Pathogenicity of DAstV-SDZZ in ducklings
Three of 20 ducklings inoculated with the virus died between 4 and 6 days post infection (dpi), with most infected ducklings exhibited signs of depression on the 2nd day. As a result, a mortality rate of 15% was achieved (3/20). At necropsy, the liver showed petechial and ecchymosis hemorrhages (Figure 3A). Besides, enlarged kidneys with congestion were variable in dead ducklings (Figure 3B). Significant histologic changes in the kidneys were noted from the infected group, included necrosis and degeneration of renal epithelial cells, accompanied by infiltration of inflammatory cells and severe haemorrhage (Figure 3D). Besides, steatosis and infiltration of inflammatory cells were observed in the livers (Figure 3C). No histological microscopic lesions were seen in the control group.
Discussion
In naturally occurring outbreaks, DVH is mainly caused by DHAV and causes great economic losses (Niu et al., 2019). Several studies have reported the existence of DAstV-1 in commercial duck farms (Fu et al., 2009; Chen et al., 2012). Unfortunately, the pathogenicity of DAstV-1 among ducklings remains unclear due to the lack of efficient in vitro culture techniques. In the present study, we report the isolation and characterization of DAstV-1 strain isolated from ducklings in China, and reproduction experiment of the disease in ducklings with the inoculating virus prepared in LMH cells has been carried out.
The infection study with DAstV-SDZZ isolate were successful reproduced. Although DAstV-1 infection has been detected in several commercial duck flocks. Unfortunately, attempts to isolate the DAstV-1 virus were unsuccessful. To the best of our knowledge, DAstV-SDZZ is the first DAstV-1 strain whose experimental infection study has been conducted. In this study, the isolated astrovirus grew well in LMH cells, the CPE appeared after inoculation within 72–96 hr. Existing studies have shown that some astroviruses can be serially passaged only after being cleaved by trypsin to form mature virions (Bass and Qiu, 2000; Mendez et al., 2007). In this study, DAstV-SDZZ isolate grew well in LMH cells without the presence of trypsin, but it does not rule out that the presence of trypsin can increase the titer of the virus to a certain extent.
In a previous study,DAstV-1 infection caused a mortality rate of about 50% in large groups, and dead duckling displaying the typical haemorrhagic lesions in the liver (Fu et al., 2009). Obviously, the disease described in the present study was distinct from the reports in the literature in terms of signs, lesions, and mortality rate. From field cases and experimental studies, major lesions were demonstrated in the kidneys which are hemorrhage and swellings. This may be due to the mutation of some key sites of the virus has caused the change of pathogenicity and tissue tropism. At the same time, the virus has a stronger adaptability to the host so that it can spread widely.
Previous studies showed that the poly (A) tail of the positive single-stranded RNA virus genome plays a very important role in its translation, replication and transcription (Bergamini et al., 2000; Kean, 2003). In addition, it was shown in another study that the length of the poly(A) tail plays a direct role in viral RNA replication in infected cells since reducing the length of the poly(A) tail resulted in a reduction in RNA infectivity (Silvestri et al., 2006). An important finding of the current study is that the length of the poly(A) tail of DAstV-SDZZ isolate is the largest among duck astroviruses so far sequenced. All of these indicate that a similar relationship may exists between DAstV-1strain infectivity and the length of poly(A) tail, which promotes the widespread of the virus. Further studies are needed to clarify those prediction.
In summary, DAstV-1 infections are more common than previously known in China. Thus, a continued surveillance of DAstV-1 is required to understand the molecular epidemiology of the viruses circulating in China. However, the limitation of our study was that we did not provide scientific evidence for the molecular epidemiology and changes in virus transmission capacity of DAstV-1 in China. Nevertheless, this study provides scientific evidence of the DAstV-1 could be one of the pathogenic factors prevalent in duck flocks in China.