Pathogenicity Evaluation and Virulence Gene Identification of an Attenuated Duck Enteritis Virus

Duck enteritis virus (DEV), an epornitic pathogen, causes substantial economic losses in the commercial duck industry and poses persistent risks to wild and migratory waterfowl populations. However, due to the large genomic capacity of the DEV, the understanding of the virulence-associated genes of DEV is still limited. In previous studies, we developed an attenuated strain E74 by serial passage of a virulent strain E1 on primary chicken embryo fibroblasts (CEFs). The bird experiment showed that the mortality rate of E1 on ducks reached 100%, and high-titered viruses were detected in all tested tissue samples. In contrast, the E74 virus has lost its pathogenicity in ducks and can only be detected at a relatively low viral load in the spleen. Furthermore, the E74 stimulated a significant increase in antibodies in the ducks at 7 days post-inoculation. To further investigate the molecular basis of the attenuation of DEV in ducks, the complete genomes of E74 and E1 were sequenced and analyzed. Compared with E1, E74 had a 5152 bp deletion in the UL region, which resulted in the lack of the hypothetical protein, LORF5, UL55 and LORF4 genes. To test the influence of the deletion on the viral pathogenicity, a rescued virus rE1-Δ5152 with the 5152 bp deletion in the UL region was generated on the E1 backbone. Animal experiments showed that the lethality of rE1-Δ5152 in ducks had disappeared. Those findings suggest that the hypothetical protein, LORF5, UL55, and LORF4 genes of DEV are associated with virus virulence, and the flexibility of this region provided excellent insertion sites for exogenous genes when DEV is used as a recombinant vaccine vector.