An update on Middle East respiratory syndrome: 2 years later

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) was first recognized in 2012 and since then has resulted in cases in 23 countries in four continents. The majority of these cases were reported from the Kingdom of Saudi Arabia. The disease caused a spectrum of illness, from asymptomatic to severe and possibly fatal disease. Recent studies showed that the transmission of MERS-CoV among family contacts remains relatively low. Currently, there are no approved vaccines or therapeutics for MERS-CoV.

Keywords:

• coronavirus
• MERS-CoV
• Middle East respiratory syndrome

The first Middle East respiratory syndrome coronavirus (MERS-CoV) case was a 60-year-old Saudi businessman, previously healthy, non-smoker. He was admitted on 10 June 2012 to a local hospital in Bisha, Kingdom of Saudi Arabia (KSA) with 1-week history of cough, fever and shortness of breath [1,2]. Community-acquired pneumonia was confirmed on admission. He subsequently left that hospital and was admitted to a hospital in Jeddah on 13 June 2012. His course was significant for the development of adult respiratory distress syndrome on day 2 and acute renal injury requiring hemodialysis on day 3. The patient had rapidly deteriorating clinical course resulting in death on 24 June 2012 [1,2]. A new β coronavirus was isolated from the patient at the Department of Viroscience of the Erasmus Medical Centre in Rotterdam, the Netherlands, 3 months later; the virus was first called human coronavirus Erasmus Medical Center virus [3] then renamed MERS-CoV by the Coronavirus Study Group of the International Committee on Taxonomy of Viruses [4]. Since the initial case of MERS-CoV infection, a total of 909 laboratory-confirmed cases of infection with MERS-CoV, including 331 (36.4%) deaths were reported to the WHO [5].

Epidemiology

Globally, as of November 2014, the virus has been reported in 23 countries in four continents. The number of countries reporting MERS-CoV in the Middle East is 10 (Saudi Arabia, United Arab Emirates, Qatar, Jordan, Oman, Kuwait, Egypt, Yemen, Lebanon, Iran), eight countries in Europe (Turkey, Austria, UK, Germany, France, Italy, Greece, The Netherlands) and other countries including Tunisia, Algeria, Malaysia, the Philippines and the USA (Table 1) [6]. In the KSA, from 1 June 2012 to 18 September 2014, there were a total of 748 MERS cases reported [7]. All cases outside the Middle East were imported following travel to the Middle East. Timeline of the major event in MERS-CoV infection is shown in Figure 1. The majority of these cases were among males (61% males vs 39% among females) and 27% were between 46 and 60 years of age (Figure 2). The male to female ratio was initially 2.8:1 to 3:1 [8–10]. The change in the ratio is likely related to the initial involvement of hemodialysis unit with predominance of males. In 2014, a large cluster of cases occurred in Jeddah, KSA. Between February and 26 April 2014, there were a total of 128 MERS-CoV patients in Jeddah treated in 14 hospitals. One hospital had 45 cases and other hospitals had fewer patients [11]. Screening of contacts showed that 7 of 554 household contacts (1.3%) were positive by polymerase chain reaction for MERS-CoV [11]. The outbreak was caused by biologically unchanged viruses in connection with nosocomial transmission [12]. The mortality rate was higher during the Al-Hasa outbreak in 2012 than the latest outbreak in Jeddah (65 vs 40%) [9,10,6]. MERS-CoV has three patterns of transmission: sporadic community cases with presumed non-human exposure [13], family clusters resulting from contact with infected family index case [14–17] and healthcare acquired infections between patients and from patients to healthcare workers [9,12,18–23]. Thus, any investigation of MERS-CoV cases should aim to investigate those contacts in the community and the healthcare setting. The first MERS-CoV case was that of a 60-year-old Saudi businessman who was admitted to a local hospital in Bisha and then he traveled to Jeddah [2]. An extensive contact investigation of this case included 53 contacts: household contacts (3 wives, 10 sons, 11 daughters, 12 grandchildren and a housemaid), healthcare worker contacts (11 nurses and 3 physicians) and 2 shepherds, as well as investigation of the camels (n = 5) that he owned (Figure 3). All obtained samples were negative for MERS-CoV by PCR.

