Turkish Journal of Medical Sciences
Turk J Med Sci
Coronaviruses and SARS-COV-2
Mustafa HASÖKSÜZ1, Selcuk KILIÇ2,*, Fahriye SARAÇ3
1Department of Virology, Faculty of Veterinary Medicine, Istanbul University-Cerrahpaşa, İstanbul, Turkey
2Microbiology Reference Lab and Biological Products Department, General Directorate of Public Health Department,
Republic of Turkey Ministry of Health, Ankara, Turkey
3Pendik Veterinary Control Institute, İstanbul, Turkey
* Correspondence: email@example.com
Before December 2019, 6 strains of coronavirus (CoVs)
were known to infect humans and cause respiratory
diseases. HCoV‐229E, HCoV‐OC43, HCoV‐NL63, and
HKU1 are coronaviruses (CoVs) that normally cause only
mild upper respiratory disease with rare severe infections
occurring in infants, young children, and elderly people
. More dangerous ones are SARS‐CoV and MERS‐CoV,
which can infect lower respiratory tract and trigger a severe
respiratory condition in humans . It is widely known
that some CoVs aect birds, bats, mice, girae, whales,
and many other wild animals, but they can also infect
livestock, causing great economic loss [3,4]. Domestic
animals can also play a role as intermediate hosts that
enable virus transmission from the natural, wild animal
hosts to humans [5,6]. In addition, domestic animals
themselves can also contract bat-borne or closely related
coronavirus diseases . Genomic sequences that are very
similar to porcine epidemic diarrhoea virus (PEDV) have
been detected in bats. In 2016, a large‐scale outbreak of a
disease in pigs in southern China that killed 24,000 piglets
were caused by an HKU2‐related Bat-CoV, swine acute
diarrhoea syndrome CoV [7,8]. is incident was the rst
documented case where a Bat-CoV caused a severe disease
in livestock .
2. Structure of coronaviruses
e family Coronaviridae is a monophyletic cluster in the
order Nidovirales members of which are enveloped with a
positive sense, single-stranded RNA genome and measures,
on average, 30 kilobases . Orthocoronavirinae subfamily
contains 4 genera (Alphacoronavirus, Betacoronavirus,
Gammacoronavirus, and Deltacoronavirus), and SARS-
CoV and SARS-CoV-2 belong to genus betacoronavirus
[10,11,12]. e coronavirus (CoV) has a single-stranded,
nonsegmented RNA genome of positive polarity, and
its virion contains 4 major structural proteins: the
nucleocapsid (N) protein, the transmembrane (M) protein,
the envelope (E) protein, and the spike (S) protein (Figure
1). However, with some coronaviruses, the full ensemble
of structural proteins is not necessary for the forming of
a complete, infectious virion; additional proteins may
be encoded with overlapping compensatory functions
Abstract: Coronaviruses (CoVs) cause a broad spectrum of diseases in domestic and wild animals, poultry, and rodents, ranging from
mild to severe enteric, respiratory, and systemic disease, and also cause the common cold or pneumonia in humans. Seven coronavirus
species are known to cause human infection, 4 of which, HCoV 229E, HCoV NL63, HCoV HKU1 and HCoV OC43, typically cause
cold symptoms in immunocompetent individuals. e others namely SARS-CoV (severe acute respiratory syndrome coronavirus),
MERS-CoV (Middle East respiratory syndrome coronavirus) were zoonotic in origin and cause severe respiratory illness and fatalities.
On 31 December 2019, the existence of patients with pneumonia of an unknown aetiology was reported to WHO by the national
authorities in China. is virus was ocially identied by the coronavirus study group as severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2), and the present outbreak of a coronavirus-associated acute respiratory disease was labelled coronavirus disease 19
(COVID-19). COVID-19’s rst cases were seen in Turkey on March 10, 2020 and was number 47,029 cases and 1006 deaths aer 1
month. Infections with SARS-CoV-2 are now widespread, and as of 10 April 2020, 1,727,602 cases have been conrmed in more than
210 countries, with 105,728 deaths.
Key words: Human coronaviruses, animal coronaviruses, history of coronaviruses, COVID-19, SARS-CoV-2
Received: 12.04.2020 Accepted/Published Online: 14.04.2020 Final Version: 00.00.2020
is work is licensed under a Creative Commons Attribution 4.0 International License.
