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Coronaviruses (CoVs), enveloped positive-sense RNA viruses, are characterized by club-like spikes that project from their surface, an unusually large RNA genome, and a unique replication strategy. Coronaviruses cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs and upper respiratory disease in chickens to potentially lethal human respiratory infections. Here we provide a brief introduction to coronaviruses discussing their replication and pathogenicity, and current prevention and treatment strategies. We also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and the recently identified Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV).
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1
Helena Jane Maier et al. (eds.), Coronaviruses: Methods and Protocols, Methods in Molecular Biology, vol. 1282,
DOI 10.1007/978-1-4939-2438-7_1, © Springer Science+Business Media New York 2015
Chapter 1
Coronaviruses: An Overview of Their Replication
and Pathogenesis
Anthony R. Fehr and Stanley Perlman
Abstract
Coronaviruses (CoVs), enveloped positive-sense RNA viruses, are characterized by club-like spikes that
project from their surface, an unusually large RNA genome, and a unique replication strategy. Coronaviruses
cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs and upper respiratory
disease in chickens to potentially lethal human respiratory infections. Here we provide a brief introduction
to coronaviruses discussing their replication and pathogenicity, and current prevention and treatment strate-
gies. We also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome
Coronavirus (SARS-CoV) and the recently identifi ed Middle Eastern Respiratory Syndrome Coronavirus
(MERS-CoV).
Key words Nidovirales , Coronavirus , Positive-sense RNA viruses , SARS-CoV , MERS-CoV
1 Classifi cation
Coronaviruses (CoVs) are the largest group of viruses belonging
to the Nidovirales order, which includes Coronaviridae ,
Arteriviridae , Mesoniviridae , and Roniviridae families. The
Coronavirinae comprise one of two subfamilies in the Coronaviridae
family, with the other being the Torovirinae . The Coronavirinae
are further subdivided into four genera, the alpha, beta, gamma,
and delta coronaviruses. The viruses were initially sorted into these
genera based on serology but are now divided by phylogenetic
clustering.
All viruses in the Nidovirales order are enveloped, non-
segmented positive-sense RNA viruses. They all contain very large
genomes for RNA viruses, with some viruses having the largest
identifi ed RNA genomes, containing up to 33.5 kilobase (kb)
genomes. Other common features within the Nidovirales order
include: (1) a highly conserved genomic organization, with a large
replicase gene preceding structural and accessory genes; (2)
expression of many non-structural genes by ribosomal
2
frameshifting; (3) several unique or unusual enzymatic activities
encoded within the large replicase–transcriptase polyprotein; and
(4) expression of downstream genes by synthesis of 3 nested sub-
genomic mRNAs. In fact, the Nidovirales order name is derived
from these nested 3 mRNAs as nido is Latin for “nest.” The major
differences within the Nidovirus families are in the number, type,
and sizes of the structural proteins. These differences cause signifi -
cant alterations in the structure and morphology of the nucleo-
capsids and virions.
2 Genomic Organization
Coronaviruses contain a non-segmented, positive-sense RNA
genome of ~30 kb. The genome contains a 5 cap structure along
with a 3 poly (A) tail, allowing it to act as an mRNA for translation
of the replicase polyproteins. The replicase gene encoding the non-
structural proteins (nsps) occupies two-thirds of the genome,
about 20 kb, as opposed to the structural and accessory proteins,
which make up only about 10 kb of the viral genome. The 5 end
of the genome contains a leader sequence and untranslated region
(UTR) that contains multiple stem loop structures required for
RNA replication and transcription. Additionally, at the beginning
of each structural or accessory gene are transcriptional regulatory
sequences (TRSs) that are required for expression of each of these
genes ( see Subheading
4.3 on RNA replication). The 3 UTR also
contains RNA structures required for replication and synthesis of
viral RNA. The organization of the coronavirus genome is
5-leader-UTR- replicase-S (Spike)-E (Envelope)-M (Membrane)-
N (Nucleocapsid)-3 UTR-poly (A) tail with accessory genes inter-
spersed within the structural genes at the 3 end of the genome
(Fig.
1 ). The accessory proteins are almost exclusively nonessential
for replication in tissue culture; however, some have been shown to
have important roles in viral pathogenesis [
1 ].
3 Virion Structure
Coronavirus virions are spherical with diameters of approximately
125 nm as depicted in recent studies by cryo-electron tomography
and cryo-electron microscopy [
2 , 3 ]. The most prominent feature
of coronaviruses is the club-shaped spike projections emanating
from the surface of the virion. These spikes are a defi ning feature
of the virion and give them the appearance of a solar corona,
prompting the name, coronaviruses. Within the envelope of the
virion is the nucleocapsid. Coronaviruses have helically symmetri-
cal nucleocapsids, which is uncommon among positive-sense RNA
viruses, but far more common for negative-sense RNA viruses.
Anthony R. Fehr and Stanley Perlman
3
Coronavirus particles contain four main structural proteins.
These are the spike (S), membrane (M), envelope (E), and nucleo-
capsid (N) proteins, all of which are encoded within the 3 end of
the viral genome. The S protein (~150 kDa), utilizes an N-terminal
signal sequence to gain access to the ER, and is heavily N-linked
glycosylated. Homotrimers of the virus encoded S protein make up
the distinctive spike structure on the surface of the virus [
4 , 5 ].
The trimeric S glycoprotein is a class I fusion protein [
6 ] and medi-
ates attachment to the host receptor [
7 ]. In most, coronaviruses,
S is cleaved by a host cell furin-like protease into two separate poly-
peptides noted S1 and S2 [
8 , 9 ]. S1 makes up the large receptor-
binding domain of the S protein, while S2 forms the stalk of the
spike molecule [
10 ].
The M protein is the most abundant structural protein in the
virion. It is a small (~25–30 kDa) protein with three transmem-
brane domains [
11 ] and is thought to give the virion its shape. It
has a small N-terminal glycosylated ectodomain and a much larger
C-terminal endodomain that extends 6–8 nm into the viral particle
[
12 ]. Despite being co-translationally inserted in the ER mem-
brane, most M proteins do not contain a signal sequence. Recent
Fig. 1 Genomic organization of representative α, β, and γ CoVs. An illustration of the MHV genome is depicted
at the top . The expanded regions below show the structural and accessory proteins in the 3 regions of the
HCoV-229E, MHV, SARS-CoV, MERS-CoV and IBV. Size of the genome and individual genes are approximated
using the legend at the top of the diagram but are not drawn to scale. HCoV-229E human coronavirus 229E,
MHV mouse hepatitis virus, SARS-CoV severe acute respiratory syndrome coronavirus, MERS-CoV Middle
East respiratory syndrome coronavirus, IBV infectious bronchitis virus
Coronavirus Introduction
4
studies suggest the M protein exists as a dimer in the virion, and
may adopt two different conformations, allowing it to promote
membrane curvature as well as to bind to the nucleocapsid [
13 ].
