INTRODUCTION
DANIEL TODD
Agri-Food & Biosciences Institute, Stormont, Belfast, UK
Circoviruses are non-enveloped, spherical viruses (20— 25 nm in diameter), that possess circular, single-stranded DNA genomes (1.7-2.3kb).
They are the smallest of the DNA viruses that affect animals and are highly resistant to physical and chemical inactivation. The family Circov- iridae contains two genera. Chicken anaemia virus (CAV) is the only species and the type species of the genus Gyrovirus, whereas the genus Circovirus presently comprises, or is soon likely to comprise, porcine circovirus types 1 and 2 (PCV1, PCV2), beak and feather disease virus (BFDV), pigeon circovirus (PiCV), goose circovirus (GoCV), canary circovirus (CaCV), duck circovirus (DuCV), gull circovirus (GuCV), finch circovirus (FiCV), starling circovirus (StCV) and swan circovirus (SwCV). Viruses belonging to the two genera can be distinguished in terms of virion size and morphology and genome organization. Viruses from both genera use a single structural protein, and also produce one or two non-structural proteins. To date, only CAV, PCV1 and PCV2 can be grown in cell culture, which has greatly restricted the characterization of the antigenic interrelationships of avian circoviruses. However, the ability to use DNA amplification methods to isolate complete genomic fragments has allowed circoviruses to be characterized and classified using nucleotide sequence-based methods. A novel circovirus will be recognized as a new member of the genus Circovirus if it shares less than 70% genome nucleotide identity and less than 75% capsid protein amino acid identity with any of the previously assigned members(1). The circovirus identified in mute swans (SwCV) is likely to be recognized as a new genus member, whereas that recognized in Australian ravens (RaCV) is not. Circovirus-like viruses have also been described in ostriches and pheasants, but these have yet to be definitively characterized.
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