HASÖKSÜZ et al. / Turk J Med Sci
e N protein is the only protein that forms the
nucleocapsid and primarily functions to bind to the
coronavirus RNA genome. While the N protein is involved
in viral genome related processes, it plays a role in the
replication of viral RNA and the host’s cellular response
to viral infection. e endoplasmic reticulum localization
of N protein carries a function for this in assembly and
budding. Furthermore, in some coronaviruses, the N
protein expression has been shown to signicantly increase
the production of virus-like particles .
Changes in the S glycoprotein are largely responsible
for the host variety of coronaviruses and the variety in
tissue tropism. e S glycoprotein is a type 1 membrane
glycoprotein with dierent functional domains near the
amino (S1) and carboxy (S2) termini. While the S2 subunit
is a transmembrane protein mediating the fusion of viral
and cellular membranes, the S1 subunit is peripheral and
is associated with receptor binding functions [13,14].
Generally speaking, the S glycoprotein facilitates viral
binding to susceptible cells, causes cell fusion, and induces
neutralizing antibodies. Of the 2 functional subunits
containing several antigenic sites, S1 and S2, the S1
monoclonal antibody appears to occur most eciently
because it has a higher level of neutralizing activity
In virus assembly, the M protein of coronavirus
plays a central role as it turns cellular membranes into
factories where virus and host factors join to make new
virus particles. e M proteins from SARS-CoV, SARS-
CoV-2, MERS-CoV, MHV, FCoV, IBV, TGEV, and BCoV
are targeted to the vicinity of the Golgi apparatus. Reverse
genetic studies and virus-like protein (VLP) assembly
studies suggest that the M protein encourages assembly
by interacting with the viral ribonucleoprotein (RNP)
and S glycoproteins at the budding site and by creating a
network of M-M interactions capable of excluding some
host membrane proteins from the viral envelope .
e smallest but also the most mysterious of the major
structural proteins is the E protein. While the E protein
is plentifully expressed inside the infected cell during the
replication cycle, only a small portion is incorporated into
the virion envelope . Most of the protein is localized at
the ER, Golgi, and ER-Golgi intermediate compartment,
the site of intracellular tracking, where it takes part in
CoV assembly and budding. According to published
studies, 3 roles have been proposed for the CoV E protein:
a) the interaction between the cytoplasmic tails of the M
and E proteins which suggests that E participates in viral
assembly; b) its hydrophobic transmembrane domain is
essential for the release of virions; and c) it is implicated in
the virus’s pathogenesis [11,18].
Interactions between the S protein and its receptor
initiate the initial attachment of the virion to the host cell.
e receptor binding domains (RBD) sites within the S1
region of a coronavirus S protein vary depending on the
virus; some have the RBD at the N-terminus of S1 (MHV),
and others (SARS- CoV and SARS- CoV-2) have the RBD
at the C-terminus of S1 . To gain entry into human
cells, many α coronaviruses (HCoV-229E, TGEV, PEDV,
FIPV, CCoV) employ aminopeptidase N (APN) as their
receptor, HCoV-NL63, SARS-CoV, and SARS-CoV-2
utilize angiotensin converting enzyme 2 (ACE2) as their
Figure 1. e Structure of SARS-CoV-2 virus and ACE2 protein .
(Contributed by Rohan Bir Singh; made with Biorender.com)
HASÖKSÜZ et al. / Turk J Med Sci
receptor, MHV enters through CEACAM1, and MERS-
CoV binds to dipeptidyl-peptidase 4 (DPP4) . Aer
the receptor binding, the virus must next gain access to
the host cell cytosol. is is usually accomplished by
acid-dependent proteolytic cleavage of the S protein by
a cathepsin, TMPRRS2, or another protease, followed by
fusion of the viral and cellular membranes (16).
3. Summary of animal coronaviruses
A wide range of animal diseases are caused by
coronaviruses, and signicant research on these viruses in
the second half of the 20th century was triggered by their
ability to cause severe disease in livestock and companion
animals such as pigs, cows, chickens, dogs, and cats
[19,20,21]. Transmissible gastroenteritis virus (TGEV) and
PEDV, for example, cause severe gastroenteritis in young
piglets that lead to signicant morbidity and mortality
and results in economic losses . e feline infectious
peritonitis virus (FIPV) results in the development of a
lethal disease called feline infectious peritonitis (FIP) .
