Tracing the origin of a new virus. . .
Epidemics made their appearance around 12,000 years ago during the Neolithic period. Starting from this period onwards humans developed gradually densely populated agricultural communities, allowing microbials to spread easier and faster through several means of transmission (water, food, air, person-to-person contact, etc.).
But while there are 10 nonillion (10 to the 31st power) viruses on our planet — more than the number of stars in the known universe! — human species manages to survive quite well and with a relatively low frequency of virus-caused illnesses in this virus-filled world. According to virologists, this has to do more with the viruses’ pickiness on the species and the cell type they infect, rather than the human body’s resilience.
Viruses are very specific with the host organisms they infect — at least initially. As a rule of thumb, a mouse virus — for example — will not be able to infect a human (at least in the beginning). Still, as the ongoing COVID-19 pandemic clearly demonstrates, outbreaks of new human viruses do happen ; they are not as unexpected as they might seem and some of them do originate from animals. Some viruses can mutate (in other words they change their genetic information and consequently some of their traits) and this — in combination with human interactions with animals — could make it easier for a virus to ‘jump’ from one species to another. This ‘hop’ of viruses (or other microorganisms) from animal species to human species is called zoonosis.
Zoonosis is the main hypothesis on how SARS-CoV-2 initiated. The story is quite fixed in the public mind; a bat virus managed to cross the species barrier and infect eventually a human. The starting source of SARS-CoV-2 transmission is thought to be the now world-famous Huanan seafood market in Wuhan, where someone was infected with the virus and passed it to other humans.
SARS-CoV-2 is the third human coronavirus responsible for a severe respiratory syndrome that has emerged in the past 20 years (after SARS-CoV in 2002 and MERS-CoV in 2012). This family of viruses circulates primarily among bats and their zoonotic transfer has occasionally triggered epidemics among humans. Scientists believe that it is highly likely that the virus came from bats but first passed through an intermediary animal in the same way that the 2002 SARS-CoV virus moved from bats to cat-like civets before infecting humans. To reach such conclusions scientists combine many experiments, with the most important being the comparison of viral genetic sequences isolated from many species and identifying the most closely related ones.
Coronaviruses have been found to infect nearly 500 species of bat. In nature, different bat populations share the same caves. According to the virologist Étienne Decroly, some viruses (different strains) can infect the same bat simultaneously. This gives these viruses the possibility to exchange their genetic material and acquire new traits (e.g. the capacity to breach the species barrier). Now, given the easy spreading and circulation of these evolved viruses among many bat populations inside caves along with the fact that bats travel distances through flying, leaving urination and guano in forests where other animals live, could explain the possible transfer of a bat virus to another animal. The evolved virus can further mutate in the new animal host and eventually acquire a trait which enables it to infect humans and also be readily transmissible among them. Should a human then interact with such an animal, an epidemic could start. . .
That is only one scenario according to the scientists. And although It has been discovered that the SARS-CoV-2 genetic sequence is 96% identical to that of a bat virus (RaTG13) collected in 2013 from the animals’ guano, there must be another virus with greater similarity than RaTG13 in a domestic animal or livestock species, in order for the researchers to safely conclude that they found the species which served as an intermediate host before this new coronavirus infected humans. However, no such strain has yet been found.
In a nutshell, the structure of our society makes it easy for a pandemic to strike, and we are always vulnerable, whether to a coronavirus, ebola or other microbials.
Once again, this highlights the importance of :
- always following hygienic (and other safety) measures
- good quality in science and
- clear and concise communication among scientists, politicians and the public.