The E protein (~8–12 kDa) is found in small quantities within
the virion. The coronavirus E proteins are highly divergent but
have a common architecture [
14 ]. The membrane topology of E
protein is not completely resolved but most data suggest that it is
a transmembrane protein. The E protein has an N-terminal ectodo-
main and a C-terminal endodomain and has ion channel activity.
As opposed to other structural proteins, recombinant viruses lack-
ing the E protein are not always lethal, although this is virus type
dependent [
15 ]. The E protein facilitates assembly and release of
the virus ( see Subheading
4.4 ), but also has other functions. For
instance, the ion channel activity in SARS-CoV E protein is not
required for viral replication but is required for pathogenesis [
16 ].
The N protein constitutes the only protein present in the
nucleocapsid. It is composed of two separate domains, an
N-terminal domain (NTD) and a C-terminal domain (CTD), both
capable of binding RNA in vitro, but each domain uses different
mechanisms to bind RNA. It has been suggested that optimal RNA
binding requires contributions from both domains [
17 , 18 ]. N
protein is also heavily phosphorylated [
19 ], and phosphorylation
has been suggested to trigger a structural change enhancing the
affi nity for viral versus nonviral RNA. N protein binds the viral
genome in a beads-on-a-string type conformation. Two specifi c
RNA substrates have been identifi ed for N protein; the TRSs [
20 ]
and the genomic packaging signal [
21 ]. The genomic packaging
signal has been found to bind specifi cally to the second, or
C-terminal RNA binding domain [
22 ]. N protein also binds nsp3
[
18 , 23 ], a key component of the replicase complex, and the M
protein [
24 ]. These protein interactions likely help tether the viral
genome to the replicase–transcriptase complex (RTC), and subse-
quently package the encapsidated genome into viral particles.
A fi fth structural protein, the hemagglutinin-esterase (HE), is
present in a subset of β-coronaviruses. The protein acts as a hemag-
glutinin, binds sialic acids on surface glycoproteins, and contains
acetyl-esterase activity [
25 ]. These activities are thought to enhance
S protein-mediated cell entry and virus spread through the mucosa
[
26 ]. Interestingly, HE enhances murine hepatitis virus (MHV)
neurovirulence [
27 ]; however, it is selected against in tissue culture
for unknown reasons [
28 ].
4 Coronavirus Life Cycle
The initial attachment of the virion to the host cell is initiated by
interactions between the S protein and its receptor. The sites of
receptor binding domains (RBD) within the S1 region of a
4.1 Attachment
and Entry
Anthony R. Fehr and Stanley Perlman
5
coronavirus S protein vary depending on the virus, with some
having the RBD at the N-terminus of S1 (MHV), while others
(SARS- CoV) have the RBD at the C-terminus of S1 [
29 , 30 ]. The
S-protein–receptor interaction is the primary determinant for a
coronavirus to infect a host species and also governs the tissue tro-
pism of the virus. Many coronaviruses utilize peptidases as their
cellular receptor. It is unclear why peptidases are used, as entry
occurs even in the absence of the enzymatic domain of these
proteins. Many α-coronaviruses utilize aminopeptidase N (APN)
as their receptor, SARS-CoV and HCoV-NL63 use angiotensin-
converting enzyme 2 (ACE2) as their receptor, MHV enters
through CEACAM1, and the recently identifi ed MERS-CoV binds
to dipeptidyl-peptidase 4 (DPP4) to gain entry into human cells
( see Table
1 for a list of known CoV receptors).
Following receptor binding, the virus must next gain access to
the host cell cytosol. This is generally accomplished by acid-
dependent proteolytic cleavage of S protein by a cathepsin,
TMPRRS2 or another protease, followed by fusion of the viral and
cellular membranes. S protein cleavage occurs at two sites within
the S2 portion of the protein, with the fi rst cleavage important for
separating the RBD and fusion domains of the S protein [
31 ] and
Table 1
Coronavirus receptors
Virus Receptor References
Alphacoronaviruses
HCoV-229E APN [ 115 ]
HCoV-NL63 ACE2 [ 116 ]
TGEV APN [
117 ]
PEDV APN [
118 ]
FIPV APN [
119 ]
CCoV APN [ 120 ]
Betacoronaviruses
MHV mCEACAM [
121 , 122 ]
BCoV N -acetyl-9- O -acetylneuraminic acid [ 123 ]
SARS-CoV ACE2 [
124 ]
MERS-CoV DPP4 [
100 ]
APN aminopeptidase N, ACE2 angiotensin-converting enzyme 2, mCEACAM murine carcinoembryonic antigen-
related adhesion molecule 1, DPP4 dipeptidyl peptidase 4, HCoV human coronavirus, TGEV transmissible gastroenteri-
tis virus, PEDV porcine epidemic diarrhea virus, FIPV feline infectious peritonitis virus, CCoV canine coronavirus,
MHV murine hepatitis virus, BCoV bovine coronavirus, SARS-CoV severe acute respiratory syndrome coronavirus,
MERS-CoV Middle East respiratory syndrome coronavirus
Coronavirus Introduction
6
the second for exposing the fusion peptide (cleavage at S2). Fusion
generally occurs within acidifi ed endosomes, but some coronavi-
ruses, such as MHV, can fuse at the plasma membrane. Cleavage at
S2 exposes a fusion peptide that inserts into the membrane, which
is followed by joining of two heptad repeats in S2 forming an anti-
parallel six-helix bundle [
6 ]. The formation of this bundle allows
for the mixing of viral and cellular membranes, resulting in fusion
and ultimately release of the viral genome into the cytoplasm.
The next step in the coronavirus lifecycle is the translation of the
replicase gene from the virion genomic RNA. The replicase gene
encodes two large ORFs, rep1a and rep1b, which express two co-
terminal polyproteins, pp1a and pp1ab (Fig.
