Genetic Variation of the VP1 Gene of the Virulent Duck Hepatitis A Virus Type 1 (DHAV-1) Isolates in Shandong Province of China

To investigate the relationship of the variation of virulence and the external capsid proteins of the pandemic duck hepatitis A virus type 1 (DHAV-1) isolates, the virulence, cross neutralization assays and the complete sequence of the virion protein 1 (VP1) gene of nine virulent DHAV-1 strains, which were isolated from infected ducklings with clinical symptoms in Shandong province of China in 2007–2008, were tested. The fifth generation duck embryo allantoic liquids of the 9 isolates were tested on 12-day-old duck embryos and on 7-day-old ducklings for the median embryonal lethal doses (ELD50s) and the median lethal doses (LD50s), respectively. The results showed that the ELD50s of embryonic duck eggs of the 9 DHAV-1 isolates were between 1.9 × 106/mL to 1.44 × 107/mL, while the LD50s were 2.39 × 105/mL to 6.15 × 106/mL. Cross-neutralization tests revealed that the 9 DHAV-1 isolates were completely neutralized by the standard serum and the hyperimmune sera against the 9 DHAV-1 isolates, respectively. Compared with other virulent, moderate virulent, attenuated vaccine and mild strains, the VP1 genes of the 9 strains shared 89.8%–99.7% similarity at the nucleotide level and 92.4%–99.6% at amino acid level with other DHAV-1 strains. There were three hypervariable regions at the C-terminus (aa 158–160, 180–193 and 205–219) and other variable points in VP1 protein, but which didn’t cause virulence of DHAV-1 change.