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.