1 ). In order to express
both polyproteins, the virus utilizes a slippery sequence
(5-UUUAAAC-3) and an RNA pseudoknot that cause ribosomal
frameshifting from the rep1a reading frame into the rep1b ORF. In
most cases, the ribosome unwinds the pseudoknot structure, and
continues translation until it encounters the rep1a stop codon.
Occasionally the pseudoknot blocks the ribosome from continuing
elongation, causing it to pause on the slippery sequence, changing
the reading frame by moving back one nucleotide, a -1 frameshift,
before the ribosome is able to melt the pseudoknot structure and
extend translation into rep1b, resulting in the translation of pp1ab
[
32 , 33 ]. In vitro studies predict the incidence of ribosomal frame-
shifting to be as high as 25 %, but this has not been determined in
the context of virus infection. It is unknown exactly why these
viruses utilize frameshifting to control protein expression, but it is
hypothesized to either control the precise ratio of rep1b and rep1a
proteins or delay the production of rep1b products until the
products of rep1a have created a suitable environment for RNA
replication [
34 ].
Polyproteins pp1a and pp1ab contain the nsps 1–11 and 1–16,
respectively. In pp1ab, nsp11 from pp1a becomes nsp12 following
extension of pp1a into pp1b. However, γ-coronaviruses do not
contain a comparable nsp1. These polyproteins are subsequently
cleaved into the individual nsps [
35 ]. Coronaviruses encode either
two or three proteases that cleave the replicase polyproteins. They
are the papain-like proteases (PLpro), encoded within nsp3, and a
serine type protease, the main protease, or Mpro, encoded by nsp5.
Most coronaviruses encode two PLpros within nsp3, except the
γ-coronaviruses, SARS-CoV and MERS-CoV, which only express
one PLpro [
36 ]. The PLpros cleave the nsp1/2, nsp2/3, and
nsp3/4 boundaries, while the Mpro is responsible for the remain-
ing 11 cleavage events.
Next, many of the nsps assemble into the replicase–transcriptase
complex (RTC) to create an environment suitable for RNA
synthesis, and ultimately are responsible for RNA replication and
transcription of the sub-genomic RNAs. The nsps also contain
4.2 Replicase
Protein Expression
Anthony R. Fehr and Stanley Perlman
7
other enzyme domains and functions, including those important
for RNA replication, for example nsp12 encodes the RNA-
dependent RNA polymerase (RdRp) domain; nsp13 encodes the
RNA helicase domain and RNA 5-triphosphatase activity; nsp14
encodes the exoribonuclease (ExoN) involved in replication fi del-
ity and N7-methyltransferase activity; and nsp16 encodes
2-O-methyltransferase activity. In addition to the replication func-
tions other activities, such as blocking innate immune responses
(nsp1; nsp16-2-O-methyl transferase; nsp3-deubiquitinase) have
been identifi ed for some of the nsps, while others have largely
unknown functions (nsp3-ADP-ribose-1-phosphatase; nsp15-
endoribo-nuclease (NendoU)). For a list of non- structural pro-
teins and their proposed functions, see Table
2 . Interestingly,
ribonucleases nsp15-NendoU and nsp14-ExoN activities are
unique to the Nidovirales order and are considered genetic markers
for these viruses [
37 ].
Viral RNA synthesis follows the translation and assembly of the
viral replicase complexes. Viral RNA synthesis produces both
genomic and sub-genomic RNAs. Sub-genomic RNAs serve as
mRNAs for the structural and accessory genes which reside down-
stream of the replicase polyproteins. All positive-sense sub-genomic
RNAs are 3 co-terminal with the full-length viral genome and
thus form a set of nested RNAs, a distinctive property of the order
Nidovirales . Both genomic and sub-genomic RNAs are produced
through negative-strand intermediates. These negative-strand
intermediates are only about 1 % as abundant as their positive-
sense counterparts and contain both poly-uridylate and anti-leader
sequences [
38 ].
Many cis-acting sequences are important for the replication of
viral RNAs. Within the 5 UTR of the genome are seven stem-loop
structures that may extend into the replicase 1a gene [
39 42 ]. The
3 UTR contains a bulged stem-loop, a pseudoknot, and a hyper-
variable region [
43 46 ]. Interestingly, the stem-loop and the pseu-
doknot at the 3 end overlap, and thus cannot form simultaneously
[
44 , 47 ]. Therefore, these different structures are proposed to
regulate alternate stages of RNA synthesis, although exactly which
stages are regulated and their precise mechanism of action are still
unknown.
Perhaps the most novel aspect of coronavirus replication is
how the leader and body TRS segments fuse during production of
sub-genomic RNAs. This was originally thought to occur during
positive-strand synthesis, but now it is largely believed to occur
during the discontinuous extension of negative-strand RNA [
48 ].