Table 1. Total number of cases in countries reporting Middle East respiratory syndrome coronavirus in 2014.

Country	Number of cases	Date of most recent report
Saudi Arabia	798	11/7/2014
Qatar	9	10/12/2014
Turkey	1	10/6/2014
Austria	1	9/29/2014
Iran	5	6/25/2014
United Arab Emirates	73	6/11/2014
Algeria	2	5/24/2014
Jordan	18	5/23/2014
The Netherlands	2	5/5/2014
United States of America	2	5/1/2014
Egypt	1	4/22/2014
The Philippines	1	4/11/2014
Greece	1	4/8/2014
Malaysia	1	4/8/2014
Yemen	1	3/14/2014

Figure 1. Timeline of major MERS Events from April 2012 to May 2014.

Figure 2. Number (and percent) of MERS-CoV cases per age group as reported from the Kingdom of Saudi Arabia from 1 June 2012 to 18 September 2014.

Figure 3. Investigation of MERS-CoV cases has multiple lines of investigation of the contacts of index case and includes contacts in the community, healthcare setting and household contact. Healthcare facility (HCF) investigation includes the investigation of all exposed healthcare workers and other exposed patients. Community investigation included the investigation of family contacts and the evaluation of any animal exposure, such as camels. Household investigation includes the investigation of all family members irrespective of the age group. The figure below shows the extent of the investigation of the first MERS-CoV case that originated in Bisha that included 53 contacts: household contacts (3 wives, 10 sons, 11 daughters, 12 grandchildren, and a housemaid), healthcare worker contacts (11 nurses and 3 physicians), and 2 shepherds, as well as investigation of the camels (N = 5) that he owned.

Clinical features & diagnosis

MERS-CoV infection causes a spectrum of illness ranging from asymptomatic to mild disease to severe and fatal disease. A full review of the clinical picture of MERS was recently published [8,24–26]. MERS-COV causes two distinct forms of disease, severe bilateral pneumonia with multi-organ failure and mild/asymptomatic disease. Severe disease is usually seen in primary index cases, immunocompromised and people with underlying comorbidities and older patients. However, few severe cases were seen in healthy and young patients as well [27]. Mild or asymptomatic disease is seen in secondary cases, usually in young patients, and in the previously healthy individuals. Table 2 shows the outcome of index cases in 14 different clusters. The all primary cases were males, with a mean age of 54 (range, 24–83) years. There were a total of 64 secondary cases.

Table 2. The outcome of index cases in different clusters.

Cluster	Location	Country	1ry case age	1ry case gender	1ry case/ outcome	Additional 1ry cases	No. 2ry cases
1		Jordan	24	M	Fatal		3
2	Riyadh	KSA	70	M	Fatal		2
3		UK	60	M	Fatal		2
4	Riyadh	KSA	39	M	Fatal		1
5	Al-Ahsa	KSA	56	M	Fatal		23^†
6		France	64	M	Fatal		1
7		Tunisia	66	M	Fatal		2
8		Italy	45	M	Recovered		0^‡
9	Taif	KSA	45	M	Fatal	68 F (Fatal)	3
10	Asir	KSA	66	M	Fatal	31 M (Recovered)	4
11	Abu Dhabi	UAE	83	M	Fatal		4
12	Medina	KSA	55	M	Fatal		4
13	Riyadh	KSA	53	M	Fatal	41F (Fatal)	7
14	Hafr al Batin	KSA	38	M	Fatal		8

^†Index case in the cluster was disputed.

^‡Two probable cases.

1ry: Primary; 2ry: Secondary.