FIP has wet and dry forms and is similar to the human
disease sarcoidosis. FIPV is macrophage tropic and may
cause aberrant cytokine and/or chemokine expression and
lymphocyte depletion, which results in a lethal disease
. e cattle industry has experienced signicant losses
from bovine coronaviruses (BCoV) that caused signicant
losses in the cattle industry, and its infection spread a
variety of ruminants including elk, girae, deer, and
camels [3,15,24]. Infectious bronchitis virus (IBV) aects
the urogenital tract of chickens, causing renal disease. Egg
production is signicantly reduced by the infection of the
reproductive tract with IBV, causing substantial industrial
losses every year . e most intensely studied animal
coronavirus is murine hepatitis virus (MHV) which causes
a variety of conditions in mice, including respiratory,
enteric, hepatic, and neurologic infections. us, MHV
is an excellent model for studying the basics of viral
replication in tissue culture cells as well as for studying the
pathogenesis and immune response to coronaviruses .
4. e history of coronaviruses
IBV was the rst coronavirus to be reported, a virus from
chickens with respiratory disease reported by Beaudette
& Hudson in 1937 . e murine and hepatitis
viruses (MHV), another group of animal’s viruses, were
rst identied by Cheever at al. in 1949 . In 1946,
transmissible gastroenteritis in swine was rst recognized.
However, it was not until aer the human coronaviruses
(HCoVs) were discovered in the 1960s and the coronavirus
genus was dened that these 3 animal diseases were found
to be related . An organ culture of human embryonic
trachea taken from a schoolboy with a cold was described
by Tyrrell & Bynoe as the rst human coronavirus (B814)
in 1965 . When examined by an electron microscope,
the virus was found to resemble avian IBV. Hamre &
Procknow recovered 5 virus strains in tissue culture taken
from medical students with colds around the same time
. Almeida & Tyrrell examined the prototype strain
HCoV 229E, and its morphology was found to be identical
to that of B814 and IBV . Using the organ culture
technique, 6 further strains were subsequently recovered
including the prototype strains HCoV OC43 as well as 3
strains considered antigenically unrelated to either OC43
or 229E .
In the Guangdong province of China during the
winter of 2002 to 2003, an unusual and oen deadly form
of pneumonia appeared, a disease subsequently labelled
severe acute respiratory syndrome (SARS) . is
disease spread to Hong Kong in late February, and, within
days, international air travel spread the virus over a wide
area, seeding outbreaks in Vietnam, Singapore, Canada,
and elsewhere. In July 2003, at the end of this outbreak,
8422 cases had been recorded, 916 (10.8 %) of them
fatal, in 29 country across 6 continents . e point of
initial emergence of SARS-CoV an animal reservoir were
the live animal markets in Guangdong, where diverse
animal species are held, traded and sold to restaurants
in response to the demand for exotic food . Small
mammals, such as civet cats, sold in these markets were
found to harbour viruses closely related to SARS-CoV, and
the initial interspecies transmission to humans probably
came from these markets [29,30]. MERS-CoV was rst
isolated in 2012 from the lung of a 60-year-old patient
who developed acute pneumonia and renal failure in Saudi
Arabia. Live MERS-CoV identical to the virus found in
humans was isolated from the nasal swabs of dromedary
camels, further indicating that camels serve as the bona
de reservoir host of MERS-CoV. As of February 14, 2020,
over 2500 laboratory conrmed cases were reported with a
high case fatality of 34.4%, making MERS-CoV one of the
most devastating viruses known to humans .
5. e history of COVID-19
A 41-year-old man was admitted to the Central Hospitalof
Wuhan on 26 December 2019, 6 days aer the onset of
disease. He had no history of hepatitis, tuberculosis,
or diabetes and reported fever, chest tightness, an
unproductive cough, pain, and weakness for 1 week on
presentation. e Wuhan Centre for Disease Control and
Prevention conducted an epidemiological investigation
and found that the patient worked at a local indoor
seafood market where, in addition to sh and shellsh, a
variety of live wild animals (including hedgehogs, badgers,
snakes, and birds) were available for sale as well as animal
carcasses. However, no bats were available for sale, and the
patient recalled no exposure to live poultry although he
HASÖKSÜZ et al. / Turk J Med Sci
might have come into contact with wild animals .
