Here is the 6th article in the series written by Dr. Mathieu Millette, see the previous articles: My mother and SARS-COV-2, An excellent virus, The Empire strikes back, I have a dream… and From Aristotle to Covid-19.
Body of a lion spitting fire, decked out to the neck and the head of a goat, finished off with the tail of a snake, the chimera was thus depicted in ancient Greece. It was the hero Bellerophon riding the winged horse, Pegasus, who brought the chimera to its end.
In 2020, humanity also faces a chimera, this time in the form of the SARS-CoV-2 virus. As of this writing, the virus has spread worldwide, in 212 countries and territories. There are more than 5 million cases and more than 325,000 people have died from the virus. Specialists have even detected several strains of the virus. Our modern chimera mutates quietly, but inexorably, like all viruses. But what is the origin of SARS-CoV-2? How did it get on Earth? Or rather, how did it come to cause so much trouble to humans?
The species barrier
Infectious disease experts agree that SARS-CoV-2 has crossed the species barrier, that is, it has been passed from animal to human. Normally a virus adapted to one species replicates and spreads within this species and does not attack another species. This species barrier is made possible because the proteins on the membrane of viruses are very specific for a receptor (another type of protein) that is found on the surface of specific cells of a particular animal; each lock has its key. Sometimes animals infected with viruses develop a disease and can even die from it, but most of the time, not. They live very well with the virus. These are called reservoirs and they contribute to the perpetuation of the virus.
However, this barrier of species is regularly crossed for several human infectious diseases such as Ebola, Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), human immunodeficiency virus (HIV), rabies and many others. It seems that this phenomenon is increasing because humans are encroaching more and more on the wild and unexplored territories in which has found a high density of terrestrial fauna. For example, the Amazon, this immense virgin forest in Brazil also called the lungs of the planet, is cleared daily to make more cultivable land or pastures to raise immense herds of animals… to satisfy our irrepressible urge to eat meat, but that’s another debate.
Many of these animal-human viral jumps have, however, gone unnoticed in human history because the density of the human population was not sufficient to maintain spread around the world. For example, the Cro-Magnon men in one area may have all died from a virus without it becoming a pandemic.
You should know that SARS-CoV-2 is part of a large family of viruses called coronaviruses. Many viruses of this family have been isolated from animals all over the planet. It is mammalian coronaviruses that worry us most, as these animals share certain molecular and biochemical similarities with humans. Microbiologists have isolated coronaviruses from camels, cats, bats, dogs, civets, turkeys, ferrets, pangolins, rats, mice, and pigs, among others. And also in humans. SARS-CoV-2 is the seventh coronavirus to spread in humans. There are SARS-CoV and MERS-CoV which cause serious and often fatal diseases, while HKU1, NL63, OC43 and 229E only cause cold-like symptoms.
Origin of SARS-CoV-2
From the outset, no, the SARS-CoV-2 was not created by humans. As proof, several experts in genetic engineering and bioinformatics have studied the genetic material of many strains of this virus and have not found the characteristic “traces” left by genetic manipulation when scientists modify viruses. Case closed. But what is the origin of this damned virus then? Quite simply, from nature itself. Do you know the famous evolution described by Charles Darwin in his book “The Origin of Species” published in 1859? Well, it’s no more and no less than his fault.
The RaTG13 coronavirus strain was isolated from a Rhinolophus affinis, more commonly known as the Horseshoe Bat. This very peculiar bat is common in China and Southeast Asia. Why am I bringing this up? Because the genetic material of this strain is very similar to that of SARS-CoV-2. In fact, the two genomes are 96% similar; which is very similar, believe me. When geneticists trace the family trees of all known coronavirus strains, RaTG13 is most similar to SARS-CoV-2. However, although very related, it seems that the portion of the viral protein (which allows entry into human cells) of RaTG13 does not have a good affinity with ACE2, the protein which acts as a lock. This means that RaTG13 cannot infect and reproduce in humans.
Other similar strains of coronavirus have been isolated, this time from Manis javanica or Javan pangolin, an insectivorous tortoiseshell mammal that lives in the same region as the Horseshoe Bats. These strains are less similar than those of the bat, but the region of the viral protein which recognizes ACE2 is identical to that of SARS-CoV-2.
We now have evidence that random mutations in nature can transform SARS-CoV-2 to make it infectious and virulent in humans. However, one question remains: how has this virus crossed the barrier between species? Whenever RNA viruses replicate, there are mutations, that is, transcription errors in the genetic material of the new viruses. Sometimes, some of these mutations are located in strategic places which give new characteristics to baby viruses. Sometimes it allows them to infect new cells. Sometimes it allows them to break through the barrier of species and spread through humans.
Scientists are now investigating two hypotheses: a series of beneficial mutations (for the virus) occurred in an animal and then infected humans, or these mutations rather took place directly in a human following several unsuccessful and silent trials and errors.
OK, OK. I know your head is spinning. Too much jargon. Too technical. To put it simply:
- SARS-CoV-2 was not created by an enlightened scientist with a divine mission to control human populations;
- the characteristics which enabled SARS-CoV-2 to infect humans and cause disease have been found in nature on bat or pangolin viruses;
- SARS-CoV-2 is an evolutionary child. It has adapted to infect the dominant species on Earth and is in the process of restoring balance;
- this chimeric virus is said to have appeared in 2019, around the city of Wuhan, in China, the site of the first documented cases of pneumonia typical of COVID-19. It is thanks to globalization and air travel that the virus has spread everywhere on Earth like wildfire.
SARS-CoV-2 is indeed a chimera, a creature not mythical, but rather microbiological and above all very real. This chimera will still be with us for a long time, like many other viruses. Unless a modern-day Bellerophon is on the horizon…
Mathieu Millette, Ph.D. Mcb.A. Doctor in microbiology and member of the Association of Microbiologists of Quebec