife market where trafficked species are kept in close proximity. In this new host, the virus could have mutated, adapting to infect other species, including humans.
Scientific Evidence and Debates
The scientific case for pangolins as an intermediate host is based on coronaviruses found in them that share similarities with SARS-CoV-2. The receptor-binding domain (RBD) of a pangolin coronavirus’s spike protein was found to be very similar to that of the human virus. The RBD is the part of the virus that allows it to lock onto and enter host cells.
However, the overall genetic makeup of the pangolin coronaviruses discovered so far is not as close to SARS-CoV-2 as the RaTG13 virus found in bats. While the RBD similarity is notable, the rest of the viral genome is more distant. This has led some researchers to question whether pangolins were the direct stepping stone to humans.
The debate continues, with several possibilities under consideration. One is that another, yet unidentified, animal served as the intermediate host. Another possibility is that the virus jumped directly from bats to humans.