**10. Pathological changes and clinical diagnosis in SARS-COV-2 infection**

Nasal droplets and saliva from infected patients function as the leading route of SARS-CoV-2 virus communicability [59]. According to Heydarloo et al., the virus accesses the alveolar-type 2 cells (AT2 cells) by attaching its viral spike (S1 and S2) proteins to the angiotensin-converting enzyme 2 (ACE2) receptor [60]. Researchers found that previous iterations of coronaviruses specifically SARS-CoV, replicated more aggressively in alveolar-type 2 cells than in alveolar type 1 cells in the lung [38]. This is significant since it has been reported that there is an 80% genetic similarity between the SARS-CoV and SARS-CoV-2 viruses [61]. SARS-CoV-2 has an extraordinary potential for binding with AT2 cells in the lungs as shown via molecular pathways [62].

The SARS-CoV-2 pandemic continues to affect much of the world and understanding its clinical diagnosis is important. Data on diagnostic testing for SARS-CoV-2 is still in its infancy, as such, understanding these tests and interpreting their results is imperative. The most frequently administered and dependable test for SARS-CoV-2 diagnosis thus far, has been the RT-PCR test completed using nasopharyngeal swabs. In some cases, alternative upper respiratory tract samples, comprising throat swabs and/or saliva have been used. Individual companies focus on a variety of RNA genes, with a significant number of tests affecting 1 or more of the envelope, RNA-dependent RNA polymerase (RdRp), and ORF1 genes [63].

In most SARS-CoV-2 patients with symptomatic infection, viral RNA in the nasopharyngeal swab becomes detectable as early as day 1 of symptoms and peaks within the first week of symptom onset. The cycle threshold (Ct) that is used to measure viral RNA, can be defined as "the number of replication cycles required to produce a fluorescent signal, with lower cycle threshold values representing higher viral RNA loads" [63]. A PCR positive is typically clinically reported as a Ct value of less than 40. By week three of infection, there is usually a decline in this positivity and subsequently becomes unnoticeable. In severely ill hospitalized SARS-CoV-2 patients, the cycle threshold values are lower than the cycle threshold values recorded in less severe cases. It is important to note, a "positive" PCR result reveals only the recognition of viral RNA and does not automatically suggest presence of viable virus [62]. It has been reported in a minority of positive test cases that viral RNA was detected by RT-PCR past week six. There have also been instances of a positive result being reported after consecutive negative PCR tests completed two days apart. Currently, it is unclear whether this is a testing error, reinfection, or recurrence.

SARSCoV-2 infection can also be identified indirectly by assessing the patients' immune response to infection. In patients who exhibit mild to moderate symptoms, serological diagnosis becomes extremely important past the first two weeks of illness onset. Serological diagnosis is an essential means of understanding the scope of SARSCoV-2 infection in the community and may assist in identifying individuals who are immune/protected from infection.

### **11. Conclusion**

Wide-ranging efforts to decrease transmission of SARSCoV-2 infection are crucial to controlling the present epidemic. Lessons learned from the SARS-CoV and MERS-CoV outbreaks offer, valuable experiences and insights into how to fight the SARSCoV-2. Specific consideration aimed at decreasing spread must be applied in vulnerable populations specifically health care workers, and the elderly. Additionally, research into the pathogenesis of human coronavirus infection is crucial for finding suitable therapeutic objectives. Presently, no specific antiviral drug is available for SARS-CoV, MERS, and SARSCoV-2.

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