AIM: To detect the effects of DNA vaccines in combination with duck IFN-γ gene on the protection of ducks against duck hepatitis B virus (DHBV) infection.METHODS: DuIFN-γ cDNA was cloned and expressed in COS-7 cells, and the antiviral activity of DuIFN-γ was detected and neutralized by specific antibodies. Ducks were vaccinated with DHBpreS/S DNA alone or coimmunized with plasmid expressing DuIFN-γ. DuIFN-γmRNA in peripheral blood mononuclear cells (PBMCs) from immunized ducks was detected by semi-quantitative competitive RT-PCR. Anti-DHBpreS was titrated by enzyme-linked immunosorbent assay (ELISA). DHBV DNA in sera and liver was detected by Southern blot hybridization, after ducks were challenged with high doses of DHBV.RESULTS: DuIFN-γ expressed by COS-7 was able to protect duck fibroblasts against vesicular stomatitis virus (VSV) infection in a dose-dependent fashion, and antiDuIFN-γ antibodies neutralized the antiviral effects. DuIFN-γin the supernatant also inhibited the release of DHBV DNA from LMH-D2 cells. When ducks were co-immunized with DNA vaccine expressing DHBpreS/S and DuIFN-γ gene as an adjuvant, the level of DuIFN-γ mRNA in PBMCs was higher than that in ducks vaccinated with DHBpreS/S DNA alone. However, the titer of anti-DHBpreS elicited by DHBpreS/S DNA alone was higher than that co-immunized with DuIFN-γ gene and DHBpreS/S DNA. After being challenged with DHBV at high doses, the load of DHBV in sera dropped faster, and the amount of total DNA and cccDNA in the liver decreased more significantly in the group of ducks co-immunized with DuIFN-γ gene and DHBpreS/S DNA than in other groups.CONCLUSION: DHBV preS/S DNA vaccine can protect ducks against DHBV infection, DuIFN-γ gene as an immune adjuvant enhances its efficacy.