The initial healthcare associated cluster in Al-Hasa was identified based on severe cases and a high mortality rate of 65% [9]. Subsequent analysis of 47 cases also showed a high mortality rate [10]. Asymptomatic contacts were not included initially in the laboratory screening, but were included later on, and this change in screening resulted in 29% of cases to be asymptomatic patients. The case fatality rate decreased from 65 to 30% [8–10,28–32], as more asymptomatic cases were included in the screening protocols. This increase in percentage is related to the change in the laboratory screening of patients with severe MERS initially to include asymptomatic contacts as well. Initial MERS-CoV cases were reported in the hemodialysis patients [9]. The presence of acute renal injury was documented in 12 patients [33]. Acute renal injury occurred after a median time to onset of 11 days [33]. The excretion of MERS-CoV in urine may be a source of transmission, especially in those with unrecognizable exposure. The diagnosis of MERS-CoV infection relies on the identification of MERS-CoV by PCR. MERS-CoV PCR has been standardized and works extremely well with respiratory samples in experienced laboratories. If MERS-CoV infection is suspected and initial testing is negative, then repeat testing is indicated. The performance of the test is best with lower respiratory tract samples than upper respiratory specimens [34]. There are multiple serology kits being developed and validated with good results [35]. Serologic evaluation of 130 blood donors sampled in 2012 in Jeddah and 226 slaughterhouse workers in October 2012 (after the virus was identified) in Jeddah and Makkah, KSA, showed only eight reactive serology against recombinant spike proteins of human coronaviruses other than MERS-CoV [36]. In another study, none of 268 tested samples, May 2010–2011 and December 2012, was positive for neutralizing MERS-CoV antibodies [37]. Another study from KSA in 2013 showed that seroprevalence among household contacts of MERS-CoV infected patients was 4% of 280 contacts [38].

Genomic analysis

Genomic analysis of the MERS-CoV from patients with known dates and locations helps answer specific questions: how fast the virus changes, when the virus began circulating in its current form, the virus adaptation to humans and those geographical patterns may help locate an animal source. Two papers from KSA reported more than 60 genome sequences. The first report described 21 sequences combined with other nine published MERS-CoV genomes [18]. Phylogenetic analysis showed that there are three distinct MERS-CoV genotypes in Riyadh and the zoonotic reservoir was geographically dispersed. It was also shown that Al-Hasa cluster [39] and the community outbreak in Hafr-Al-Batin [16] was caused by more than one introduction of the virus. The second paper estimated the rate of MERS-CoV evolution to be 1.12 × 10^–3 substitutions/site per year (95% CI of 8.76 × 10^–4 to 1.37 × 10^–3) [39]. The time of the Most Recent Common Ancestor was estimated to be March 2012 (95% CI, December 2011; June 2012) [39]. In another estimate, it is thought Most Recent Common Ancestor is the end of 2010 [40]. Phylogenetic analysis showed that the source is geographically dispersed [18,39].

Animal (putative) reservoirs & risk factors for zoonotic transmission

The analysis of an African bat virus related to MERS-CoV showed evidence that this virus was the ancestor of camel MERS-CoV and that camels may act as a mixing vessel for MERS-CoV [41]. The contribution of camels to the epidemiology of MERS-CoV was recently reviewed [8,24–26]. Of all the studies, between 0 and 100% of tested camel sera (depending on country of origin) were positive for MERS-CoV antibodies [8,24–26] depending on the country of the study [42–52]. MERS-CoV was also isolated from camels by PCR in few instances [44,53,54]. Experimental infection of three camels with human isolates of MERS-CoV was recently published [55]. The infection caused transient upper respiratory tract infection and camels shed viable MERS-CoV in large quantities up to 7 days and viral PCR was positive for 35 days [55]. Juvenile camels had a lower seroprevalence of MERS-CoV antibodies than adult (>2 years) camels, indicating that camels get infection later in life [8,24–26]. Since not all primary cases have a link to camels, another source of infection may yet be revealed. In a recent report of 70 human MERS-CoV infections, only 1 (1.7%) had contacts with camels [30]. In a previous summary of 161 cases, 49 (30.4%) had contact with animals, 7 (4.3%) had contact with camels and another 7 (4.3%) had contact with sheep [29]. Recent reports showed that a patient was in contact with a diseased camel [56], and one patient treated a sick camel and that two MERS-CoV from the patient and the camel were identical [53,54].