On 31 December 2019, the WHO China Country
Oce was informed that cases of pneumonia with an
unknown aetiology had been detected in Wuhan City,
in the Hubei province of China . From 31 December
2019 through 3 January 2020, a total of 44 patients with a
pneumonia of unknown aetiology were reported to WHO
by the national authorities in China. No causal agent was
identied during this reporting period. en, on 11 and
12 January 2020, WHO received further details from the
National Health Commission in China that the outbreak
had been associated with one of the seafood markets in
Wuhan City. On 7 January 2020, the Chinese, isolated and
identied a new type of coronavirus so that other countries
could develop specic diagnostic kits. On 12 January 2020,
China shared the genetic sequence of the novel coronavirus
. On 13 January 2020, the Ministry of Public Health of
ailand reported the rst imported case of lab-conrmed
novel coronavirus (2019-nCoV) from Wuhan, Hubei
Province, China. On 15 January 2020, the Ministry of
Health, Labour and Welfare of Japan (MHLW) reported
an imported case of laboratory-conrmed 2019-novel
coronavirus (2019-nCoV) from the same source location
. On 20 January 2020, the National IHR Focal Point
(NFP) for the Republic of Korea reported the rst case of
novel coronavirus, also from Wuhan, China . COVID-
19’s rst cases were seen in Turkey on March 10, 2020
and it was 47,029 cases and 1006 deaths aer 1 month.
Infections with SARS-CoV-2 are now widespread, and as
of 10 April 2020, 1,727,602 cases have been conrmed in
more than 210 countries, with 105,728 deaths .
In an early study, phylogenetic tree showed that 2019‐
nCoV (previous naming of SARS-CoV-2) signicantly
clustered with bat SARS-like coronavirus sequence isolated
in 2015, whereas structural analysis revealed mutation in
Spike glycoprotein and nucleocapsid protein. us, it is
clear that the new 2019‐nCoV is distinct from the SARS
virus which, aer a mutation that gave it the ability to
infect humans, was probably transmitted from bats .
Based on the phylogenetic tree, taxonomy, and established
practice, this virus was ocially identied as severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2) by
the Coronavirus Study Group, and the current outbreak
of a coronavirus-associated acute respiratory disease was
called Coronavirus Disease-19 (COVID-19) .
Human epidemics with variable clinical severity
featuring respiratory and extra-respiratory manifestations
have been caused by a number of CoVs: SARS-CoV,
SARS-CoV-2, and MERS-CoV (betaCoVs of the B and C
lineage, respectively) . With SARS-CoV and MERS-
CoV, mortality rates have been seen up to 10% and 35%,
respectively which places SARS-CoV-2 in the betaCoVs
category . is variety has a round or elliptic, and oen
pleomorphic form and a diameter of approximately 60–
140 nm. Like other CoVs, it is sensitive to ultraviolet rays
and heat . Another eective means of inactivation is
by lipid solvents including ether (75%), ethanol, chlorine-
containing disinfectant, peroxyacetic acid, and chloroform
except for chlorhexidine [43,44].
SARS-CoV-2 and SARS-CoV-1 have similar stability.
Both viruses could be detected in aerosols up to 3 h post-
aerosolization, up to 4 h on copper, up to 24 h on cardboard
and up to 2 to 3 days on plastic and stainless steel. In
aerosols, SARS-CoV-2, and SARS-CoV-1 exhibited
similar half-lives with median estimates of around 2.7 h
. With the median half-life estimate for SARS-CoV-2
being around 13 h on stainless steel and around 16 h on
polypropylene, both viruses exhibit relatively long viability
on these surfaces compared to copper or cardboard .
Chan et al.  have shown that, in genetic terms, the
genome of the new HCoV, isolated from a cluster-patient
who presented with atypical pneumonia aer visiting
Wuhan, had a 89% nucleotide identity with bat SARS-like-
CoVZXC21 and 82% with that of human SARS-CoV .
For this reason, the new virus was called SARS-CoV-2,
its single-stranded RNA genome containing 29,903
nucleotides encoding for 9860 amino acids (Figure 2) .
While its origins are not completely understood, these
genomic analyses suggest that SARS-CoV-2 probably
evolved from a strain found in bats. However, the possible
intermediate between bats and humans, the potential
amplifying mammalian is not known. It is not even certain
that this intermediary exists since the virulence toward
humans could have been directly triggered by the mutation
in the original strain .
