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With a steadily growing swirl of news (and rumors) about the novel coronavirus (SARS-CoV-2) that has been spreading across the globe, it may be helpful too have a general idea of what coronaviruses are and how the current one — SARS-CoV-2 — compares with other strains. We’re hoping to cover the main points and dispel some misconceptions surrounding the virus’s origins.
Coronaviruses are nothing new. Current research indicates that all of the currently known strains have a most recent common ancestor (MRCA) somewhere around 8000 BCE. There are four primary lines (also known as Genera) of coronaviruses:
- Alphacoronavirus (emerged ~2400 BCE)
- Betacoronavirus (emerged ~3300 BCE)
- Deltacoronavirus (emerged ~3000 BCE)
- Gammacoronavirus (emerged ~2800 BCE)
Not all coronaviruses act the same, especially in terms of infections. Only Alpha- and Betacoronaviruses are capable of infecting mammals (such as bats), including humans. They result in respiratory illnesses among humans and gastroenteritis in animals. On the other hand, Delta- and Gammacoronaviruses primarily infect birds and do not jump to humans or other mammals.
Until recently, coronaviruses were not considered highly pathogenic. Among the strains of coronavirus capable of infecting humans, they mostly resulted in the common cold. While inconvenient, it was seldom fatal and basically an inconvenience. But that all changed in 2002 when the SARS virus emerged from China.
The Severe acute respiratory syndrome-related coronavirus (SARS-CoV) is a Betacoronavirus that infects cells in the respiratory tract of humans. It consists of a single strand of RNA, as opposed to the more common double-stranded DNA, that contains the instructions for the structrural proteins and internal machinery involved in replication.
The virus is shaped like a ball with spikes protruding from the surface.
Each spike consists of a lower area and a more variable upper area (the crown from which the name coronavirus comes from) which contains what is known as its receptor binding domain (RBD). If you imagine a lock and key, the RBD is a key in search of the lock it was made for. In the case of SARS-CoV, the lock is a type of cell commonly found in the upper respiratory tracts of human beings. It is called the angiotensin-converting enzyme 2, or ACE2. Once the coronavirus finds ACE2 it basically slides into place and begins working its way into the cell.
When that process is finishes, infection is complete. The coronavirus then makes itself at home, borrows some cell resources, and begins making copies of itself (see video below). This happens over and over during the course of an infection with ACE2 the weak point.
SARS was found to be transmissible from person-to-person via droplets from coughs and sneezes and very contagious. It’s basic reproductive number (how many people one infected person can infect) ranged from 0.43-2.41 with a median of 1.10. At its higher end, each SARS patient infected roughly three people. (For reference, any reproductive number greater than 1 means that infections will continue to spread until measures are taken to stop it.)
In 2012, a different coronavirus emerged in the Middle East that was named Middle East respiratory syndrome coronavirus, or MERS-CoV.
While similar to the SARS virus, it differed in the lock its RBD was looking for. Rather than ACE2, it fit a protein called dipeptidyl peptidase 4, or DPP4.
Other than having a different lock and key, MERS-CoV makes copies of itself in essentially the same way as SARS. MERS-CoV’s basic reproductive number tended to be slightly lower than SARS, though an outbreak in South Korea had a range around 4.
The novel coronavirus is called Severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2. It is closely related to the SARS virus that first emerged in 2002.
Its RBD binds to ACE2 and mostly infects cells in the respiratory tract. The similarity between their RBD targets suggest that the two coronaviruses are closely related, a fact backed up by their genetic makeup. They are not, however, identical. This is because the crown part of the spike is considered the most variable part of the entire virus.
While there are many estimates of SARS-CoV-2’s basic reproductive number circulating, we’re going to steer clear of that figure and allow parties better placed to calculate it.
In terms of immediate origins, SARS-CoV was found to have made the jump from animal to human (zoonosis) in market civet cats sold in Chinese wetmarkets.
MERS-CoV made the jump from dromedary camels.
Both viruses are believed to have ultimately originated in bats, with cats and camels being intermediates.
Current research indicates that SARS-CoV-2 most likely originated in bats as well, with pangolins playing the intermediary role in the transmission to humans.
As an aside… Many people have wondered why both SARS viruses came from China. The answer most likely lies in the fact that a single cave in Yunnan province is a coronavirus diversity hotspot. Within that one location, every strain of CoV around China can be found.
We hope this brief review of coronaviruses helps you understand what’s going on in the news a little more. If you have anything you’d like to see included or clarified, give us a shout @sci_inquirer on Twitter.
IMAGE SOURCE: Creative Commons