Genus Avihepadnavirus

Type Species	Duck hepatitis B virus	(DHBV)

Distinguishing Features

Virions of DHBV are spherical, 46-48  nm in diameter, with a nucleocapsid that is 35  nm in diameter and exhibits projections. Empty particles composed of excess envelope material are pleomorphic and up to 60  nm diameter. The single stranded gap in the virion DNA is usually very short (8 bases). DHBV lacks an X gene containing a conventional initiation codon.

Virus particles have only the largest (Mr 36 10³) and smallest (Mr 17 10³) S proteins. Transmission is predominantly vertical.

Heron hepatitis B virus (HHBV) differs from DHBV in that a highly conserved ORF is present upstream of C in a position analogous to the X gene of orthohepadnaviruses, and that the S protein and not the L protein possesses a potential myristylation site.

List of Species Demarcation Criteria in the Genus

The species demarcation criteria in the genus are:

	Nucleotide sequence diversity: HHBV/DHBV 21.6%,
	Presence in HHBV but not in DHBV of a highly conserved ORF upstream of the C gene in a position analogous to that of the mammalian hepadnaviruses,
	Potential myristylation site located in the S gene (HHBV) or in the preS gene (DHBV).

A number of other less well characterized viruses have been isolated from geese and ducks with reported sequences more closely related to that of DHBV than HHBV.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Duck hepatitis B virus	[K01834, X58567-9, M21953, M32990-1, M60677]	(DHBV)
Heron hepatitis B virus	[M22056]	(HHBV)

Tentative Species in the Genus

None reported.

List of Unassigned Viruses in the Family

Ross’ goose hepatitis B virus	[M95589]	(RGHBV)

Phylogenetic Relationships within the Family

The orthohepadnaviruses and avihepadnaviruses are distinguished by the following criteria :

	Low nucleotide sequence identity,
	Differences in genome size ( 3.2  kb for orthohepadnaviruses and 3.0  kb for avihepadnaviruses),
	Host range restricted to either mammals or birds respectively.

Similarity with Other Taxa

Reverse transcription as an essential step in replication is a common feature of hepadnaviruses, retroviruses and caulimoviruses. Hepadnaviruses and retroviruses also contain three major genes each with the same function and in the same order (ie core-polymerase-pre S/S and gag-pol-env respectively); a fundamental distinction is that, with hepadnaviruses, the form of the genome in extracellular virions is DNA and reverse transcription takes place during the efferent or outgoing arm of the replication cycle, while the reverse hold true for retroviruses. Many other aspects are distinctly different in both virus families, partly due to the extremely small size of the hepadnaviral genome and the need to efficiently exploit this restricted genetic space by using considerable overlap of both coding regions and regulatory elements.

Derivation of Names

Avi: from Latin avis, “bird”,

dna: sigla for deoxyribonucleic acid,

Hepa: from Greek hepar, “liver”,

Ortho: from Greek orthos, “straight”.