The current model proposes that the RdRp pauses at any one of
the body TRS sequences (TRS-B); following this pause the RdRp
either continues elongation to the next TRS or it switches to ampli-
fying the leader sequence at the 5 end of the genome guided by
4.3 Replication
and Transcription
Coronavirus Introduction
8
complementarity of the TRS-B to the leader TRS (TRS-L). Many
pieces of evidence currently support this model, including the
presence of anti-leader sequence at the 3 end of the negative-
strand sub-genomic RNAs [
38 ]. However, many questions remain
to fully defi ne the model. For instance, how does the RdRp bypass
all of the TRS-B sequences to produce full-length negative-strand
genomic RNA? Also, how are the TRS-B sequences directed to the
Table 2
Functions of coronavirus non-structural proteins (nsps)
Protein Function References
nsp1 Promotes cellular mRNA degradation and blocks host cell
translation, results in blocking innate immune response [ 125 128 ]
nsp2 No known function, binds to prohibitin proteins [
129 , 130 ]
nsp3 Large, multi-domain transmembrane protein, activities include:
Ubl1 and Ac domains, interact with N protein
ADRP activity, promotes cytokine expression
PLPro/Deubiquitinase domain, cleaves viral polyprotein
and blocks host innate immune response
Ubl2, NAB, G2M, SUD, Y domains, unknown functions
[
131 138 ]
nsp4 Potential transmembrane scaffold protein, important for proper
structure of DMVs [
139 , 140 ]
nsp5 Mpro, cleaves viral polyprotein [
141 ]
nsp6 Potential transmembrane scaffold protein [ 142 ]
nsp7 Forms hexadecameric complex with nsp8, may act as
processivity clamp for RNA polymerase [
143 ]
nsp8 Forms hexadecameric complex with nsp7, may act as
processivity clamp for RNA polymerase; may act as primase [
143 , 144 ]
nsp9 RNA binding protein [
145 ]
nsp10 Cofactor for nsp16 and nsp14, forms heterodimer with
both and stimulates ExoN and 2-O-MT activity [ 146 , 147 ]
nsp12 RdRp [
148 ]
nsp13 RNA helicase, 5 triphosphatase [ 149 , 150 ]
nsp14 N7 MTase and 3-5 exoribonuclease, ExoN; N7 MTase adds
5 cap to viral RNAs, ExoN activity is important for
proofreading of viral genome
[
151 154 ]
nsp15 Viral endoribonuclease, NendoU [
155 , 156 ]
nsp16 2-O-MT; shields viral RNA from MDA5 recognition [
157 , 158 ]
Ubl ubiquitin-like, Ac acidic, ADRP ADP-ribose-1-phosphatase, PLPro papain-like protease, NAB nucleic acid bind-
ing, SUD SARS-unique domain, DMVs double-membrane vesicles, Mpro main protease , RdRp RNA-dependent
RNA polymerase, MTase methyltransferase, Exo N viral exoribonuclease, Nendo U viral endoribonuclease, 2-O-MT
2-O-methyltransferase, MDA5 melanoma differentiation associated protein 5
Anthony R. Fehr and Stanley Perlman
9
TRS-L and how much complementarity is necessary [ 49 ]? Answers
to these questions and others will be necessary to gain a full perspec-
tive of how RNA replication occurs in coronaviruses.
Finally, coronaviruses are also known for their ability to recom-
bine using both homologous and nonhomologous recombination
[
50 , 51 ]. The ability of these viruses to recombine is tied to the
strand switching ability of the RdRp. Recombination likely plays a
prominent role in viral evolution and is the basis for targeted RNA
recombination, a reverse genetics tool used to engineer viral
recombinants at the 3 end of the genome.
Following replication and sub-genomic RNA synthesis, the viral
structural proteins, S, E, and M are translated and inserted into the
endoplasmic reticulum (ER). These proteins move along the secre-
tory pathway into the endoplasmic reticulum–Golgi intermediate
compartment (ERGIC) [
52 , 53 ]. There, viral genomes encapsid-
ated by N protein bud into membranes of the ERGIC containing
viral structural proteins, forming mature virions [
54 ].
The M protein directs most protein–protein interactions
required for assembly of coronaviruses. However, M protein is not
suffi cient for virion formation, as virus-like particles (VLPs) cannot
be formed by M protein expression alone. When M protein is
expressed along with E protein VLPs are formed, suggesting these
two proteins function together to produce coronavirus envelopes
[
55 ]. N protein enhances VLP formation, suggesting that fusion
of encapsidated genomes into the ERGIC enhances viral envelop-
ment [
56 ]. The S protein is incorporated into virions at this step,
but is not required for assembly. The ability of the S protein to
traffi c to the ERGIC and interact with the M protein is critical for
its incorporation into virions.
While the M protein is relatively abundant, the E protein is
only present in small quantities in the virion. Thus, it is likely that
M protein interactions provide the impetus for envelope maturation.
It is unknown how E protein assists M protein in assembly of the
virion, and several possibilities have been suggested. Some work
has indicated a role for the E protein in inducing membrane cur-
vature [
57 59 ], although others have suggested that E protein
prevents the aggregation of M protein [
60 ]. The E protein may
also have a separate role in promoting viral release by altering the
host secretory pathway [
61 ].
The M protein also binds to the nucleocapsid, and this interac-
tion promotes the completion of virion assembly. These interactions
have been mapped to the C-terminus of the endodomain of M
with CTD of the N-protein [
62 ]. However, it is unclear exactly
how the nucleocapsid complexed with virion RNA traffi cs to the
ERGIC to interact with M protein and become incorporated into
the viral envelope. Another outstanding question is how the N
protein selectively packages only positive-sense full-length genomes
4.4 Assembly
and Release
Coronavirus Introduction
10
among the many different RNA species produced during infection.
A packaging signal for MHV has been identifi ed in the nsp15 cod-
ing sequence, but mutation of this signal does not appear to affect
virus production, and a mechanism for how this packaging signal
works has not been determined [
22 ]. Furthermore, most corona-
viruses do not contain similar sequences at this locus, indicating
that packaging may be virus specifi c.
Following assembly, virions are transported to the cell surface
in vesicles and released by exocytosis. It is not known if the virions
use the traditional pathway for transport of large cargo from the
Golgi or if the virus has diverted a separate, unique pathway for its
own exit. In several coronaviruses, S protein that does not get
assembled into virions transits to the cell surface where it mediates
cell–cell fusion between infected cells and adjacent, uninfected
cells. This leads to the formation of giant, multinucleated cells,
which allows the virus to spread within an infected organism with-
out being detected or neutralized by virus-specifi c antibodies.
5 Pathogenesis
Coronaviruses cause a large variety of diseases in animals, and their
ability to cause severe disease in livestock and companion animals
such as pigs, cows, chickens, dogs, and cats led to signifi cant
research on these viruses in the last half of the twentieth century.
For instance, Transmissible Gastroenteritis Virus (TGEV) and
Porcine Epidemic Diarrhea Virus (PEDV) cause severe gastroen-
teritis in young piglets, leading to signifi cant morbidity, mortality,
and ultimately economic losses. PEDV recently emerged in North
America for the fi rst time, causing signifi cant losses of young pig-
lets. Porcine hemagglutinating encephalomyelitis virus (PHEV)
mostly leads to enteric infection but has the ability to infect the
nervous system, causing encephalitis, vomiting, and wasting in
pigs. Feline enteric coronavirus (FCoV) causes a mild or asymp-
tomatic infection in domestic cats, but during persistent infection,
mutation transforms the virus into a highly virulent strain of FCoV,
Feline Infectious Peritonitis Virus (FIPV), that leads to development
of a lethal disease called feline infectious peritonitis (FIP). FIP has
wet and dry forms, with similarities to the human disease, sarcoid-
osis. FIPV is macrophage tropic and it is believed that it causes
aberrant cytokine and/or chemokine expression and lymphocyte
depletion, resulting in lethal disease [
63 ]. However, additional
research is needed to confi rm this hypothesis. Bovine CoV, Rat
CoV, and Infectious Bronchitis Virus (IBV) cause mild to severe
respiratory tract infections in cattle, rats, and chickens, respectively.
