**7. Pathological changes and clinical diagnosis in MERS-COV infection**

The understanding of the pathological findings related to MERS-CoV infection have relied on a paucity of autopsy cases. Notwithstanding the limited number of autopsy cases, several studies have assessed the pathological features of MERS-CoV infection in human tissue. The pathogenesis of MERS-CoV infection in human tissue ex vivo revealed exudative diffuse alveolar damage (DAD) with hyaline membranes, interstitial pneumonia (which was primarily lymphocytic), pulmonary edema, multinucleate syncytial cells, and type II pneumocyte hyperplasia [38]. Researchers also observed bronchial submucosal gland necrosis in diseased lung tissue, where these bronchial lesions make up the pathologic origin for respiratory failure and radiologic anomalies of MERS-CoV infection [38]. Some of the cells in the lungs targeted by the MERS-CoV infection include: pneumocytes, multinucleated epithelial cells, and bronchial submucosal gland cells [39]. Microstructurally, viral particles were discovered in the pulmonary macrophages, pneumocytes, renal proximal tubular epithelial cells and macrophages infiltrating the skeletal muscles [38, 40]. Consistent with the microstructural results in the kidney, renal biopsies revealed acute tubulointerstitial nephritis and acute tubular sclerosis with proteinaceous cast formation [40].

Researchers discovered comparable replication of kinetics and cellular tropism in a study comparing the replication of camel-isolated MERS-CoV strains to human-isolated MERS-CoV strains. Non-ciliated bronchial epithelium and alveolar epithelial cells including type II pneumocytes were infected by all strains. It is important to note that no infection of the pulmonary macrophages was present [39]. Infection of several cell types including vascular endothelial cells, renal tubular cells, and podocytes was established in studies examining kidney explants [41]. Exploratory infection of small intestine tissue samples with MERS-CoV confirmed that infection was restricted to the surface enterocytes and formation of syncytial cells [6]. It has been observed that infected patients shed virus in their urine and stool, which is consistent with these findings.

RT-PCR has functioned as the main clinical laboratory diagnostic test throughout transmission events. Critical to the success of these tests is an understanding of the viral kinetics and tissue tropism discovered in MERS-CoV cases. Numerous studies have acknowledged that lower respiratory tract samples contain the highest viral loads, while upper respiratory swabs, whole blood or serum, feces, and urine may also contain significant viral load [35]. Samples from the upper respiratory tract, urine and blood may offer further diagnostic usefulness by delivering a convenient sample type, notwithstanding 10 to 100 times lower virus levels. Measurable viremia at the point of diagnosis has been linked with an increase in patient death due to the necessity for mechanical ventilation, despite blood only being positive in approximately one-half to one-third of cases [42]. The reduced viremia rate in MERS-CoV samples in comparison to SARS-CoV is significantly different, where RT-PCR on blood can be beneficial for preliminary diagnosis and is normally the primary positive site identified. Analyses of upper and lower respiratory samples as well as blood samples for MERS-CoV patients, has shown that it may benefit in maximizing the sensitivity while also stratifying risk [34]. Two RT-PCR testing approaches were approved for emergency use authorization by the FDA during the MERS-CoV outbreak: both targeted a region upstream of the envelope gene (principal target of the humoral immune response). Of these two tests, one additionally targets a specific region of the ORF1a gene, while the other targets two regions inside the nucleocapsid gene [6].

MERS-CoV serology tests share comparable kinetics to that of SARS-CoV infections. About 2–3 weeks following the onset of symptoms, a significant number of patients develop measurable levels of IgM and IgG antibodies. However, in many cases the detection of IgG has superior diagnostic value when compared to IgM [34]. Some researchers posit that if serologic testing is used to detect current infection, "a neutralization assay and 4-fold increase in titer after 14 days should be used to confirm a specific immune response" [6, 42]. Disease severity may affect antibody responses as numerous studies have established; PCR-positive patients exhibiting only mild disease symptoms often do not generate measurable quantities of antibodies, especially when monitored during the post-acute phase of disease [34].

## **8. SARS-CoV-2**

#### **8.1 Etiology, epidemiology, and clinical presentation**

The coronavirus (SARS-CoV-2) (also known as the novel coronavirus) outbreak has reached pandemic proportions with a large global footprint [43, 44]. In late December 2019, SARS-CoV-2 was first reported in Wuhan, Hubei Province, China among clusters of patients with pneumonia of unknown etiology [43, 44]. In early

*Severe Acute Respiratory Syndromes and Coronaviruses (SARS-CoV, MERS-CoV… DOI: http://dx.doi.org/10.5772/intechopen.97564*

January 2020, the National Health Commission of People's Republic of China released information regarding the causative agent of an enigmatic pneumonia identified as a novel coronavirus (SARS-CoV-2). The novel coronavirus (SARS-CoV-2) was verified by several independent laboratories located in China [45, 46]. The World Health Organization (WHO) provisionally named the causative virus as 2019 novel coronavirus [2019-nCoV/SARS-CoV-2] [46]. Coronaviruses are known to cause respiratory, hepatic, and neurologic diseases and are generally spread among humans and animals [3]. The SARS-Cov-2 virus is illustrated by a spherical shape, and a characteristic "crown" appearance, and they belong to the family of coronaviruses of positive-stranded RNA viruses [47].

Genetically, SARS-CoV-2 has a closer resemblance to SARS-CoV than the Middle East respiratory syndrome coronavirus [MERS-CoV] [48]. Nevertheless, the span of the incubation period, clinical severity, and transmissibility of SARS-CoV-2 differs from SARS-CoV [49]. Public health and government efforts aimed at curbing the spread by implementing social practices through social distancing, mask wearing, isolating/quarantining and non-pharmacological and preventive treatments for psychophysical wellbeing, has been relatively successful in part, but SARS-CoV-2 has continued to increase globally [50, 51]. By the end of January 2021, SARS-CoV-2 accounted for more than two million deaths and more than 100 million confirmed cases of the disease [52]. Radiologically, SARS-CoV-2 has distinctive imaging features that constitute a visual identity. Besides, SARS-CoV-2 negatively impacts other organs in addition to the lungs. As a result of these developments, SARS-CoV-2 has grown exponentially with nearly 2000 articles being published per week [50].
