Porcine circovirus 2 (PCV2), a single-stranded DNA virus associated with post-weaning multisystemic wasting syndrome of swine, has two potential open reading frames, ORF1 and ORF2, greater than 600 nucleotides in length. ORF1 is predicted to encode a replication-associated protein (Rep) essential for replication of viral DNA, while ORF2 contains a conserved basic amino acid sequence at the N terminus resembling that of the major structural protein of chicken anaemia virus. Thus far, the structural protein(s) of PCV2 have not been identified. In this study, a viral structural protein of 30 kDa was identified in purified PCV2 particles. ORF2 of PCV2 was cloned into a baculovirus expression vector and the gene product was expressed in insect cells. The expressed ORF2 gene product had a molecular mass of 30 kDa, similar to that detected in purified virus particles. The recombinant ORF2 protein self-assembled to form capsid-like particles when viewed by electron microscopy. Antibodies against the ORF2 protein were detected in samples of sera obtained from pigs as early as 3 weeks after experimental infection with PCV2. These results show that the major structural protein of PCV2 is encoded by ORF2 and has a molecular mass of 30 kDa.

Infection of animals with a molecular viral clone is critical to study the genetic determinants of viral replication and virulence in the host. Type 2 porcine circovirus (PCV2) has been incriminated as the cause of postweaning multisystemic wasting syndrome (PMWS), an emerging disease in pigs. We report here for the first time the construction and use of an infectious molecular DNA clone of PCV2 to characterize the disease and pathologic lesions associated with PCV2 infection by direct in vivo transfection of pigs with the molecular clone. The PCV2 molecular clone was generated by ligating two copies of the complete PCV2 genome in tandem into the pBluescript SK (pSK) vector and was shown to be infectious in vitro when transfected into PK-15 cells. Forty specific-pathogen-free pigs at 4 weeks of age were randomly assigned to four groups of 10 each. Group 1 pigs served as uninoculated controls. Pigs in group 2 were each inoculated intranasally with about 1.9 ؋ 10 5 50% tissue culture infective doses of a homogeneous PCV2 live virus stock derived from the molecular clone. Pigs in group 3 were each injected intrahepatically with 200 g of the cloned PCV2 plasmid DNA, and pigs in group 4 were each injected into the superficial iliac lymph nodes with 200 g of the cloned PCV2 plasmid DNA. Animals injected with the cloned PCV2 plasmid DNA developed infection resembling that induced by intranasal inoculation with PCV2 live virus stock. Seroconversion to PCV2-specific antibody was detected in the majority of pigs from the three inoculated groups at 35 days postinoculation (DPI). Viremia, beginning at 14 DPI and lasting 2 to 4 weeks, was detected in the majority of the pigs from all three inoculated groups. There were no remarkable clinical signs of PMWS in control or any of the inoculated pigs. Gross lesions in pigs of the three inoculated groups were similar and were characterized by systemically enlarged, tan lymph nodes and lungs that failed to collapse. Histopathological lesions and PCV2-specific antigen were detected in numerous tissues and organs, including brain, lung, heart, kidney, tonsil, lymph nodes, spleen, ileum, and liver of infected pigs. This study more definitively characterizes the clinical course and pathologic lesions exclusively attributable to PCV2 infection. The data from this study indicate that the cloned PCV2 genomic DNA may replace infectious virus for future PCV2 pathogenesis and immunization studies. The data also suggest that PCV2, although essential for development of PMWS, may require other factors or agents to induce the full spectrum of clinical signs and lesions associated with advanced cases of PMWS.Porcine circovirus (PCV) was originally isolated as a cell culture contaminant of a porcine kidney cell line (PK-15) (56,60). PCV is a small, nonenveloped virus that contains a singlestranded circular DNA genome of about 1.76 kb. PCV is classified in the family of Circoviridae, which consists of three other animal circoviruses (chicken anemia virus [CAV], psittacine beak and feather disease v...

