Recently, we identified a novel porcine circovirus type 2-like agent P1 isolate from swine. The present study represents the first survey of P1 prevalence in swine herds from Jiangsu, China, by using PCR targeting the complete genome of P1. Prevalences of 50% and 19% were found among 6 herds and 248 animals, respectively. The results indicate a high prevalence of P1 in China pig populations.

Porcine circovirus type 2 (PCV2) belongs to the genus Circovirus of the family Circoviridae, and it has been associated with porcine circovirus (associated) disease (PCVD or PCVAD) in pigs. PCVAD is the generic term for a series of disease syndromes that have caused economic losses to the pig industry worldwide. Since the discovery of PCV2 in the late 1990s, the virus has continued to evolve, and novel genotypes have continued to appear. Moreover, there has been recombination between different genotypes of PCV2. This review attempts to illustrate some progress concerning PCV2 in genome rearrangement and genomic recombination with non-PCV2-related nucleic acids, particularly focusing on the porcine circovirus-like virus P1 formed by the recombination of PCV2. The presence of rearranged PCV2 genomes can be demonstrated both in vivo and in vitro, and these subviral molecules ranged from 358 to 1,136 bp. Depending on whether it has the ability to encode a protein, the agents formed by PCV2 recombination can be divided into two categories: porcine circovirus-like viruses and porcine circovirus-like mini agents. We mainly discuss the porcine circovirus-like virus P1 regarding genomic characterization, etiology, epidemiology, and pathogenesis. Further research needs to be conducted on the pathogenicity of other porcine circovirus-like viruses and porcine circovirus-like mini agents and the effects of their interactions with PCV2, especially for the porcine circovirus-like mini agents that do not have protein-coding functions in the genome.

Recently identified porcine circovirus-like virus P1 has the smallest DNA viral genome. In this study, we identified the viral genes and their corresponding mRNA transcripts. The RNAs of P1, synthesized in porcine kidney cells, were examined with northern blotting and PCR analyses. Eight virus-specific RNAs were detected. Four mRNAs (open reading frames (ORFs) 1, 2, 4, and 5) are encoded by the viral (−) strand and four (ORFs 3, 6, 7, and 8) are encoded by the viral (+) strand. All proteins encoded by the ORFs of the P1 virus are less than 50 amino acids in length, except that encoded by ORF1 (113 amino acids). We show a very complex viral transcription pattern in P1-infected cells.

Porcine circovirus-like virus P1 is a newly discovered virus. To date, there has been no specific serological assay for use in the diagnosis of P1 infection. Because P1 has high homology to porcine circovirus type 2 (PCV2) at the nucleotide level, the C-terminal portion of the capsid protein (amino acids 73–114), a discriminative antigen, was expressed in a prokaryotic expression system. The recombinant product (rctCap), composed of three identical repeated domains, was shown to be strongly immunoreactive to P1-specific serum. This assay was validated by comparison with an indirect immunofluorescence assay (IFA). The diagnostic sensitivity and specificity of the rctCap enzyme-linked immunosorbent assay (ELISA) developed in this study are 93.6 % and 98.3 %, respectively, compared with the results from IFAs on 450 sera samples from pigs. The indirect ELISA that we developed with rctCap, the recombinant capsid fragment containing the 217–342 nt repeat domain, was sensitive, specific, and suitable for the large-scale detection of P1 infections in swine.