Transmission of MERS-CoV among family contacts

The transmission of MERS-CoV among family contacts seems to be limited. Initial data suggested that the transmission of MERS-CoV among family members was 11–19% based on screening of a small number of contacts of 28–42 family members [14–16]. Screening of 462 family members revealed a positivity rate of only 3% by PCR [57]. Further screening of household contacts utilizing both PCR and serology revealed a positivity of 12 out of 280 contacts, showing a positivity rate of only 4% [38].

Transmission of MERS-CoV within the healthcare facility

In KSA, from 1 June 2012 to 18 September 2014, a total of 748 MERS cases were reported, and 27% were among healthcare workers [7]. Studies of the transmission of MERS-CoV in healthcare setting revealed a low transmission rate. The investigation of healthcare worker contacts of the first MERS case in KSA showed that none of the 48 case-contact and 48 non case-contacts was positive for MERS-CoV by HKU5.2N nucleocapsid enzyme immunoassay [58]. Upper respiratory symptoms developed among 27% of case-contact compared to 33% of non-case-contacts [58]. Of the 48 case-contact, 87.5% had surgical mask and 33% had N-95 mask [58]. Contact investigation of healthcare workers (HCWs) of MERS-CoV patients also revealed no transmission in two other studies [59,60]. Transmission of MERS-CoV to HCWs was confirmed in other studies [9]. In an investigation of >200 HCW, two laboratory-confirmed cases were identified [9].

The retrospectively identified Jordanian outbreak took place in April 2012 and involved 13 people (including 10 healthcare workers) [21]. The investigation revealed two laboratory-confirmed and 11 probable cases [21]. Further serologic investigation of this outbreak showed that 7 of 124 (6%) contacts were positive [20]. Of the exposed healthcare workers, 6 (10%) were positive and none recalled developing respiratory symptoms [20]. During Al-Hasa outbreak, between 1 April and 23 May 2013, a total of 23 cases with MERS-CoV were identified, and all confirmed cases and 11 probable cases were part of a single cluster involving four healthcare facilities [9]. The most recent outbreak in Jeddah (February–April 2014) involved 128 MERS-CoV patients in 14 hospitals [11], where the infection was amplified in the healthcare setting [12,61].

Controversies exist regarding the best infection control measures for MERS-CoV. WHO advocates contact and droplet precautions [62], whereas the US and the European CDC calls for additional airborne infection isolation precautions [63,64]. This recommendation is also favored by WHO when dealing with aerosol-generating procedures [62]. The Al-Hasa outbreak involved a hemodialysis unit, and the outbreak was controlled with the use of contact and droplet precautions [9].

Molecular mechanism by which MERS-CoV enters target cells

A number of cell lines are susceptible to MERS-CoV and include human-derived Calu-3, HFL, Caco-2, Huh-7, HEK and His-1 cell lines [65]. Dipeptidyl peptidase 4 was found to be a critical factor for MERS-CoV entry into cells [66]. It seems that MERS-CoV prefers human dipeptidyl peptidase 4 over bat dipeptidyl peptidase 4 [67]. In addition, cellular proteases such as Type II transmembrane serine protease and members of the cathepsin family are believed to be activators of the viral spike (S) glycoprotein [68]. MERS-CoV entry into host cells depends on the viral spike protein and MERS-CoV spike protein is a substrate for proprotein convertases [69]. MERS-CoV spike protein site is subjected to proteolytic process by furin during protein biosynthesis, [70],. The use of the viral S protein allows MERS-CoV to enter cells within endosomes or at the plasma membrane depending on cell type [71]. Camostat, a serine protease inhibitor, is able to block MERS-CoV cell entry in vitro [72]. Furthermore, inhibition of MERS-CoV is blocked by interferon-inducible transmembrane proteins [73].

