In November 2002, mysterious pneumonia was
detected by scientists in the Guangdong Province of China. The first case of
this new type of pneumonia wasn't reported until February 2003, when it took
only a couple of months to spread to more than 25 countries in Asia, Europe,
and North and South America. This newly emergent pneumonia was labeled as
"Severe Acute Respiratory Syndrome" (SARS), and its causative agent
was identified as a previously unrecognized coronavirus (CoV), the SARS-CoV.
Almost 10% of the roughly 8,000 infected people with SARS died. However, once
the epidemic was contained, the virus appeared to "die out," and
except for a few mild, sporadic cases in 2004, no additional cases have been
identified. Now the question emerges that from where does this newly emergent
virus come? And why did this viral disease disappear?
Coronaviruses are large, enveloped viruses with
positive-strand RNA genomes. They are known to infect a variety of mammals and
birds. In 2003, WHO announced that a new microbe, member of the coronavirus
family never before seen in humans suspected that SARS-CoV had
"jumped" from its animal host to humans. To this hypothesis, various
samples of animals at open markets in Guangdong were taken for nucleotide sequencing.
These studies revealed that catlike animals called masked palm civets (Paguma
larvata) harbored variants of the SARS-CoV. Thousands of civets were then
slaughtered to stop the spread of this virus in humans; further studies failed
to find widespread infection of domestic or wild civets. Besides, experimental
infection of civets with human SARS-CoV strains made these animals ill, making
the Civet an unlikely candidate for the reservoir species. Such a species would
be expected to harbor SARS-CoV as an asymptomatic so that it could efficiently
spread the virus.
Bats are the reservoir hosts of several zoonotic viruses
(viruses spread from animals to humans). Thus it had been perhaps not too
surprising when in 2005 two groups of international scientists independently
demonstrated that Chinese horseshoe bats (genus Rhinolophus) are the natural
reservoir of a SARS-like coronavirus. When the genomes of the human and bat
SARS-CoV are aligned, 92% of the nucleotides are identical. More revealing is
the alignment of the translated amino acid sequences of the proteins encoded by
each virus. The amino acid sequences are 96% to 100% identical for all proteins
except the receptor-binding spike proteins which are only 64% identical. The
SARS-CoV spike protein mediates both host cell surface attachment and membrane
fusion.
Thus a mutation of the spike protein allowed the virus to
"jump" from bat host cells to those of another species. It is not
clear if the SARS-CoV was transmitted directly to humans (bats are eaten as a
delicacy, and bat faeces are a traditional Asian cure for asthma) or if
transmission to humans occurred through infected civets.
The region of the SARS-CoV spike protein that binds to the
host receptor, angiotensin-converting enzyme-2 (ACE2), forms a shallow packet
into which ACE2 rests. The region of the spike protein that makes this pocket
is called the receptor-binding domain (RBD). Of the approximately 220 amino
acids within the RBD, only 4 differ between civet and human. Two of these amino
acids appear to be critical. As shown in the figure, compared to the spike RBD
in the SARS-CoV that caused the 2002-2003 epidemic, the civet spike has a
serine(S) substituted for a threonine(T) at position 487(T487S) and lysine (K)
at position 479, instead of asparagine (N), N479K. This causes a thousand-fold
decrease in the capacity of the virus to bind to human ACE2. Furthermore, the
spike found in SARS-CoV isolated from patients in 2003 and 2004 also has a
serine at position 487 as well as a Proline(P) for Leucine(L) substitution at
position 472 (L472P). These amino acid substitutions could be responsible for
the reduced virulence of the virus found in these more recent infections. In
other words, these mutations could be the reason the SARS virus appears to have
"died out". There could be many other reasons like hot summer
weather, strict quarantine of all infected individuals. Viruses are like that,
they are unpredictable.
Presently the current coronavirus (COVID-19, which some
people are calling SARS-CoV's cousin) may also come to an end like this. As
this coronavirus goes through constant genetic mutation, it is difficult to
produce a vaccine. Scientists are
getting confused by the mutation power of this virus. It mutates at a very fast
rate and can accustom itself to the surroundings. The adaptability power of
this virus is so powerful that it can adapt to a large range of temperatures.
Coronaviruses have been named such due to the presence of
spikes around the surface. The coronavirus is divided into main four sub-groups
known as alpha, beta, gamma, and delta.
It was discovered around in the mid-1960s that humans can be affected by
coronavirus. There are 7 types of coronaviruses that affect humans, which
includes:-
COMMON HUMAN CORONAVIRUSES:-
- 229E (alpha coronavirus)
- NL63 (alpha coronavirus)
- OC43 (beta coronavirus)
- HKU1 (beta coronavirus)
OTHER HUMAN CORONAVIRUSES
- MERS-CoV (the beta coronavirus which is responsible for Middle East Respiratory Syndrome, or MERS)
- SARS-CoV (the beta coronavirus causing the severe acute respiratory syndrome, or SARS)
- SARS-CoV-2 (the novel coronavirus that caused a pandemic, coronavirus disease 2019, or COVID-19)
People around the world generally get infected by some of
the common human coronaviruses like 229E, NL63, OC43, and HKU1. There have been
many examples where it has been seen that coronavirus infects animals and when
humans get infected by those animals somehow, a new coronavirus emerges.
Leaving all these things behind, then main important is that,
thousands are being infected each and every day posing the greatest threats to
the human race may be more than the second world war. Although scientists are
trying their best to bring vaccine in the market as fast as possible, almost all
the countries have plunged themselves in the various clinical trials for
vaccine. We have got various fruitful results from many parts of the world, but
we have to still wait till a recognized and globally accepted vaccine becomes
available in the market. Till that time, we have to be at our most safety and
security, to protect ourselves from getting infected by these type if viruses.
Stay Home and Stay Safe.
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