Postweaning multisystemic wasting syndrome of swine associated with porcine circovirus (PCV) is a recently reported and economically important disease. Simple and reliable diagnostic methods are needed for detecting antibodies to PCV type 2 (PCV2) for monitoring of PCV infection. Here, we report the development of two modified indirect enzyme-linked immunosorbent assays (ELISAs): a PCV2 ELISA based on cell-culturepropagated PCV2 and an ORF2 ELISA based on recombinant major capsid protein. PCV2 and ORF2 ELISA detected antibodies to PCV2 and the capsid protein, respectively, in sera from pigs experimentally infected with PCV2 as early as 14 and 21 days postinoculation (dpi). The kinetics of the antibody response to PCV2 and the major capsid protein were similar. Repeatability tests revealed that the coefficients of variation of positive sera within and between runs for both assays were less than 30%. To validate the assays, PCV2 and ORF2 ELISAs were performed with 783 serum samples of young and adult pigs collected from different herds in the Midwestern United States and compared with an indirect immunofluorescent assay (IIF). Six out of 60 samples collected from nursery and growing pigs in 1987 were positive by both ELISA and IIF. Compared with IIF, the diagnostic sensitivity, specificity, and accuracy of PCV2 and ORF2 ELISAs were similar (>90%). The tests showed no cross-reactivity with antibodies to porcine parvovirus and porcine reproductive and respiratory syndrome virus. There was good agreement between the two ELISAs and between the ELISAs and IIF. The availability of the two ELISAs should accelerate our understanding of the host immune response to PCV2 and facilitate the development of prevention and control strategies by elucidating the ecology of PCV2 within swine populations.

Abstract. An immunohistochemical method for the detection of type 2 porcine circovirus (PCV2) in paraffin-embedded tissue was developed. Rabbits were inoculated with purified PCV2 to obtain a polyclonal antiserum. Antiserum was applied to sections of porcine tissue that contained lesions consistent with postweaning multisystemic wasting syndrome and in which PCV2 genome had been demonstrated by in situ hybridization. In all cases (18/18), the density and distribution of positive cells detected by in situ hybridization or immunohistochemistry were identical. The immunohistochemical method is more rapid and less expensive than in situ hybridization and is thus more suitable for routine diagnostic use.A newly emerged porcine disease syndrome, postweaning multisystemic wasting syndrome (PMWS), has been reported throughout North America and Europe and is associated with infection by a novel strain of porcine circovirus (PCV). 2,3,5,7-10 PMWS is characterized clinically by progressive dyspnea, emaciation, and occasionally icterus and pathologically by a wide range of inflammatory lesions that most often include lymphohistiocytic to granulomatous lymphadenitis, interstitial pneumonia, hepatitis, and interstitial nephritis. 2,3 The hallmark of PMWS is depletion of lymphoid tissues and the consistent presence in those tissues of a newly identified strain of PCV. 2,3,5,[7][8][9][10] The genotype and serotype of the PCV isolated from PMWS lesions (PCV2) is markedly different from that of PCV1, an apparently ubiquitous, nonpathogenic virus initially identified over 20 years ago as a contaminant of the PK15 cell line. 1,6,11,12 Although pigs with PMWS are consistently infected with PCV2, it remains uncertain whether PCV2 infection alone is sufficient to induce PMWS. 4 Detection of PCV2 in tissues has been achieved by 1) identification of macrophages with large basophilic to amphophilic intracytoplasmic inclusion bodies that ultrastructurally correspond to arrays of PCV particles, 7 2) demonstration of viral genome via in situ hybridization (ISH), 2,9 or 3) demonstration of viral antigen via immunohistochemistry. 3,4,7 Because characteristic inclusion bodies are an inconsistent feature of PCV2 infection, reliable methods for the detection of PCV2 genome or antigen are highly desirable. Compared with immunohistochemistry, ISH is more complex and less suited to a busy diagnostic laboratory. Others have reported the immunohistochemical detection of PCV in tissues using rabbit antisera raised against PCV1 3,4 or monoclonal antibodies to PCV1. 4 Immunohistochemistry using rabbit antisera to a presumptive PCV2 isolate was previously reported, 7 but detailed methodology was not presented. Herein, the production of polyclonal Received for publication April 19, 1999. rabbit antiserum to purified PCV2 and utilization of this antiserum in an immunohistochemical assay suitable for routine diagnostic use is described. A well-characterized PCV2 isolate (ISU-31) was propagated in glucosamine-treated PCV-free PK15 cells. 9 When an indirec...

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