Development of duck hepatitis A virus type 1 attenuated vaccine E23-SP80 and its protective efficacy evaluation against DHAV-1 infection in ducks.

Duck Hepatitis A Virus type 1 (DHAV-1) is a highly pathogenic virus that causes severe mortality in ducklings and results in substantial economic losses to the global duck industry. Live-attenuated DHAV vaccine remains one of the most effective strategies for controlling this disease. We developed a safe and effective live attenuated vaccine candidate E23-SP80 adapted to specific-pathogen-free (SPF) chicken embryos by serial passage of a field isolate. The E23-SP80 exhibited an adaptive growth capacity in SPF chicken embryos with a viral titer of 10^7.25 ELD₅₀/0.2 mL and lost its pathogenicity in 2-day-old Cherry Valley ducklings. The vaccine strain maintained its attenuation and showed no virulence reversion after back propagation into 2-day-old ducklings for five rounds. An immunizing dose of only 10^3.0 ELD₅₀ of E23-SP80 could provide 100% protection against challenge with lethal parental DHAV-1 strain. After a single intramuscular vaccination, virus-neutralizing antibody titers exceeded 9 log₂ from 7 to 28 days post-vaccination and the titers were markedly higher than those of a commercial vaccine. Genomic analysis of E23-SP9 and E23-SP80 revealed fifteen amino acid substitutions, most of which were located in VP1 and 2A2 proteins, and the hypervariable region of VP1 (T180I and D193N) might contribute to attenuation. These results suggest that E23-SP80 strain is a promising commercial vaccine candidate for the prevention and control of DHAV-1 infection.