SARS-CoV, MERS-CoV, and 2019-nCoV viruses: an overview of origin, evolution, and genetic variations.

Coronaviruses are single stranded RNA viruses usually present in bats (reservoir hosts), and are generally lethal, highly transmissible, and pathogenic viruses causing sever morbidity and mortality rates in human. Several animals including civets, camels, etc. have been identified as intermediate hosts enabling effective recombination of these viruses to emerge as new virulent and pathogenic strains. Among the seven known human coronaviruses SARS-CoV, MERS-CoV, and SARS-CoV-2 (2019-nCoV) have evolved as severe pathogenic forms infecting the human respiratory tract. About 8096 cases and 774 deaths were reported worldwide with the SARS-CoV infection during year 2002; 2229 cases and 791 deaths were reported for the MERS-CoV that emerged during 2012. Recently ~ 33,849,737 cases and 1,012,742 deaths (data as on 30 Sep 2020) were reported from the recent evolver SARS-CoV-2 infection. Studies on epidemiology and pathogenicity have shown that the viral spread was potentially caused by the contact route especially through the droplets, aerosols, and contaminated fomites. Genomic studies have confirmed the role of the viral spike protein in virulence and pathogenicity. They target the respiratory tract of the human causing severe progressive pneumonia affecting other organs like central nervous system in case of SARS-CoV, severe renal failure in MERS-CoV, and multi-organ failure in SARS-CoV-2. Herein, with respect to current awareness and role of coronaviruses in global public health, we review the various factors involving the origin, evolution, and transmission including the genetic variations observed, epidemiology, and pathogenicity of the three potential coronaviruses variants SARS-CoV, MERS-CoV, and 2019-nCoV.