Hepadnaviruses of Birds

Abstract

The avian hepadnaviruses belong to the genus Avihepadnavirus in the family Hepadnaviridae. Assigned species include the duck (DHBV) and heron hepatitis B viruses. These avian viruses are related phylogenetically to human hepatitis B virus (HBV). The major site of hepadnavirus infection is the hepatocyte. Hepadnaviruses contain a relaxed circular double-stranded DNA genome which is converted in the nucleus to covalently closed circular DNA (cccDNA), which is used as the virus transcriptional template to produce 'pregenomic' RNA and virus mRNA. Hepadnaviruses replicate via reverse transcription of pregenomic RNA into DNA. Virus replication and release are noncytopathic and disease activity is attributed to the host immune response. The avian hepadnaviruses are transmitted in ovo via vertical transmission, resulting in persistent infection. Although experimental DHBV infection of newly hatched ducks also leads to persistent infection, DHBV-infected ducks do not develop severe liver disease, cirrhosis, or liver cancer but provide a reproducible experimental system for studying virus kinetics and immune clearance. Much of what we know of the hepadnavirus replication strategy has been discovered from studies of DHBV. DHBV-infected ducks are also used to evaluate new antiviral therapies and vaccines with the ultimate aim of applying the same strategies to the treatment of HBV infections in humans.