key: cord-1022482-lgqvfh7s
authors: Subbaram, Kannan; Kannan, Hemalatha; Khalil Gatasheh, Mansour
title: Emerging Developments on Pathogenicity, Molecular Virulence, Epidemiology and Clinical Symptoms of Current Middle East Respiratory Syndrome Coronavirus (MERS-CoV)
date: 2017-09-14
journal: Hayati
DOI: 10.1016/j.hjb.2017.08.001
sha: fc766f4da6dcc43946f06f65cde8d79dcb740c11
doc_id: 1022482
cord_uid: lgqvfh7s

Middle East respiratory syndrome coronavirus (MERS-CoV) is a recently reported virus that is associated with severe, life threatening and rapidly spreading primarily respiratory illness called the Middle East respiratory syndrome. MERS-CoV possesses a unique positive-sense single-stranded RNA and can undergo rapid mutation in the viral genome. This results in antigenic switching and genetic variation, finally leading to the emergence of novel and new MERS-CoV subtypes which are uncontrollable by vaccines. Researchers are also finding difficulties to sort out therapeutic intervention strategies for MERS-CoV. This virus can spread from human to human, but transmission from dromedary camels to humans plays a crucial epidemiological significance. Dromedary camel acts as “gene mixing vessels” for MERS-CoV and these virus particles undergo rapid change in them. Viral receptors called dipeptidyl peptidase-4 are important receptors for attachment and spread of MERS-CoV in humans. The current method of laboratory confirmation is through real-time polymerase chain reaction on bronchoalveolar lavage, sputum and tracheal aspirates. Unfortunately, till today there are no definite anti-viral drugs available for MERS-CoV.

Middle East respiratory syndrome coronavirus (MERS-CoV) is a unique positive-sense single-stranded RNA (þssRNA) virus belonging to the genus Betacoronavirus (World Health Organization, 2014) . Previously, it was called novel coronavirus 2012 because it was isolated in 2012 by genomic sequencing from sputum of a patient, during the 2012 outbreak of new flu (Zaki et al. 2012) .

Until August 2015, MERS-CoV outbreaks were reported in more than 21 nations like Saudi Arabia, Jordan, Egypt, Qatar, UAE, Turkey, Kuwait, Oman, Bangladesh, Indonesia, Algeria, South Korea, Philippines, Thailand, USA, UK and Austria (Abdullah et al. 2016) . According to the European Center for Disease Prevention and Control, it was estimated that out of a total of 1082 MERS-CoV cases, there were 439 deaths thereby mortality rate leading to whooping 41% . As per the World Health Organization estimates, the fatality rate surged to 37%. MERS-CoV patients have acquired this deadly infection through various sources, namely, infected humans, camels, bats, other domesticated animals and pets (World Health Organization, 2015) . The very nature and special feature of coronavirus is its ability to undergo rapid genetic rearrangement and profound molecular variation in its ssRNA. This rearrangement and variation of the viral genome resulting in new antigenic subtypes, leads to difficulties in developing vaccines and therapeutic interventions (Chu et al. 2016) . Hence, MERS-CoV poses serious threat not only to the Middle East region, but also throughout the world, as a whole.

Genomes of MERS-CoV are phylogenetically grouped into clade A and clade B. Initially, reported MERS-CoV cases belonged to clade A (Jordan-N3/2012), recently, the newly reported cases were shown to be genetically different, as such, they were named as clade B. Based on molecular, genetic and antigenic studies, it was established that MERS-CoV is unique and different from severe acute respiratory syndrome (SARS) coronavirus and common cold coronaviruses. Because of this fact, earlier MERS-CoV was called "Saudi SARS" (Azhar et al. 2016) . This notorious virus was first identified and isolated in Saudi Arabia by Dr Ali Mohamed Zaki from a patient suffering from pneumonia-like illness, and these isolates exhibited cytopathic effects like syncytia formation. Through molecular clock analysis, zoonotic transmission was established from dromedary camels (Camelus dromedaries), particularly prevailing in Middle East countries.

Both MERS-CoV and SARS coronavirus have similar þssRNA in their genome.

This ssRNA in MERS-CoV encodes structural proteins (SPs), membrane protein (M), envelope (E), nucleocapsid (N), two nonstructural replicase polyproteins (ORF1a and ORF1b) and spike (S) (Reusken et al. 2016) . The two non-structural proteins (NSPs) initiate genomic replication and RNA synthesis. The huge replicase gene surrounds 5 0 proximal side of RNA genome. Translation of ORF1a gives rise to polyprotein 1a (pp1a) and ribosomal frame shifting makes translation of ORF1b to give pp1ab. MERS-CoV uses one or two papain-like protease to release NSPs. It is established that MERS-CoV possesses 16 NSPs (Firth and Brierley, 2015) .

