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Circoviruses are small, non-enveloped, icosahedral viruses that are unique among animal viruses in having circular, single-stranded DNA genomes. Their genomes are also the smallest possessed by animal viruses. The circovirus family currently comprises three members, chicken anaemia virus, porcine circovirus, and psittacine beak and feather disease virus, with pigeon circovirus being classified as a tentative member. Infections with each of the four circoviruses are associated with potentially fatal diseases in which virus-induced damage to lymphoid tissue and immunosuppression are common features. Experience with other animal virus families suggests that additional animal species will be infected by, as yet undiscovered, circoviruses and that these may display similar tissue tropism and disease-causing potential. Recent reports describing the association of circovirus-lik e viruses with immunodeficiency-related diseases of geese and southern black-backed gulls suggest that circovirus infections of avian species may be more common than previously recognized, and prompt the question of whether novel circoviruses infect poultry to cause clinical and/or subclinical diseases that may be economically important. This review has three purposes. First, it is designed to summarize the currently available information about the classified circoviruses and viruses that are regarded as circovirus-like. Second, it aims to alert the readership to the possibility that other avian species, including commercial poultry, may be infected with novel circoviruses. Finally, possible methods for discovering novel circoviruses and for controlling infections by such viruses are suggested.

Cell culture isolates of salmon pancreas disease virus (SPDV) of farmed Atlantic salmon and sleeping disease virus (SDV) of rainbow trout were compared. Excluding the poly(A) tracts, the genomic nucleotide sequences of SPDV and SDV RNAs include 11,919 and 11,900 nucleotides, respectively. Phylogenetic analysis places SPDV and SDV between the New World viruses of Venezuelan equine encephalitis virus and Eastern equine encephalitis virus and the Old World viruses of Aura virus and Sindbis virus. When compared to each other, SPDV and SDV show 91.1% nucleotide sequence identity over their complete genomes, with 95 and 93.6% amino acid identities over their nonstructural and structural proteins, respectively. Notable differences between the two viruses include a 24-nucleotide insertion in the C terminus of nsP3 protein of SPDV and amino acid sequence variation at the C termini of the capsid and E1 proteins. Experimental infections of Atlantic salmon and rainbow trout with SPDV and SDV confirmed that the disease lesions induced by SPDV and SDV were similar in nature. Although infections with SPDV and SDV produced similar levels of histopathology in rainbow trout, SDV induced significantly less severe lesions in salmon than did SPDV. Virus neutralization tests performed with sera from experimentally infected salmon indicated that SPDV and SDV belonged to the same serotype; however, antigenic variation was detected among SDV and geographically different SPDV isolates by using monoclonal antibodies. Although SPDV and SDV exhibit minor biological differences, we conclude on the basis of the close genetic similarity that SPDV and SDV are closely related isolates of the same virus species for which the name Salmonid alphavirus is proposed.

The complete nucleotide sequence (1759 nt) of the ssDNA genome of porcine circovirus (PCV) was determined from a cloned dsDNA replicative form isolated from PCV-infected cells. Sequence analysis detected no significant nucleic acid or protein similarity with another animal circovirus, chicken anaemia virus (CAV) but, surprisingly, the highest protein similarity was obtained between the product of the largest predicted PCV ORF (ORF1 ; encoding a potential protein of 35n7 kDa) and a putative protein encoded by the plant circovirus banana bunchy top virus (BBTV). High protein similarity was also detected with the other plant circoviruses subterranean clover stunt virus (SCSV) and coconut foliar

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