Bovine CoV causes signifi cant losses in the cattle industry and also
has spread to infect a variety of ruminants, including elk, deer, and
camels. In addition to severe respiratory disease, the virus causes
5.1 Animal
Coronaviruses
Anthony R. Fehr and Stanley Perlman
11
diarrhea (“winter dysentery” and “shipping fever”), all leading to
weight loss, dehydration, decreased milk production, and depres-
sion [
63 ]. Some strains of IBV, a γ-coronavirus, also affect the
urogenital tract of chickens causing renal disease. Infection of the
reproductive tract with IBV signifi cantly diminishes egg produc-
tion, causing substantial losses in the egg- production industry
each year [
63 ]. More recently, a novel coronavirus named SW1
has been identifi ed in a deceased Beluga whale [
64 ]. Large num-
bers of virus particles were identifi ed in the liver of the deceased
whale with respiratory disease and acute liver failure. Although,
electron microscopic images were not suffi cient to identify the
virus as a coronavirus, sequencing of the liver tissue clearly identi-
ed the virus as a coronavirus. It was subsequently determined to
be a γ-coronavirus based on phylogenetic analysis but it has not
yet been verifi ed experimentally that this virus is actually a caus-
ative agent of disease in whales. In addition, there has been intense
interest in identifying novel bat CoVs, since these are the likely
ancestors for SARS-CoV and MERS-CoV, and hundreds of novel
bat coronaviruses have been identifi ed over the past decade [
65 ].
Finally, another novel family of nidoviruses, Mesoniviridae , has
been recently identifi ed as the fi rst nidoviruses to exclusively infect
insect hosts [
66 , 67 ]. These viruses are highly divergent from
other nidoviruses but are most closely related to the roniviruses.
In size, they are ~20 kb, falling in between large and small nidovi-
ruses. Interestingly, these viruses do not encode for an endoribo-
nuclease, which is present in all other nidoviruses. These attributes
suggest these viruses are the prototype of a new nidovirus family
and may be a missing link in the transition from small to large
nidoviruses.
The most heavily studied animal coronavirus is murine hepatitis
virus (MHV), which causes a variety of outcomes in mice, including
respiratory, enteric, hepatic, and neurologic infections. These
infections often serve as highly useful models of disease. For
instance, MHV-1 causes severe respiratory disease in susceptible
A/J and C3H/HeJ mice, A59 and MHV-3 induce severe hepati-
tis, while JHMV causes severe encephalitis. Interestingly, MHV-3
induces cellular injury through the activation of the coagulation
cascade [
68 ]. Most notably, A59 and attenuated versions of JHMV
cause a chronic demyelinating disease that bears similarities to mul-
tiple sclerosis (MS), making MHV infection one of the best models
for this debilitating human disease. Early studies suggested that
demyelination was dependent on viral replication in oligodendro-
cytes in the brain and spinal cord [
69 , 70 ]; however, more recent
reports clearly demonstrate that the disease is immune-mediated.
Irradiated mice or immunodefi cient (lacking T and B cells) mice
do not develop demyelination, but addition of virus-specifi c T cells
restores the development of demyelination [
71 , 72 ]. Additionally,
demyelination is accompanied by a large infl ux of macrophages
Coronavirus Introduction
12
and microglia that can phagocytose infected myelin [ 73 ], although
it is unknown what the signals are that direct immune cells to
destroy myelin. Finally, MHV can be studied under BSL2 labora-
tory conditions, unlike SARS-CoV or MERS-CoV, which require
a BSL3 laboratory, and provides a large number of suitable animal
models. These factors make MHV an ideal model for studying the
basics of viral replication in tissue culture cells as well as for study-
ing the pathogenesis and immune response to coronaviruses.
Prior to the SARS-CoV outbreak, coronaviruses were only thought
to cause mild, self-limiting respiratory infections in humans. Two
of these human coronaviruses are α-coronaviruses, HCoV-229E
and HCoV-NL63, while the other two are β-coronaviruses,
HCoV-OC43 and HCoV-HKU1. HCoV-229E and HCoV-OC43
were isolated nearly 50 years ago [
74 76 ], while HCoV-NL63 and
HCoV-HKU1 have only recently been identifi ed following the
SARS-CoV outbreak [
77 , 78 ]. These viruses are endemic in the
human populations, causing 15–30 % of respiratory tract infections
each year. They cause more severe disease in neonates, the elderly,
and in individuals with underlying illnesses, with a greater inci-
dence of lower respiratory tract infection in these populations.
HCoV-NL63 is also associated with acute laryngotracheitis (croup)
[
79 ]. One interesting aspect of these viruses is their differences in
tolerance to genetic variability. HCoV-229E isolates from around
the world have only minimal sequence divergence [
80 ], while
HCoV-OC43 isolates from the same location but isolated in dif-
ferent years show signifi cant genetic variability [
81 ]. This likely
explains the inability of HCoV-229E to cross the species barrier to
infect mice while HCoV-OC43 and the closely related bovine
coronavirus, BCoV, are capable of infecting mice and several rumi-
nant species. Based on the ability of MHV to cause demyelinating
disease, it has been suggested that human CoVs may be involved in
the development of multiple sclerosis (MS). However, no evidence
to date suggests that human CoVs play a signifi cant role in MS.
SARS-CoV, a group 2b β-coronavirus, was identifi ed as the
causative agent of the Severe Acute Respiratory Syndrome (SARS)
outbreak that occurred in 2002–2003 in the Guangdong Province
of China. It is the most severe human disease caused by any coro-
navirus. During the 2002–2003 outbreak approximately 8,098
cases occurred with 774 deaths, resulting in a mortality rate of 9 %.
This rate was much higher in elderly individuals, with mortality
rates approaching 50 % in individuals over 60 years of age.
Furthermore, the outbreak resulted in the loss of nearly $40 billion
dollars in economic activity, as the virus nearly shut down many
activities in Southeast Asia and Toronto, Canada for several
months. The outbreak began in a hotel in Hong Kong and
ultimately spread to more than two dozen countries. During the
epidemic, closely related viruses were isolated from several exotic
5.2 Human
Coronaviruses
Anthony R. Fehr and Stanley Perlman
13
animals including Himalayan palm civets and raccoon dogs [ 82 ].
