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Type Species |
(CAV) |
Virions exhibit icosahedral symmetry and do not possess an envelope. Ranges of reported virion sizes for Chicken anemia virus (CAV), Porcine circovirus (PCV) and Beak and feather disease virus (BFDV) are 19.1-26.5 nm, 17-20.7 nm and 12-20.7 nm respectively. Diameters of virus particles are approximately 20% greater when negatively stained using uranyl acetate as opposed to the more commonly used phosphotungstate. A capsid structure consisting of 32 hollow morphological subunits arranged in a T = 3 icosahedron has been proposed for CAV (Fig. 1).
Physicochemical and Physical Properties
The buoyant density of virions in CsCl is 1.33-1.37g/cm3. CAV and PCV possess sedimentation coefficients of 91S and 57S respectively. CAV and PCV are resistant to inactivation by treatment at pH 3 and both viruses can withstand incubation at 70°C for 15 minutes. CAV is resistant to treatment with chloroform and ether; PCV is resistant to treatment with chloroform. Both CAV and PCV are at least partially resistant to sodium dodecyl sulfate. The effects of pH, solvents and detergents on the infectivity of BFDV are not known due to the lack of an in vitro culture system.
Virions contain circular ssDNA. The genomes of CAV and PCV contain 2,298 and 1,759 bases respectively. The BFDV genome is about 2.0 kb in size. The CAV genome is of negative sense. Information concerning the sense of the PCV and BFDV genomes has not been reported.
The PCV genome contains a nonanucleotide sequence motif (TAGTATTAC), which is found at the apex of a potential stem loop and which is identical or highly similar to those found in bacterial and plant viruses with circular, ssDNA genomes (Microviridae, Nanovirus and Geminiviridae).
CAV and PCV virions contain one protein, Mr = 50 and 36 
103 respectively. BFDV is reported to contain three proteins, Mr = 26.3, 23.7, and 15.9 103. CAV has two non-structural proteins, Mr = 24 103 (VP2) and 13.6 103 (VP3), the smaller of which causes apoptosis in vitro. The non-structural proteins of PCV have not been characterized.
The N-terminal of the CAV CP shares homologies with histone proteins consistent with it having a DNA-binding role within the virion. The protein (Mr = 35.7 103) encoded by the largest PCV ORF has amino acid sequence homology with the replication-associated proteins of plant viruses with circular, ssDNA genomes.
None reported.
None reported.
Genome Organization and Replication
CAV and PCV DNA replicate using circular ds replicative form (RF) DNAs. Nucleic acid and protein homologies shared with plant geminiviruses are consistent with PCV DNA replicating by a rolling circle mechanism. The origin of replication of PCV DNA has been mapped.
Only one strand of the CAV RF is transcribed to produce a polycistronic messenger RNA ( ~ 2.1 kb) which contains 3 partially overlapping ORFs encoding proteins of Mr = 52 103 (VP1, CP), 24 103 (VP2) and 13.6 103 (VP3). All three proteins are detected in electron dense bodies within the nuclei of virus-infected cell cultures. CAV particles have not been directly observed within infected cells. The PCV RF sequence has 6 ORFs larger than 200 nts, which occur in both positive and negative sense orientations (Fig. 2).
Thin sections of feather inclusions in birds with beak and feather disease demonstrate viral particles in the nuclei of feather epithelium with inclusions, and viral particles in the cytoplasm of macrophage and intracytoplasmic inclusions.
No common antigens have been reported between CAV, PCV and BFDV. BFDV exhibits hemagglutination.
Viruses appear to be specific for species of origin. Modes of transmission and possible vectors are not known. The viruses have a worldwide distribution. CAV causes transient anemia and immunosuppression in baby chicks. BFDV causes chronic and ultimately fatal disease in large psittacine birds. PCV-like viruses have been associated with a recently described condition of pigs known as post-weaning multisystemic wasting syndrome. Cells of the hematopoietic system are infected by CAV and BFDV.
List of Species Demarcation Criteria in the Genus
The list of species demarcation criteria is:
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Differences in nucleotide and gene product sequences, |
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Differences in host range, |
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Differences in viral antigens. |
At present all assigned members of the family have been classified within a single genus. However differences in virion size, genome size and genome organization may provide the basis for definition of more than one genus in the future.
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:
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Beak and feather disease virus |
(BFDV) | |
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Chicken anemia virus |
[M55918, M81223] |
(CAV) |
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Porcine circovirus |
[U49186, Y09921] |
(PCV) |
Tentative Species in the Genus
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Pigeon circovirus |
(PiCV) |
List of Unassigned Viruses in the Family
None.
Phylogenetic Relationships within the Genus
Not available.
PCV is similar to members of the families Geminiviridae and Microviridae, in that it exhibits nucleic acid and protein homologies related to rolling circle DNA replication. The animal circoviruses are similar to plant nanoviruses such as Banana bunchy top virus, Coconut foliar decay virus and Subterranean clover stunt virus, which possess non-enveloped, icosahedral capsids (18-20 nm in diameter) and circular, ssDNA genomes (0.85-1.3 kb in size). These plant viruses, formerly regarded as “unassigned viruses in the family Circoviridae”, are now classified in an unassigned genus Nanovirus.
Circo: sigla to indicate that the viral DNA has a circular conformation.
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