4 DISCUSSION
So far, seven different human CoVs, including SARS-CoV, MERS-CoV, human coronavirus NL63 (HCoV-NL63), HCoV-229E, HCoV-OC43, HCoV-HKU1 and SARS-CoV-2, have been identified. Bat was deemed to be the natural host for SARS-CoV, MERS-CoV, HCoV-NL63, HCoV-229E, and rodents may the natural host for HCoV-OC43 and HCoV-HKU1 (Khan et al., 2020). The intermediate hosts for SARS-CoV, MERS-CoV, HCoV-229E, and HCoV-OC43 were found to be palm civets, dromedary camels, alpacas and cattle, respectively. However, the intermediate hosts of SARS-CoV-2 remain unknown.
Since SARS-CoV-2 is genetically close to SARS-CoV, it has been proposed that bats could be the natural host (Phan 2020). Recent study shows that the common ancestor of SARS-CoV-2 and SARS-CoV is similar to bat coronavirus HKU9-1 (Zhou et al., 2020). Pangolins was suspected to be direct animal source of SARS-CoV-2 for humans since the SARS-CoV-2 related CoVs were isolated from Malayan pangolins which shared 97.4% similarity with SARS-CoV-2 in virus receptor binding domain in S gene (Zhang et al., 2020). To date, findings from experimental infection studies suggest that poultry and pigs are not susceptible to SARS-CoV-2 infection (Shi et al., 2020). Of the animal species investigated, cats are the most susceptible species for SARS-CoV-2, which can transmit between cats via respiratory droplets (Jiang et al., 2012). In the laboratory setting ferrets were susceptible to infection. The susceptibility of minks was documented by a report from the Netherlands on an outbreak of SARS-CoV-2 infection in farmed minks (Oreshkova et al., 2020). Golden Syrian hamsters, as well as cynomolgus and rhesus macaques can be consistently infected with SARS-CoV-2 and may show clinical signs. Dogs appear to be susceptible to infection but appear to be less affected than ferrets or cats. Both virological and serological testing found evidence for natural SARS-CoV-2 infection in two dogs from households with human cases of COVID-19 in Hong Kong (Sit et al., 2020).
Our data showed that all samples collected from 14 provinces of China (including Hubei) from domestic poultry in 2019 were negative for SARS-CoV-2. Besides, Shi et al . found that poultry such as chickens and ducks were not susceptible to SARS-CoV-2 (Shi et al., 2020), which was consistent with our results.
In addition, we conducted a large-scale surveillance of avian CoV for the 3160 samples using a conserved RT-PCR assay. The conserved RT-PCR assay in this study detected avian CoVs, including IBVs, DuCoVs and PiCoVs. Our results demonstrated that IBVs, DuCoVs, and PiCoVs belonged to distinct lineages, even though they all belonged to theGammacoronavirus , with the emerging SARS-CoV-2 belongs to theBetacoronavirus of Coronaviridae. SARS-CoV-2 showed higher genetic distance compared with the avian CoV.
Our results also suggest that IBV was detected in all provinces investigated in this study, with DuCoV and PiCoV were detected in most provinces. IBV is harmful to the poultry industry and were listed as notifiable disease by OIE, but the pathogenicity of DuCoV and PiCoV to poultry is still unclear, which needs further research. Besides, the positive rates of IBVs, DuCoVs, and PiCoVs were 15.35%, 2.28% and 1.14%, respectively. And these CoVs are of high prevalence in LPMs, and meanwhile also suggest high prevalence in slaughterhouses. Therefore, each links from breeding, marketing to slaughter (poultry farms, LPMs, and slaughterhouses) may likely play an important role in the circulation of CoVs in poultry, as they do in the circulation of AIVs(Jiang et al., 2012).
In conclusion, the possibility of SARS-CoV-2 originating from the known avian-origin CoVs can be preliminarily ruled out according to the above analysis. Moreover, continuous surveillance on animal-origin coronaviruses should be enhanced for better understanding the diversity, distribution, cross-species transmission and clinical significance of CoVs in nature.