While it is believed that bats and palm civets were
the natural and intermediate reservoirs for SARS-CoV,
respectively, it has been isolated from animals and adapted
to lab cell culture [46,47]. It is also believed that SARS-
CoV was transmitted from palm civets to humans in an
animal market in southern China, and while the animal
source of the outbreak is currently unknown, SARS-CoV-2
also reportedly infected humans in an animal market
in Wuhan. Critically, the capacity for SARS-CoV-2 to
transmit from human to human has been conrmed .
An envelope-anchored spike protein mediates coronavirus
entry into host cells by rst binding to a host receptor and
then fusing viral and host membranes .
When the SARS-CoV-2 virus was compared with
S gene of SARS-CoV, bat-CoV (As6526), bat-CoV
(RaTG13), mink-CoV, and pangolin-CoV, it was found
71.41%, 68.17%, 92,86%, 30,89%, and 90% similarity,
respectively . When the homology of SARS-CoV-2
and these coronaviruses is less than 75%, it would be
HASÖKSÜZ et al. / Turk J Med Sci
presumed that SARS-CoV-2 is not the same virus like
the coronaviruses arising from these wild animals . It
is also clear from these results that the 2 viruses, SARS-
CoV-2 and bat coronavirus RaTG13, are closely related
. It was also reported that the SARS-CoV-2 virus did
not come directly from pangolins (24), so the relationship
between SARS-CoV-2 and pangolin coronavirus and
whether the pangolin is the intermediate host of SARS-
CoV-2 requires further investigation [51,52].
Spike protein contains 2 subunits, S1 and S2 .
S1 contains a receptor binding domain (RBD), which is
responsible for recognizing and binding with the cell
surface receptor. S2 subunit is the “stem” of the structure,
which contains other basic elements needed for the
membrane fusion . e spike protein is the common
target for neutralizing antibodies and vaccines. It has been
reported that COVID-19 can infect the human respiratory
epithelial cells through interaction with the human
ACE2 receptor (Figure 3). Indeed, the recombinantSpike
proteincan bind with recombinantACE2protein [54,55].
e ACE2 protein is reportedly present in type 1 and
type 2 pneumocytes, enterocytes of all parts of the small
intestine, the brush border of the proximal arteries, and
veins of all tissues studied, and arterial smooth muscle
cells . is localization of ACE2 explains the tissue
tropism of SARS-CoV for the lung, small intestine, and
kidney. However, notable discrepancies include virus
replication in colonic epithelium, which has no ACE2, and
no virus infection in endothelial cells, which have ACE2,
other receptors, or co-receptors such as LSIG that explain
such discrepancies .
e pathophysiology and virulence mechanisms of
CoVs– and thus of SARS-CoV-2–have links to the function of
the nonstructural protein (nsps) and structural proteins. For
Figure 2. Genomic structure of SARS-CoV-2 Wuhan Hu-1 strain. Accession: NC_045512 
Figure 3. Binding, viral entry, and replication cycle of SARS-CoV-2 . (Contributed by
Rohan Bir Singh, made with Biorender.com).
HASÖKSÜZ et al. / Turk J Med Sci
instance, a nsp can block the host innate immune response
of the host, according to the research. It is known that the
envelope, among the functions of structural proteins, plays a
crucial role in virus pathogenicity because it promotes viral
assembly and release, but many features (e.g., those of nsp 2
and 11) have not been described yet .
Infectious disease experts and multiple international
and domestic human and animal health organizations
(CDC, OIE, and WHO) agree that there is no evidence at
this point indicating that pets can spread COVID-19 to
other animals, including people. Although there has not
been reports of pets becoming sick with COVID-19, out of
an abundance of caution,it is recommended that those of
ill with COVID-19 should limit contact with animals until
more information is known about the virus. On the other
hand, Shi et al.  report that cats and ferrets can be
experimentally infected with SARS-CoV-2, but not dogs,
pig, chickens, and ducks. In this study, which is the only
one in this subject, they claim that cats can spread SARS-
CoV-2 by respiratory and other cats can be infected .
Ultimately, novel coronaviruses are likely to emerge
periodically in humans because of frequent cross-species
infections and occasional spillover events (Figure 4), given
the high prevalence and wide distribution of coronaviruses,
the large genetic diversity, and frequent recombination of
their genomes, and an increasing level of human-animal
Mustafa Hasöksüz and Selçuk Kılıç are the members of
COVID-19 Advisory Committee of Ministry of Health of
Figure 4. Animal origins of human coronaviruses .
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