However, it is widely accepted that SARS-CoV originated in bats
as a large number of Chinese horseshoe bats contain sequences of
SARS-related CoVs and contain serologic evidence for a prior
infection with a related CoV [
83 , 84 ]. In fact, two novel bat SARS-
related CoVs have been recently identifi ed that are more similar to
SARS-CoV than any other virus identifi ed to date [
85 ]. They were
also found to use the same receptor as the human virus, angioten-
sin converting enzyme 2 (ACE2), providing further evidence that
SARS-CoV originated in bats. Although some human individuals
within wet animal markets had serologic evidence of SARS-CoV
infection prior to the outbreak, these individuals had no apparent
symptoms [
82 ]. Thus, it is likely that a closely related virus circulated
in the wet animal markets for several years before a series of factors
facilitated its spread into the larger population.
Transmission of SARS-CoV was relatively ineffi cient, as it only
spread through direct contact with infected individuals after the
onset of illness. Thus, the outbreak was largely contained within
households and healthcare settings [
86 ], except in a few cases of
superspreading events where one individual was able to infect
multiple contacts due to an enhanced development of high viral
burdens or ability to aerosolize virus. As a result of the relatively
ineffi cient transmission of SARS-CoV, the outbreak was controlla-
ble through the use of quarantining. Only a small number of SARS
cases occurred after the outbreak was controlled in June 2003.
SARS-CoV primarily infects epithelial cells within the lung.
The virus is capable of entering macrophages and dendritic cells
but only leads to an abortive infection [
87 , 88 ]. Despite this,
infection of these cell types may be important in inducing pro-
infl ammatory cytokines that may contribute to disease [
89 ]. In
fact, many cytokines and chemokines are produced by these cell
types and are elevated in the serum of SARS-CoV infected patients
[
90 ]. The exact mechanism of lung injury and cause of severe dis-
ease in humans remains undetermined. Viral titers seem to dimin-
ish when severe disease develops in both humans and in several
animal models of the disease. Furthermore, animals infected with
rodent-adapted SARS-CoV strains show similar clinical features to
the human disease, including an age-dependent increase in disease
severity [
91 ]. These animals also show increased levels of proin-
ammatory cytokines and reduced T-cell responses, suggesting a
possible immunopathological mechanism of disease [
92 , 93 ].
While the SARS-CoV epidemic was controlled in 2003 and the
virus has not since returned, a novel human CoV emerged in the
Middle East in 2012. This virus, named Middle East Respiratory
Syndrome-CoV (MERS-CoV), was found to be the causative agent
in a series of highly pathogenic respiratory tract infections in Saudi
Arabia and other countries in the Middle East [
94 ]. Based on the
high mortality rate of ~50 % in the early stages of the outbreak, it
Coronavirus Introduction
14
was feared the virus would lead to a very serious outbreak. However,
the outbreak did not accelerate in 2013, although sporadic cases
continued throughout the rest of the year. In April 2014, a spike
of over 200 cases and almost 40 deaths occurred, prompting fears
that the virus had mutated and was more capable of human-to-
human transmission. More likely, the increased number of cases
resulted from improved detection and reporting methods com-
bined with a seasonal increase in birthing camels. As of August
27th, 2014 there have been a total of 855 cases of MERS-CoV,
with 333 deaths and a case fatality rate of nearly 40 %, according to
the European Center for Disease Prevention and Control.
MERS-CoV is a group 2c β-coronavirus highly related to two
previously identifi ed bat coronaviruses, HKU4 and HKU5 [
95 ].
It is believed that the virus originated from bats, but likely had an
intermediate host as humans rarely come in contact with bat
secreta. Serological studies have identifi ed MERS-CoV antibodies
in dromedary camels in the Middle East [
96 ], and cell lines from
camels have been found to be permissive for MERS-CoV replication
[
97 ] providing evidence that dromedary camels may be the natural
host. More convincing evidence for this comes from recent studies
identifying nearly identical MERS-CoVs in both camels and human
cases in nearby proximities in Saudi Arabia [
98 , 99 ]. In one of
these studies the human case had direct contact with an infected
camel and the virus isolated from this patient was identical to the
virus isolated from the camel [
99 ]. At the present time it remains
to be determined how many MERS-CoV cases can be attributed to
an intermediate host as opposed to human-to-human transmis-
sion. It has also been postulated that human-to-camel spread con-
tributed to the outbreak.
MERS-CoV utilizes Dipeptidyl peptidase 4 (DPP4) as its
receptor [
100 ]. The virus is only able to use the receptor from
certain species such as bats, humans, camels, rabbits, and horses to
establish infection. Unfortunately for researchers, the virus is
unable to infect mouse cells due to differences in the structure of
DPP4, making it diffi cult to evaluate potential vaccines or antivi-
rals. Recently, a small animal model for MERS-CoV has been
developed using an Adenoviral vector to introduce the human
DPP4 gene into mouse lungs [
101 ]. This unique system makes it
possible to test therapeutic interventions and novel vaccines for
MERS-CoV in any animal sensitive to adenoviral transductions.
6 Diagnosis, Treatment, and Prevention
In most cases of self-limited infection, diagnosis of coronaviruses is
unnecessary, as the disease will naturally run its course. However,
it may be important in certain clinical and veterinary settings or in
epidemiological studies to identify an etiological agent. Diagnosis
Anthony R. Fehr and Stanley Perlman
15
is also important in locations where a severe CoV outbreak is
occurring, such as, at present, in the Middle East, where MERS-
CoV continues to circulate. The identifi cation of cases will guide
the development of public health measures to control outbreaks. It
is also important to diagnose cases of severe veterinary CoV-
induced disease, such as PEDV and IBV, to control these patho-
gens and protect food supplies. RT-PCR has become the method
of choice for diagnosis of human CoV, as multiplex real-time
RT-PCR assays have been developed, are able to detect all four
respiratory HCoVs and could be further adapted to novel CoVs
[
102 , 103 ]. Serologic assays are important in cases where RNA is
diffi cult to isolate or is no longer present, and for epidemiological
studies.
To date, there are no antiviral therapeutics that specifi cally
target human coronaviruses, so treatments are only supportive.
