Primary reservoirs and hosts of coronaviruses
The source of origination and transmission are important to be determined in order to develop preventive strategies to contain the infection. In the case of SARS-CoV, the researchers initially focused on raccoon dogs and palm civets as a key reservoir of infection. However, only the samples isolated from the civets at the food market showed positive results for viral RNA detection, suggesting that the civet palm might be secondary hosts [15]. In 2001 the samples were isolated from the healthy persons of Hongkong and the molecular assessment showed 2.5% frequency rate of anti-bodies against SARS-coronavirus. These indications suggested that SARS-coronavirus may be circulating in humans before causing the outbreak in 2003 [16]. Later on, Rhinolophus bats were also found to have anti-SARS-CoV antibodies suggesting the bats as a source of viral replication [17]. The Middle East respiratory syndrome (MERS) coronavirus first emerged in 2012 in Saudi Arabia [9]. MERS-coronavirus also pertains to beta-coronavirus and having camels as a zoonotic source or primary host [18]. In a recent study, MERS-coronavirus was also detected in Pipistrellus and Perimyotis bats [19], proffering that bats are the key host and transmitting medium of the virus [20], [21]. Initially, a group of researchers suggested snakes be the possible host, however, after genomic similarity findings of novel coronavirus with SARS-like bat viruses supported the statement that not snakes but only bats could be the key reservoirs (Table 1 ) [22], [23]. Further analysis of homologous recombination revealed that receptor binding spike glycoprotein of novel coronavirus is developed from a SARS-CoV (CoVZXC21 or CoVZC45) and a yet unknown Beta-CoV [24]. Nonetheless, to eradicate the virus, more work is required to be done in the aspects of the identification of the intermediate zoonotic source that caused the transmission of the virus to humans.
Table 1 Comparative analysis of biological features of SARS-CoV and SARS-CoV-2.
Features SARS-CoV SARS-CoV-2 Reference
Emergence date November 2002 December 2019 [37], [79], [80], [81]
Area of emergence Guangdong, China Wuhan, China
Date of fully controlled July 2003 Not controlled yet
Key hosts Bat, palm civets and Raccon dogs Bat [22], [82], [83]
Number of countries infected 26 109 [84]
Entry receptor in humans ACE2 receptor ACE2 receptor [22], [55], [85]
Sign and symptoms fever, malaise, myalgia, headache, diarrhoea, shivering, cough and shortness of breath Cough, fever and shortness of breath [12], [23], [85]
Disease caused SARS, ARDS SARS, COVID-19 [85], [86]
Total infected patients 8098 123882 [84]
Total recovered patients 7322 67051
Total died patients 776 (9.6% mortality rate) 4473 (3.61% mortality rate)