The Bat, the Pangolin and the City: A Tale of COVID-19
Costanza Manes | Research Fellow at the One Health Center
It is highly probable that most human coronaviruses that have caused epidemics, and now even pandemics, have originally come from bats. It is safe to say that they should have stayed in bats. This informative sheet will go over the current COVID-19 pandemic from a wildlife biology and conservation prospective, offering insights on the possible underlying dynamics of this pandemic. As of now, scientists are offering a potential cross-species pathway for SARS-CoV-2: from bats, via pangolins, to humans.
There is a 96% DNA match between SARS CoV 2 and a coronavirus in bats
Researchers have found that SARS-CoV-2 in humans shares about 96% of its genome sequence with a coronavirus found in bats (Cyranoski, 2020). Bats have often been indicated as the most likely animal reservoir candidate for coronaviruses in human populations. Bats harbor a large variety of viruses, including coronaviruses. Severe Acute Respiratory Syndrome (2002) and Middle East Respiratory Syndrome (2012) coronaviruses both originated from bat reservoirs. The highly likelihood of bats being natural reservoirs of viral diseases comes from their biological adaptations.
In the course of evolution, bats have developed a unique ability that no other mammal has: flying. Flight is an extremely complex activity, which requires enormous physical effort. Such effort causes metabolic rate acceleration, which leads to high body temperature. This febrile state allows bats to share their organism with viral communities, while simultaneously being protected from their potential harmful effects (O’Shea et al., 2014; Brook and Dobson, 2015). While a febrile state debilitates humans, it is a natural state for bats. Bats have been living with some of these viruses for millions of years. The problem occurs when such pathogens make a “spillover”, meaning they jump from a reservoir species (bats) into another host species. Those viruses that live so peacefully in bats, might exert a completely different effect after a spillover into another host. Bats are also very good at transmitting viruses that they bear.
First of all, they have the social habit of clustering together. The high contact rates keep pathogens flowing from one individual to the other within a population. Secondly, as we mentioned, bats can fly. This adaptation reveals itself beneficial once again for pathogens, as it allows spread over long distances. Bats’ ability to harbor diseases and spread them so efficiently makes them optimal reservoirs. This is why we observe this species mentioned so often whenever we are talking of zoonotic epidemics.
There is a 90% DNA match between SARS CoV 2 and a coronavirus in pangolins
Researchers have found that SARS-CoV-2 in humans shares about 90.3% of its genome sequence with a coronavirus found in pangolins (Cyranoski, 2020). This statement informs us of the possibility that a virus has spilled over from a very rare and shy reptile-looking mammal to human beings. It sounds improbable but it is actually quite likely.
There are eight species of pangolins: four in Asia and four in Africa. Since the early 1900s, pangolins have been harvested for human use. CITES (Convention on International Trade in Endangered Species) data indicate that between 1977 and 2012 an estimated 576,303 Asian pangolins were internationally traded (Challender, Harrop and MacMillan, 2015). From this and other numbers we can assume that the demand for wild pangolin is high.
This is explained by the typical customs of certain Asian cultures. In Chinese traditional medicine, pangolin scales are dried and crushed, and used as natural remedies against arthritis and lactation difficulties. Moreover, pangolin meat is considered a delicacy across ultra-wealthy classes across China and Vietnam. This high demand has led pangolins to be poached extensively, almost to the point of extinction. The largest ever seizure of pangolin scales was registered in Singapore in 2019: 12.7 tons of pangolin parts were intercepted while traded between Nigeria and Vietnam. That equals to about 36,000 pangolins (The Telegraph, 2019). The supply of pangolins is then often sold in urban settings: in wet markets.
And now humans
Differently from “dry” markets, that sell dry goods such as furniture, clothing and textiles, “wet” markets are dedicated, completely or partially, to the sale of meat, fish, produce and other perishable goods. Illegal wildlife trade for consumption ends up mostly here. In main urban centers, crowded with citizens, animals are forced and mixed into a limited space. This comprises a scenario of multiple species that possibly never shared a habitat before now sharing the same space. This variegated multi-species urban scenario provides more opportunity for a spillover event. A virus now has the chance to jump into a new host, where it might be successful. The main 96% and 90.3% similarities between the coronaviruses lay in the Receptor Binding Protein (RBP), which is the part of the virus able to bind the host cell and gain access.
In an animal reservoir many mutants arise, some which might have traits that enable them to replicate and bind to cell in other species. This might have happened in the RBP of SARS-CoV-2, granting it access to new host species. This information is not yet confirmed, more solid scientific evidence can only be given by discovering the actual virus inside an animal host, which usually takes years and resources. Typically, the true animal reservoir of a human disease is only found years after an epidemic is over. Wet markets as the origin of the COVID-19 outbreak are still only an hypothesis and it would not be fair to condemn all wet markets and products of Asian culture. The issue lays in the underlying smuggling and illegal trade of wild animals and the unsustainable demand that fuels it.
Thus, we cannot say for certain that SARS-CoV-2 jumped from a bat, to a pangolin, to human beings. However, these data report a significantly high chance of that scenario. We encroach on wild animal populations, trade them, mix them together and therefore create various opportunities for virus spillover. One of those may have been SARS-CoV-2, which leaped to a human host, and caused what is now a global pandemic. This is because we provided the possibility.
In bats, pangolins and possibly other animals, nature had created separate coronavirus viral populations for a reason. Our incessant intervention brought those populations to mix together and move them where they were not supposed to thrive. Not only are we growing in number, but humanity is treating the planet like its own. Urbanization and globalization are shaking the very bases of biology and the dynamics of planet Earth. We have to understand that we are one: humans, animals and the environment. This pandemic is the opportunity for us to see what humans actions can cause if we do not make the collective decision of living a more sustainable and respectful life.
The cause is not the animals, it is how we relate to animals.
- Cyranoski, D., 2020. Mystery deepens over animal source of coronavirus. Nature, 579(7797), pp.18-19.
- O’Shea, T., Cryan, P., Cunningham, A., Fooks, A., Hayman, D., Luis, A., Peel, A., Plowright, R. and Wood, J., 2014. Bat Flight and Zoonotic Viruses. Emerging Infectious Diseases, 20(5), pp.741-745.
- Brook, C. and Dobson, A., 2015. Bats as ‘special’ reservoirs for emerging zoonotic pathogens. Trends in Microbiology, 23(3), pp.172-180.
- Challender, D., Harrop, S. and MacMillan, D., 2015. Understanding markets to conserve trade-threatened species in CITES. Biological Conservation, 187, pp.249-259.
- The Telegraph, 2019. 36,000 Endangered Pangolins Thought To Have Been Killed After Singapore Makes Record Seizure Of The Animals’ Scales. [online] Available at: <https://www.telegraph.co.uk/news/2019/04/09/singapore-makes-record-seizure-endangered-pangolin-scales/> [Accessed 25 March 2020].