In vitro, interferons (IFNs) are only partially effective against coro-
naviruses [
104 ]. IFNs in combination with ribavirin may have
increased activity in vitro when compared to IFNs alone against
some coronaviruses; however, the effectiveness of this combination
in vivo requires further evaluation [
105 ]. The SARS and MERS
outbreaks have stimulated research on these viruses and this
research has identifi ed a large number of suitable antiviral targets,
such as viral proteases, polymerases, and entry proteins. Signifi cant
work remains, however, to develop drugs that target these processes
and are able to inhibit viral replication.
Only limited options are available to prevent coronavirus
infections. Vaccines have only been approved for IBV, TGEV, and
Canine CoV, but these vaccines are not always used because they
are either not very effective, or in some cases have been reported
to be involved in the selection of novel pathogenic CoVs via recom-
bination of circulating strains. Vaccines for veterinary pathogens,
such as PEDV, may be useful in such cases where spread of the
virus to a new location could lead to severe losses of veterinary
animals. In the case of SARS-CoV, several potential vaccines have
been developed but none are yet approved for use. These vaccines
include recombinant attenuated viruses, live virus vectors, or
individual viral proteins expressed from DNA plasmids. Therapeutic
SARS-CoV neutralizing antibodies have been generated and could
be retrieved and used again in the event of another SARS-CoV
outbreak. Such antibodies would be most useful for protecting
healthcare workers. In general, it is thought that live attenuated
vaccines would be the most effi cacious in targeting coronaviruses.
This was illustrated in the case of TGEV, where an attenuated variant,
PRCV, appeared in Europe in the 1980s. This variant only caused
mild disease and completely protected swine from TGEV. Thus,
this attenuated virus has naturally prevented the reoccurrence of
severe TGEV in Europe and the U.S. over the past 30 years [
106 ].
Despite this success, vaccine development for coronaviruses faces
Coronavirus Introduction
16
many challenges [ 107 ]. First, for mucosal infections, natural infection
does not prevent subsequent infection, and so vaccines must either
induce better immunity than the original virus or must at least
lessen the disease incurred during a secondary infection. Second,
the propensity of the viruses to recombine may pose a problem by
rendering the vaccine useless and potentially increasing the evolu-
tion and diversity of the virus in the wild [
108 ]. Finally, it has been
shown in FIPV that vaccination with S protein leads to enhanced
disease [
109 ]. Despite this, several strategies are being developed
for vaccine development to reduce the likelihood of recombina-
tion, for instance by making large deletions in the nsp1 [
110 ] or E
proteins [
111 ], rearranging the 3 end of the genome [ 112 ],
modifying the TRS sequences [
113 ], or using mutant viruses with
abnormally high mutation rates that signifi cantly attenuate the
virus [
114 ].
Owing to the lack of effective therapeutics or vaccines, the best
measures to control human coronaviruses remain a strong public
health surveillance system coupled with rapid diagnostic testing
and quarantine when necessary. For international outbreaks, coop-
eration of governmental entities, public health authorities, and
health care providers is critical. During veterinary outbreaks that
are readily transmitted, such as PEDV, more drastic measures such
as destruction of entire herds of pigs may be necessary to prevent
transmission of these deadly viruses.
7 Conclusion
Over the past 50 years the emergence of many different coronavi-
ruses that cause a wide variety of human and veterinary diseases has
occurred. It is likely that these viruses will continue to emerge and
to evolve and cause both human and veterinary outbreaks owing to
their ability to recombine, mutate, and infect multiple species and
cell types.
Future research on coronaviruses will continue to investigate
many aspects of viral replication and pathogenesis. First, under-
standing the propensity of these viruses to jump between species,
to establish infection in a new host, and to identify signifi cant
reservoirs of coronaviruses will dramatically aid in our ability to
predict when and where potential epidemics may occur. As bats
seem to be a signifi cant reservoir for these viruses, it will be inter-
esting to determine how they seem to avoid clinically evident disease
and become persistently infected. Second, many of the non-struc-
tural and accessory proteins encoded by these viruses remain
uncharacterized with no known function, and it will be impor-
tant to identify mechanisms of action for these proteins as well as
defi ning their role in viral replication and pathogenesis. These
studies should lead to a large increase in the number of suitable
Anthony R. Fehr and Stanley Perlman
17
therapeutic targets to combat infections. Furthermore, many of
the unique enzymes encoded by coronaviruses, such as ADP-
ribose-1-phosphatase, are also present in higher eukaryotes, mak-
ing their study relevant to understanding general aspects of
molecular biology and biochemistry. Third, gaining a complete
picture of the intricacies of the RTC will provide a framework for
understanding the unique RNA replication process used by these
viruses. Finally, defi ning the mechanism of how coronaviruses
cause disease and understanding the host immunopathological
response will signifi cantly improve our ability to design vaccines
and reduce disease burden.
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Coronavirus Introduction
... Coronaviruses are the largest group belonging to the Nidovirales order. All viruses in this order are non-segmented positive-sense RNA [9]. Replicase gene translation from the RNA genome of the virus is an important stage in the life cycle of coronaviruses. ...
... Coronaviruses encode three proteases including Papain-Like protease (PLpro), serine protease, and the Main protease (Mpro) which is also called "3CLpro". The PLpro separates the first four non-structural proteins of the polyprotein by cleaving the nsp1/2, nsp 2/3, and nsp 3/4 boundaries while the Mpro is responsible for cleavage at 11 remaining positions [9]. ...
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Background: The acute respiratory syndrome named “COVID-19” is caused by a novel coronavirus called Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Lack of specific antiviral drugs or proper vaccination has led to the development of new therapeutic methods against this virus. Objective The Mpro 3Clpro is the main protease of the SARS-CoV-2 which plays an important role in replication and transcription of the virus. Therefore, targeting this enzyme is a valuable approach for drug development. Methods: In the present study, the structural properties of 69 anti-migraine and 212 anti-HIV drugs were first obtained from Drug Bank database. To select the appropriate drugs for the enzyme inhibition, the AutoDock Vina software was used. The molecular dynamics (MD) simulation method was applied for better recognition of the structural changes. Results: We identified Rimegepant (PubChem ID: 51049968), Dihydroergotamine (PubChem ID: 10531) and Ergotamine (PubChem ID: 8223) as potential inhibitors of Mpro 3Clpro. These complexes were equilibrated after 70 ns. Conclusion: Among these compounds, the anti-migraine drug “Rimegepant” showed the highest affinity for binding to the Mpro 3Clpro (-60.8 kJ/mol). This study provides enough evidence for further accomplishment of the identified compounds in the development of effective therapeutics methods against COVID-19.