Currently available antiviral agents & vaccines

Currently, there are no proven therapies for MERS-CoV infection. Useful antiviral agents for (MERS-CoV) infection include neutralizing antibody, convalescent plasma, polyclonal human immunoglobulin, Equine F(ab’)2 antibody fragments, anti-S monoclonal antibodies and interferons [74]. Other medications could be repurposed for use against MERS-CoV, including ribavirin, protease inhibitors (lopinavir, nelfinavir), cyclophilin inhibitors (cyclosporine, alisporivir), chloroquine, mycophenolic acid, nitazoxanide, recombinant human mannose-binding lectin and siRNA to key MERS-CoV genes [74]. Approved antiviral agents can be repurposed for use against emerging viral infections to shorten the time from virus discovery to treatment availability [75]. The use of interferon and ribavirin was tried in five patients with MERS-CoV infection. The median time from admission to therapy with these two agents was 19 days, and thus any benefit effect might have not been evident [76]. A larger number of 20 patients received ribavirin and interferon treatment at a median of 3 days [77]. The therapy resulted in improved 14-day survival of 70% (14 of 20 patients) in the treatment group, compared to 29% (7 of 24 of patients) in the comparison group (p = 0·004) with no survival advantage at 28 days (30 vs 17%, p = 0·54) [77].

In an experimental model of MERS-CoV infection in mice, a recombinant receptor-binding domain (rRBD) of MERS-CoV spike (S) glycoprotein in association with Fc of human IgG (RBD-Fc) induced neutralizing antibodies [78,79]. Intranasal and subcutaneous MERS-CoV RBD-Fc vaccination resulted in similar systemic humoral immune responses [80]. A higher systemic cellular immune response and local mucosal immune responses were achieved with intranasal vaccination [80]. Other studies showed similar findings of antigenic property of a rRBD protein of MERS-CoV spike (S) [78,81–83]. Moreover, three adjuvants–alum, IFA, CpG and poly (I:C)–in rRBD subunit vaccines were effective in producing RBD-specific cellular and humoral immune responses [84]. Since the N-terminus RBD of (S) glycoprotein is conserved in currently circulating MERS-CoV strains, this terminus is considered a possible vaccine candidate [85].

Unanswered questions & future directions

Person-to-person transmission of MERS-CoV is definite. However, the exact route of transmission needs to be better defined as it is still not clear. What contribution asymptomatic patients have to the transmission cycle of infection and how long patients remain infectious still needs to be defined? In an evaluation of the pattern of transmission of MERS-CoV, it was speculated that the current epidemiology of the disease may be secondary to human to human transmission with a large proportion of undetected cases [59]. Since screening of a large number of household contacts revealed relatively low rate of positive cases [36], the contribution of those cases to the transmission might have been overlooked [86]. In an analysis of the viral shedding of MERS-CoV, about one-third of the contacts shed the virus for 30 days [87]. Further studies and research are needed to develop effective vaccines for MERS-CoV infection.

Expert commentary & five-year view

In the following months or years, it is important to understand key elements in the transmission of MERS-CoV and therapy. It is also important to identify the exact source and the role of camels in the transmission. The exact means of transmission from camels to humans needs to be elucidated. The contribution of any intermediate hosts should be further studied. The factors associated with the risk of transmission from animal to humans are essential to understand to further prevent the disease. It is essential to comprehend what happened that led to camel–human transmission. Understanding the risk factors for such transmission within the community would further enhance the ability to prevent animal to human transmission of MERS-CoV. The factors leading to transmissions within healthcare facilities should be further delineated. The best strategies to prevent the transmission within healthcare facilities are of paramount importance to prevent the transmission and amplification of the infection in these settings. Enhancing preventive measures within healthcare facilities would contribute to the understanding of the contribution of asymptomatic individuals in the propagation of the disease. The development of effective vaccines and therapies is of paramount importance to further limit the disease. Using new or repurposed medications for the therapy and potentially as a prophylaxis is highly welcomed to curtail the spread of disease. Identifying the risk factors for progression to severe disease and those factors important to render infection short will be facilitated by elucidating the pathogenesis of MERS-CoV infection.

Key issues

• Middle East respiratory syndrome coronavirus (MERS-CoV) continues to cause disease with amplification in healthcare setting.

• The best effective therapy for MERS remains unknown.

• There are three main patterns of transmission of MERS-CoV.

• Phylogeographic analyses suggested that the zoonotic reservoir was geographically disperse.

• The recognition of asymptomatic cases decreased the case fatality rate.

• Transmission of MERS-CoV among household contacts seems to be low.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Notes