The COVID-19 pandemic is a ‘Public Health Emergency’ of a scale like no other in the past couple of decades. Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), earlier known by the provisional name 2019 Novel Coronavirus (2019-nCoV), is a virus, which causes mild to moderate respiratory illness in most individuals. However, the immunocompromised are much more likely to develop severe illness.
Coronaviruses (CoVs) are a group of related RNA viruses in which this protein coat has projections(thus making it appear like a crown; In Latin, corōna means crown). Also, this virus is zootonic in nature, like some other disease-causing viruses (like ones causing the Ebola, H1N1 and SARS outbreaks), which means that they can ‘jump’ between animals and people. This jump is called a viral spill. Viruses are known to mutate fast (i.e.it required fewer generations for a specific mutation to manifest in the population); which also makes making vaccines against viral diseases such a daunting task. The explanation of such ‘viral spills’ between organisms is probabilistic, and it is fascinating to conceive this from an evolutionary point of view. A probabilistic model is based on the fact that randomness plays a role in predicting future events, and such randomness characterizes the realm of life sciences.
Bats are rightly infamous to be pools of several zoonotic viruses, including corona viruses (CoVs) that cause disease in humans and animals. Bats being the only mammal that flies, requires a high amount of activity for them, which has resulted in their immune systems to become very specialized. This character enables it to host pathogens in its body and to equip these pathogens the ability to bear higher body temperatures (as flying increases their body temperatures, these pathogens evolve to withstand it). Typically, viruses found in bats require an intermediate host before spilling over into humans, like what was observed with SARS-CoV and civets or MERS and camels. Closer the host is in its phylogenetic history with humans; more are the similarities in the genomes of the two organisms, fewer mutations the host-virus might need to infect us, hence making the viral spill more likely. However, molecular epidemiology and phylogenetic studies on CoVs highlight the plasticity of their genome, which makes these viral spills all the more unpredictable.
The receptor-binding domain(RBD) of the virus allows the virus to clasp on to and enter a cell. Some pangolin coronaviruses exhibit substantial similarity to SARS-CoV-2 in the receptor-binding domain (RBD), including all six key amino acids that makes it unique and substrate specific. The six RBD amino acids decide the host range of similar coronaviruses. This match clearly shows that the optimization of SARS-CoV-2 for latching onto human-like receptors is the consequence of natural selection. Researchers suggest two scenarios that can explain the origin of SARS-CoV-2 in nature: (i) natural selection in an animal host before zoonotic transfer; and (ii) natural selection in humans after the viral spill. In the first case, as the new coronavirus evolved in its natural hosts, perhaps bats or pangolins, its spike proteins mutated to attach to molecules similar in structure to the human receptor protein (ACE2), hence allowing it to infect human cells. In the other scenario, the disease is a result of gradual evolutionary changes over the years; the virus ultimately gained the ability to spread from human-to-human and cause a life-threatening disease. We cannot rule out either of the possibilities as of now.
The question to ponder upon is such diseases moved from bat communities which seldom are in direct contact with humans, to spread across Earth. The answers advocate the need to rethink how we treat the planet altogether. As one may observe, the tropical rainforests of Africa were claimed to be the originating grounds of the deadly Ebola. While the origin of COVID-19 and SARS can be traced back to specific wet markets in Asia. So a crucial question to ask is why all these deadly diseases new to us were emerging from biodiversity hot spots.
Humans have been changing landscapes and invading newer ecosystems at a pace like never before. And with new organisms, we come in contact with unknown viruses. When we disturb ecosystems, it is like us shaking loose viruses from their original hosts. When that befalls, they need another host. Usually, that is some other animal, and via them, these viruses can reach us and cause havoc in human populations. Thus, we observe that damage to the planet can also harm people more quickly and severely than the gradual shifts of climate change.
To summarize the scenario at present, we have little control over these diseases to prevent them from ever spilling over to Homo sapiens, i.e., us. All we can do is to minimize the disease penetrance in the population. We can study and analyze distribution, patterns, and determinants of health and disease conditions from previous outbreaks to understand and monitor COVID-19. One can start by asking the right questions. We first want to establish a case definition (identifies people involved in the outbreak by person, place, and time) to identify cases accurately. That is, asking when and where it is happening. Then, we want to identify and characterize the pathogen. That is, asking, what is responsible for the outbreak? Is it a bacterial infection, for which we can create antibiotics, or does a virus cause it, if so, is it similar to any existing ones for which we have developed a vaccine? Next, we could ask why the outbreak is occurring; to pinpoint the cause of the outbreak. In the case of COVID-19, we could backtrack to a particular wet market in Wuhan, as all of the initial patients had been in contact with that in the recent past before they fell ill. After analyzing the scenario in its entirety, it is time now we take action to counter the outbreak. Many wet markets all across China, Vietnam and some other countries have been shut down. And people are instructed to follow ‘social distancing’ to prevent the further spread of the disease-causing virus in the community as contact with affected individuals is the primary reason due to which this small outbreak took a much more extensive form of a global pandemic.
Diseases relinquished from animals to humans are on a steep rise, as the world continues to see an unprecedented disruption of wild habitats by human activity. Habitat loss and fragmentation, illegal trade, pollution, invasive species, and climate change, humans are manipulating the environment massively. These degraded ecosystems might promote faster evolutionary processes and disease diversification, as pathogens spread quickly to livestock and humans. As an example, to see how fast we change the environment, we can look around us right now. It has only been some time that lockdowns and shutdowns have become popular policies undertaken across the world to prevent the spread of the virus. Within this time, air quality and water quality of rivers, lakes, etc., have increased significantly. This could be achieved in such a short period of limited pollution activities. Therefore, one can easily scale the magnitude of the burden that we put on the natural environment around us through our normal activities on a daily basis. Sadly, once the pandemic is over, human beings will return to the normal pace of work and pollution will rapidly scale up, yet again. Doreen Robinson, the Chief of Wildlife at the United Nations Environment Program (UNEP), very rightly said, “Humans and nature are part of one connected system, and nature provides the food, medicine, water, clean air, and many other benefits that have allowed people to thrive. Yet like all systems, we need to understand how it works so that we don’t push things too far and face increasingly negative consequences”. And COVID-19 may be the beginning of many such pandemics.
We should always keep in mind that nature has existed and evolved on this planet for billions of years since the formation of the Earth. Species have emerged and got extinct from time to time, but nature has always prevailed. If we do not want to be the next species to be wiped away from the face of this planet, we have to accept and respect this superiority of nature with fear and start acting sensibly at the earliest.
By Juee Dhar, BS-MS(1st year), IISER Kolkata.
Edited by: Diptatanu Das, Department of Biological Science, IISER Kolkata
About the author: Juee is a first-year BS-MS student at IISER Kolkata, a science enthusiast who just loves to learn, may it be ecology or mechanics, she enjoys it all. Other than that, one can catch her playing at IISERK, may it be TT, badminton, basketball, or tennis. Also is being an avid reader, she devours the best of both worlds, the high hit of adrenaline of sports and quiet hours spent with a good book.
This is Juee’s first blog for TQR.