Viral components like NSPs, SPs, M, E, N, S and other proteins, glycoproteins and enzymes play a crucial role in virulence in establishing and exaggerating the disease (Figure 1 ).

MERS-CoV's E protein is responsible for the attachment of virus particles to host cell receptors. S protein is in charge of fusion and entry in to the respiratory epithelia .

Virus-induced proteases help viral particles to spread to lower parts of the lungs, thus enhancing the severity of the infection. It results in inflammation and suppression of anti-viral interferons (IFN). At this point of the infection, there is excessive anti-viral immune responses like interleukins (IL-6, IL-8 and TNF-a), humoral IgG and IgM are inefficient to control the viral spread and replication in the lungs. The very nature of MERS-CoV virus ability to overcome and suppress host immune challenge is due to its þssRNA, viral SPs and NSPs .

MERS-CoV exhibits its degree of pathogenicity based on the host. It elicits maximum pathogenic potential, especially in humans. This is due to the fact that MERS-CoV shows a strong tropism for bronchial non-ciliated epithelia. During this time, it is proved that virus also arrests host bronchial IFN synthesis (Coleman et al. 2015; Mielech et al. 2016) . At this juncture, it should be noted that most of other viruses causing respiratory diseases attack and damage epithelial cilia, including influenza type A. Studies revealed that cellular receptors for MERS-CoV is exopeptidase (angiotensin converting enzyme 2) (Coleman et al. 2015) . Moreover, it was found that neutralization of angiotensin converting enzyme 2 by specific antibodies did not arrest the spread of infection into bronchus and lung alveolus. Extensive studies showed that another functional cellular receptors called dipeptidyl peptidase-4 were also involved in the severity of disease spread in the lungs due to MERS-CoV (de Wit et al. 2015) . Moreover, receptors for dipeptidyl peptidase-4 are also located in nephrons of kidneys and heart. During the acute stage of the MERS-CoV infection, there is severe viremia, leading to the spread of MERS-CoV virus particles in blood stream. At this point, MERS-CoV exhibits the hall mark of infection leading to not only to the damage of lungs but also kidneys and heart, thereby resulting in respiratory, renal and cardiac failure, ultimately leading to coma and death (vanBoheemen et al. 2016) . The severity is worsened by concurrent secondary bacterial infections. Recent research showed that bacterial infections due to Staphylococcus aureus, Group A Streptococcus, Streptococcus pneumoniae and Haemophilus influenzae type b augment the pathogenic potential of MERS-CoV, particularly in humans. These bacteria particularly dwell in the oral cavities, tonsils and pharynx of humans . Another interesting fact is that the reservoir host dromedary camels and other animals do not exhibit the presence of dipeptidyl peptidase-4 receptors in their kidneys and heart. In humans, after the entry of MERS-CoV viral particles in to lung alveoles, alveolar macrophages fail to contain the spread of infection. The strong host cellular immune response and cytokine release lead to inflammation and fluid accumulation in lungs. This will result in the characteristic symptoms of MERS-CoV; high fever, chills, rigors, severe cough, dyspnoea and hypovolemic shock . Many patients exhibited common symptoms of pneumonia, whereas a significant number of patients expressed symptoms of chronic obstructive pulmonary syndrome like disease. Another large number of patients also revealed laryngotracheobronchitis, which is called "croup". The patterns of disease due to MERS-CoV also differed among different age groups. Clinical cases, morbidity and mortality rates due to MERS-CoV infections are more common and high (80%) in the age group of 35e45 years. Astonishingly, the severity of MERS-CoV is lower (40%) in the higher age groups, particularly above 55 years (Lei et al. 2015) .

Epidemiology and mode of spread of MERS-CoV play a crucial role in morbidity and fatality due to these life-threatening viruses. Different ways of transmission of MERS-CoV have been documented (Corman et al. 2015) . They include dromedary camel to human mode, bats to camels, among camels, cattle to man, dogs to humans, cats to man, bats to humans and finally man to man transmission. Human to human spread is very effective and common in close contacts and crowded surroundings. Nosocomial, hospital-borne outbreaks have also been reported in many instances. There have been reported incidences of spread from patients to health care workers (Song et al. 2015) . In MERS-CoV epidemiology, dromedary camels play an important role because these animals act as reservoir host. Dromedary camels also act as "gene mixing vessels". When two different strains of MERS-CoV, from two different sources, infect dromedary camels, these two genetically different MERS-CoV exchange their þssRNA resulting in the development (mutation) of new subtypes of MERS-CoV. The stunning features of these new subtypes of MERS-CoV have new antigens and novel virulence genes. In Saudi Arabia, there were possible cases of human transmission after drinking camel milk. But, there are no cases documented in humans through ingestion of camel meat (de Wit et al. 2016) .

