Experimental infection study
The aim of the study was conducted to determine the pathogenicity of the isolate. The specific pathogen free (SPF) duck embryos were obtained from a hatchery (Harbin Veterinary Research Institute). Newly hatched SPF ducklings were moved in SPF negative pressured isolators. Twenty 1-day-old SPF ducklings were infected by subcutaneous injection with 0.2 ml of the isolated virus (DAstV-SDZZ strain; 5×104.8TCID50/0.1ml) as the experimental group, while the other ducklings (n = 20) were inoculated with sterile phosphate buffered saline (0.2 mL) in the same manner as the control group. Clinical signs and mortality of ducklings were monitored daily. Survival birds were euthanized at 15 days after inoculation. Dead birds were necropsied immediately for histopathological observation and virus identification. All ducklings were bred and cared for in accordance with humane procedures.