Rescue of a duck circovirus from an infectious DNA clone in ducklings

Duck circovirus may predispose the host to immunosuppression and may serve as an immunological trigger for further complicated disease progression. Due to the lack of a cell culture system for propagating DuCV, little is known regarding the molecular biology and pathogenesis of DuCV. The aim of this study was to describe the construction and initial in vivo characterization of full-length DNA clones of DuCV (pIC-Mu2DuCV) and its infectivity under in vivo conditions.

The constructed pIC-Mu2DuCV contained two copies of the whole DuCV genome and an introduced Xho I restriction enzyme site. Eighty-one 10-day-old conventional ducklings that were free of DuCV were randomly divided equally into three groups (1, 2 and 3). The ducklings in groups 1, 2 and 3 were inoculated intramuscularly with pIC-Mu2DuCV, wild-type virus GH01 and PBS, respectively. Subsequently, all of the ducklings were examined clinically, which were each given a physical condition score, and their rectal temperatures were taken daily during the experimental period. DuCV genomes in serum samples and in various tissues from all of the ducklings at 0, 1, 3, 5, 7, 10, 15, 21 and 28 DPC were detected by PCR and real-time quantitative PCR, respectively.

The average daily weight gain (ADWG) of group 3 was significantly higher than those of groups 1 and 2, and the temperature of all ducklings was stable between 41.7 °C and 42.2 °C. The clinical values (physical condition scores) of groups 1, 2 and 3 were 12.5, 15.6 and 0, respectively. In addition, viremia occurred at 15 and 10 days post-challenge (DPC) in groups 1 and 2, and antibodies could be detected in these ducklings at 21 and 15 DPC. Proliferation ability analysis showed that the viral titers of group 1 were lower than those of their parental viruses in group 2.

This study shows that the rescued viruses are not significantly different but exhibit lower pathogenicity and proliferation ability compared with the parental virus. The results will facilitate future studies on DuCV pathogenesis and biology.