This is still a matter of great controversy how the Covid-19 virus spread from China into the entire globe! The SARS-CoV-2 (the virus causing Covid-19) pandemic has created an urgent need for a better understanding of how viruses jump from animals to humans. This type of leap is called zoonotic spillover or zoonotic transmission.

Zoonotic leap is not very uncommon and in 2016, it happened in case of Ebola virus where a new species of Ebola infected two bat species that roost in people’s homes in the Bombali region of Sierra Leone. In the following year, the new virus popped up in the same bat species in Guinea and Kenya. Although researchers not yet confirm whether this new Bombali virus infects people, but lab experiments suggest that it could. The protein that helps the Bombali virus to enter host cells can also bind to the receptor of lab-grown human cells. This is not yet been proved officially and scientists are still trying to reach a conclusion about how this spill-over happened from bats into humans. But, in general, whether a virus can make such a species leap and trigger an outbreak like Covid-19, depends on a complex cascade of factors relating to ecology, viral evolution, and human immunity.

The oversimplification of scientific facts and misinterpretations have triggered an unwanted blame against bats, prompting wildlife biologists and conservationists to clear the air on Covid-19 and bat-borne viruses. Many parts of the globe facing a silent unrest demanding bats evacuation from that area. Wildlife volunteers of the municipal corporation of an Indian metropolis in Karnataka received a flurry of calls from citizens to clear bats from trees. This is happening in other parts of the world as well such as in Switzerland. People are uncomfortable having bats around them. Even if a cat catches a bat and brings the bat to the house, people are getting scared and panicked.

The exact origin of SARS-CoV-2 in bats is not yet been proved. But as mentioned earlier, the oversimplification of scientific data has created an unexpected unrest against bats which seems very unfortunate. And as a result, the shy, flying mammals have come under intense scrutiny followed by misleading interpretations of preliminary research that marked bats as the most likely reservoir for Covid-19 as it is very similar to a bat Coronavirus. There are varied opinions about exactly how many different viruses exist in nature, but one recent estimate suggests that as many as 1.67 million still-unknown viruses infect mammals and birds, and that between 631,000 and 827,000 of those have the potential to make a zoonotic leap. Regardless of the actual number, zoonotic viral species are truly rare—especially ones that cause large epidemics.

Zoonotic transmission is not that simple, there are multiple barriers to a successful zoonotic leap. Many factors have to be considered for a virus to successfully jump from a host species to humans such as:

  • Does original host animal carry the virus?
  • Does original host shed the virus?
  • To what extent are humans exposed to original host?
  • Can the virus infect humans?
  • How well does the virus replicate in humans?
  • How effectively do humans spread the virus?

There are many conditions to meet for a virus to spill over from animals to humans. First, the original host—the species that serves as the virus’s so-called reservoir—needs to rub elbows with people at a time when it is shedding enough virus for people to get exposed. Next, the virus must be equipped with the molecular machinery to enter human cells. Specifically, it needs the right protein to bind to a receptor on a human cell, allowing it to slip inside. Then, it has to be able to replicate and to infect other cells, disrupting human immune system. Different viruses balance these factors differently, but the basic mechanism is the same. They get into the cells, and replicate, and then get out of the cells, and keep infecting other cells. In the meantime, they might undergo mutations that could help them hiding their actual identity.

In the case of SARS-CoV-2, the species route to humans is very much uncertain. Researchers speculated that the virus may have jumped from horseshoe bats to pangolins before infecting humans. More recent thinking suggests that pangolins simply became infected the same way that humans did. It is speculated that two changes to the genome of a bat-infecting coronavirus made SARS-CoV-2 an effective human pathogen. The first was to adapt the receptor binding protein (RBD) so it could bind to the human angiotensin converting enzyme (ACE-2) receptor. The second was to create a site in the virus’s spike protein that could be cleaved by a human cell enzyme called furin, which helps get the virus ready to enter a cell. ACE2 is a protein on the surface of many cell types. It is an enzyme that generates small proteins – by cutting up the larger protein angiotensinogen – that then go on to regulate functions in the cell. Using the spike-like protein on its surface, the SARS-CoV-2 virus binds to ACE2 – like a key being inserted into a lock – prior to entry and infection of cells. Hence, ACE2 acts as a cellular doorway – a receptor – for the virus that causes Covid-19. But researchers still have much to learn about SARS-CoV-2’s zoonotic leap—and how to prevent viral spillover in general.

References

Kuldeep Dhama et.al., SARS-CoV-2 jumping the species barrier: Zoonotic lessons from SARS, MERS and recent advances to combat this pandemic virus, Travel Med Infect Dis. 2020 September-October; 37: 101830.
Alla Katsnelson, How do viruses leap from animals to people and spark pandemics? C&EN, August 30, 2020, volume 98, issue 33.
Sahana Ghosh, April 2020, Bats not the enemy in the fight against Covid-19.
Covid-19 Reignites a Contentious Debate Over Bats and Disease.
Study explains the controversial link between bats, rats, and Covid-19.
Cohen J., Mining coronavirus genomes for clues to the outbreak’s origins.
The Global Virome Project, Science 2018.
Mohamed E.El Zowalaty and Josef D.Järhult, From SARS to Covid-19: A previously unknown SARS-related coronavirus (SARS-CoV-2) of pandemic potential infecting humans- Call for a One Health approach. One Health, Volume 9, June 2020, 100124.