Both, PCV1 and PCV2 are members of the Circoviridae family (Todd et al., 2005). The Circoviridae family is divided into the genera Circovirus (Circo indicates that the virus has a circular conformation) and Gyrovirus (Gyro is a derivation from the Greek work “gyrus” meaning “ring” or “circuit”). The genus Circovirus contains the following species: psittacine beak and feather disease virus (BFDV), canary circovirus (CaCV), goose circovirus (GoCV), pigeon circovirus (PiCV), PCV1, and PCV2. The species tentatively placed in the genus Circovirus are duck circovirus (DuCV), finch circovirus (FiCV), and gull circovirus (GuCV). The genus Gyrovirus contains only chicken anemia virus (CAV) (Todd et al., 2005).
Viruses that belong to the Circoviridae family have characteristic virions that exhibit icosahedral symmetry and do not posses an envelope. The genomes are covalently closed, circular, ssDNAs, which range in size from 1.8 to 2.3 kb. The genome organization of CAV is negative sense, whereas those of the other circoviruses are ambisense (Todd et al., 2005). CAV, PCV2, and BFDV were found to have an icosahedral T=1 structure containing 60 capsid protein molecules arranged in 12 pentamer clustered units (Crowther et al., 2003). Circoviruses are host-specific or exhibit a narrow host range and the majority of those reported to date infect avian species (Todd et al., 2005). Subclinical infections are common; however, circovirus infections are associated with clinical disease in some cases such as with chicken anemia virus, psittacine beak and feather disease, circovirus disease of pigeons, and PCV2-associated disease in pigs. Circovirus infections in all species cause varying degrees of lymphoid depletion and are thought to be immunosuppressive (Todd et al., 2005).
Phylogenetic analysis of PCV1, avian circovirus, plant geminiviruses, and nanoviruses classified PCV1 as most closely related to BFDV and were intermediate between the two plant viral groups (Niagro et al., 1998). Furthermore, it has been proposed that a predecessor to PCV1 and BFDV may have originated from a plant nanovirus that infected a vertebrate host and recombined with a vertebrate-infecting RNA virus, most likely a calicivirus (Gibbs and Weiller, 1999).
Crowther RA, Berriman JA, Curran WL, Allan GM, Todd D:Comparison of the structures of three circoviruses: chicken anemia virus, porcine circovirus type 2, and beak and feather disease virus. J Virol. 77:13036-13041, 2003
Gibbs MJ, Weiller GF: Evidence that a plant virus switched hosts to infect a vertebrate and then recombined with a vertebrate-infecting virus. Proc Natl Acad Sci USA. 96:8022-8027, 1999
Niagro FD, Forsthoefel AN, Lawther RP, Kamalanathan L, Ritchie BW, Latimer KS, Lukert PD: Beak and feather disease virus and porcine circovirus genomes: intermediates between the geminiviruses and plant circoviruses. Arch Virol. 143:1723-1744, 1998
Todd D, Bendinelli M, Biagini P, Hino S, Mankertz A, Mishiro S, Niel C, Okamoto H, Raidal S, Ritchie BW, Teo GC:Circoviridae.In: C. M. Fauquet, M. A. Mayo, J. Maniloff, U. Desselberger, L.A. Ball (Eds.): Virus taxonomy: Eighth report of the international Committee on Taxonomy of Viruses, Elsevier Academic Press, San Diego:327-334, 2005