2.2. Natural Host
For both SARS outbreaks, bat was suspected as a natural host for SARS-CoVs. It was claimed that SARS-CoV virus originated from horseshoe bats in a cave of Yunnan Province [22].
In 2005, SARS-like COVs (SL-CoVs) were found in wild Chinese horseshoe bats (Rhinolophus sinicus) collected from a cave in Yunnan Province of China [22]. In 2013, live SL-CoV was isolated from Vero E6 cells incubated in bat feces [23]. The isolated virus showed more than 95% genome sequence identity with human and civet SARS-CoVs. SL-CoV possesses the ability to infiltrate cells using its S protein to combine with ACE2 receptors [24]. This observation indicated that SARS-CoV originated from Chinese horseshoe bats and that SL-CoV isolated from bats poses a potential threat to humans without the involvement of any intermediate hosts. Between 2015 and 2017, 334 bats were collected from Zhoushan city, Zhejiang Province, China. A total of 26.65% of those bats were detected as having a conserved coronaviral protein RNA-dependent RNA polymerase (RdRp). Full genomic analyses of two SL-CoVs (bat-SL-CoV ZC45 and bat-SL-CoV ZXC21) showed 81% nucleotide identity with human/civet SARS CoVs. These viruses reproduced and caused disease in suckling rats, with virus-like particles being observed in the brains of suckling rats by electron microscopy [25].
Thus, prior to 2018, bats collected in some areas of China have been shown to carry CoVs capable of directly infecting humans.
A recent study showed that SARS-CoV-2 has 96% homology at the whole genome level with bat coronavirus. Pairwise protein sequence analysis of seven conserved non-structural proteins showed that this virus belongs to the species of SARS-CoV [26]. In phylogenetic analysis, SARS-CoV and SARS-CoV-2 not only share a common ancestor, but also have an amino acid identity of 82.3% [27,28,29].