... The SARS-CoV-2 Spike protein is involved in cell receptor identification and membrane fusion [9]. The Spike protein is made up of two subunits: the S1 receptor-binding domain (RBD), which recognizes and binds to the host receptor angiotensin-converting enzyme 2 (ACE2), and the S2 subunit, which is in charge of facilitating viral cell membrane fusion [10][11][12][13][14]. During viral infection, the Spike protein is split into S1 and S2 subunits, with S1 subunits released during the transition to the post-fusion conformation [15][16][17][18][19][20]. ...
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Background The coronavirus disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection became an international pandemic and created a public health crisis. The binding of the viral Spike glycoprotein to the human cell receptor angiotensin-converting enzyme 2 (ACE2) initiates viral infection. The development of efficient treatments to combat coronavirus disease is considered essential. Methods An in silico approach was employed to design amino acid peptide inhibitor against the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein. The designed inhibitor (SARS-CoV-2 PEP 49) consists of amino acids with the α1 helix and the β4 - β5 sheets of ACE2. The PEP-FOLD3 web tool was used to create the 3D structures of the peptide amino acids. Analyzing the interaction between ACE2 and the RBD of the Spike protein for three protein data bank entries (6M0J, 7C8D, and 7A95) indicated that the interacting amino acids were contained inside two regions of ACE2: the α1 helical protease domain (PD) and the β4 - β5 sheets. Results Molecular docking analysis of the designed inhibitor demonstrated that SARS-CoV-2 PEP 49 attaches directly to the ACE2 binding site of the Spike protein with a binding affinity greater than the ACE2, implying that the SARS-CoV-2 PEP 49 model may be useful as a potential RBD binding blocker.
... Coronaviruses are enveloped viruses containing a positive-sense single-stranded RNA (+ssRNA) genome, belonging to the subfamily Coronavirinae in Coronaviridae family [4]. There are four genera of coronaviruses including α, β, γ, and δ coronavirus with some specific mutations and recombination. ...
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SARS-CoV-2 as a zoonotic virus has significantly affected daily life and social behavior since its outbreak in late 2019. The concerns over its transmission through different media directly or indirectly have evoked great attention about the survival of SARS-CoV-2 virions in the environment and its potential infection of other animals. To evaluate the risk of infection by SARS-CoV-2 and to counteract the COVID-19 disease, extensive studies have been performed to understand SARS-CoV-2 biogenesis and its pathogenesis. This review mainly focuses on the molecular architecture of SARS-CoV-2, its potential for infecting marine animals, and the prospect of drug discovery using marine natural products to combat SARS-CoV-2. The main purposes of this review are to piece together progress in SARS-CoV-2 functional genomic studies and antiviral drug development, and to raise our awareness of marine animal safety on exposure to SARS-CoV-2.
... Porcine epidemic diarrhoea (PED) virus (PEDV) is a single-stranded RNA virus that belongs to the α genera of the Coronaviridae family [1]. PEDV primarily infects swine intestinal epithelial cells. ...
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Autophagy plays an important role in defending against invading microbes. However, numerous viruses can subvert autophagy to benefit their replication. Porcine epidemic diarrhoea virus (PEDV) is an aetiological agent that causes severe porcine epidemic diarrhoea. How PEDV infection regulates autophagy and its role in PEDV replication are inadequately understood. Herein, we report that PEDV induced complete autophagy in Vero and IPEC-DQ cells, as evidenced by increased LC3 lipidation, p62 degradation, and the formation of autolysosomes. The lysosomal protease inhibitors chloroquine (CQ) or bafilomycin A and Beclin-1 or ATG5 knockdown blocked autophagic flux and inhibited PEDV replication. PEDV infection activated AMP-activated protein kinase (AMPK) and c-Jun terminal kinase (JNK) by activating TGF-beta-activated kinase 1 (TAK1). Compound C (CC), an AMPK inhibitor, and SP600125, a JNK inhibitor, inhibited PEDV-induced autophagy and virus replication. AMPK activation led to increased ULK1S777 phosphorylation and activation. Inhibition of ULK1 activity by SBI-0206965 (SBI) and TAK1 activity by 5Z-7-Oxozeaenol (5Z) or by TAK1 siRNA led to the suppression of autophagy and virus replication. Our study provides mechanistic insights into PEDV-induced autophagy and how PEDV infection leads to JNK and AMPK activation.
... N proteins are therefore regarded as possible therapeutic targets. The RNA-binding domain of the N proteins is around 140 amino acids long and acts as a "bead on a string" to bind viral RNA (85). N protein sequence of SARS-CoV-2 showed high similarity with the SAR-CoV and thus it is speculated that antibodies developed against the former would be likely to detect the latter. ...
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Viruses are submicroscopic, obligate intracellular parasites that carry either DNA or RNA as their genome, protected by a capsid. Viruses are genetic entities that propagate by using the metabolic and biosynthetic machinery of their hosts and many of them cause sickness in the host. The ability of viruses to adapt to different hosts and settings mainly relies on their ability to create de novo variety in a short interval of time. The size and chemical composition of the viral genome have been recognized as important factors affecting the rate of mutations. Coronavirus disease 2019 (Covid-19) is a novel viral disease that has quickly become one of the world’s leading causes of mortality, making it one of the most serious public health problems in recent decades. The discovery of new medications to cope with Covid-19 is a difficult and time-consuming procedure, as new mutations represent a serious threat to the efficacy of recently developed vaccines. The current article discusses viral mutations and their impact on the pathogenicity of newly developed variants with a special emphasis on Covid-19. The biology of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), its mutations, pathogenesis, and treatment strategies are discussed in detail along with the statistical data.
... Coronavirus infection is primarily associated with mild to moderate upper respiratory illnesses (Fehr and Perlman, 2015). The World Health Organisation called this new coronavirus infection COVID-19 with documented confirmed cases in more than 110 countries of the world (WHO, 2019;Perrella et al., 2020). ...
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Replicative capacity of Middle East respiratory syndrome coronavirus (MERS-CoV) was assessed in cell lines derived from livestock and peridomestic small mammals on the Arabian Peninsula. Only cell lines originating from goats and camels showed efficient replication of MERS-CoV. These results provide direction in the search for the intermediate host of MERS-CoV.
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