Patients suffering from MERS exhibited low white blood cell count in particular of low lymphocytes. Confirmation and rapid molecular testing can be performed using real-time polymerase chain reaction (RT-PCR) test. RT-PCR testing can be performed on bronchoalveolar lavage, sputum and tracheal aspirates. Serological test like immunofluorescence technique (IFT) is also useful. But, the main issue with IFT is many other serologically related viruses may cross-react with MERS-CoV antibodies. So, IFT is unsuitable for confirmation. World Health Organization recommends RT-PCR for the confirmation of MERS-CoV in clinical samples (B aez-Santos et al. 2014 ).

Because the main transmission is through camels, people should be advised to handle camels with precautions, especially camels with severe nasal discharge. Other general health care measures should be taken to arrest nosocomial hospital-borne MERS-CoV outbreaks. For the treatment of MERS-CoV cases, administration of ribavirin and IFN-a2b was found to be promising, but not very effective. Researchers are recently investigating many ways to overcome the outbreak of MERS-CoV. Many drugs are under evaluation (IFN, chloroquine, chlorpromazine, loperamide, camostat, mycophenolic acid and lopinavir (de Wit et al. 2016) .

MERS-CoV causes severe, life threatening and rapidly spreading Middle East respiratory syndrome not only in Middle East region, but throughout the world. Dromedary camels and bats play an important role in the spread of MERS-CoV. Several nosocomial infections due to MERS-CoV have also been reported worldwide. Several viral SPs and NSPs are critical for the molecular virulence of MERS-CoV. Because of the þssRNA, MERS-CoV can undergo molecular rearrangement leading to antigenic change, genetic variation and mutation. This attributes to the emergence of new and novel subtypes of MERS-CoV in the human populations. These unique subtypes are difficult to control by vaccinations and therapeutic intervention strategies. Another important issue faced by scientists and molecular virologists is related to these new MERS-CoV viral particles which possess enhanced molecular virulence, thus leading to increased pathogenicity and a more rapid spread than their parental strains. Studies revealed that dromedary camels act as "gene mixing vessels" for MERS-CoV. Prevention is mainly based on avoiding contact with dromedary camels and focussing on nosocomial infection outbreaks. Researchers are constantly trying to develop anti-viral drugs to combat MERS-CoV, but till today there is no breakthrough. The perspective of what can we expect in the future of this field is developing effective vaccine, conducting detailed studies on molecular research on mechanism of mutation and genetic recombination occurring in MERS-CoV.

Increase in MERS-CoV cases in Saudi Arabia linked to hospital outbreak with continued circulation of recombinant virus

Evidence for camel-to-human transmission of MERS coronavirus

Catalytic function and substrate specificity of the PLpro domain of nsp3 from the Middle East respiratory syndrome coronavirus (MERS-CoV)

Productive replication of Middle East respiratory syndrome coronavirus in monocyte-derived dendritic cells modulates innate immune response

Wild-type and innate immune-deficient mice are not susceptible to the Middle East respiratory syndrome coronavirus

Characterization of a novel betacoronavirus related to Middle East respiratory syndrome coronavirus in European hedgehogs

Middle East respiratory syndrome coronavirus (MERS-CoV) causes transient lower respiratory tract infection in rhesus macaques

The Middle East respiratory syndrome coronavirus (MERS-CoV) does not replicate in Syrian hamsters

Middle East respiratory syndrome coronavirus in dromedary camels: an outbreak investigation

Assessing activity and inhibition of Middle East respiratory syndrome coronavirus papain-like and 3C-like proteases using luciferase-based biosensors

Delayed induction of proinflammatory cytokines and suppression of innate antiviral response by the novel Middle East respiratory syndrome coronavirus: implications for pathogenesis and treatment

Crystal structure of the papain-like protease of MERS coronavirus reveals unusual, potentially druggable active-site features

Structural and functional characterization of MERS coronavirus papain-like protease

MERS-CoV papain-like protease has deISGylating and deubiquitinating activities

Middle East respiratory syndrome coronavirus (MERS-CoV) RNA and neutralizing antibodies in milk collected according to local customs from dromedary camels

Middle East respiratory syndrome coronavirus spike protein delivered by modified vaccinia virus Ankara efficiently induces virus-neutralizing antibodies

Middle East respiratory syndrome coronavirus (MERS-CoV): update (United Arab Emirates)

There is no conflict of interest.

We would like to acknowledge University Grants Commission (UGC), New Delhi for the financial grant supporting this work through their grant number 112050678/2016.