INTRODUCTION
Porcine circoviruses (PCVs) are small, non-enveloped DNA viruses belonging to the Circoviridae family. The PCVs genomes are single-stranded circular DNA, 1,700 to 2,000 nucleotides (nt) long. At present, PCV can be classified into four types: PCV1, PCV2, PCV3, and PCV4. PCV1 is considered to have no pathogenicity in pigs (Tischer et al., 1986), whereas PCV2 has been considered to be associated with PCV2-associated diseases (PCVAD), which cause huge economic losses to the swine industry worldwide (Harding & Clark, 1998). Recently, PCV3 was identified in pigs with cardiac and multi-systemic inflammation, and in pigs with clinical signs of porcine dermatitis and nephropathy syndrome (PDNS) in the United States (Phan et al., 2016; Palinski et al., 2017). PCV4 was recently identified in pigs with severe clinical disease in China (Zhang et al., 2019).
Porcine circovirus-like agents are agents with truncated PCV2 genomes, 200 to 1,000 nt long, containing extraneous nucleotide sequences. At present, these agents can be divided into two types: porcine circovirus-like viruses and porcine circovirus-like mini agents. The former comprises four viruses, tentatively named P1, P2, ZJ-R (P3), and P4, whose genomes can encode virus proteins (Wen et al., 2008, 2012, 2015, 2018a). The latter also includes four agents, tentatively named PCVL258, PCVL264, PCVL201, and PCVL347, whose genomes have no ability to encode a protein (Wen et al., 2019). The detection rate of porcine circovirus-like virus P1 is significantly higher than that of other porcine circovirus-like agents, so more research has been conducted on P1. Genomic sequence analysis has revealed that the novel P1 virus possesses a single-stranded circular DNA genome of 648 nt containing eight open reading frames (ORFs) (Wen et al., 2014). It has higher homology with the partial Cap of a PCV2 strain except for 16 consecutive nucleotides (CGTTACTAGTGGATCC). The P1 capsid protein (Cap) encoded by ORF1 contains 114 amino acid (aa) residues, 120–121 aa fewer than PCV2. To date, two types of P1 virus have been identified, which are 648 and 647 nt long (based on the whole genome); 114 and 122 aa (based on the full-length capsid), respectively (Wen et al., 2017, 2018c). All porcine circovirus-like agents except for PCVL201 and PCVL347 were originally detected in diseased pigs in China. More recently, P1 DNA has also been detected in cattle, goats, and rabbits (Wen et al., 2018b). The goal of this study was to investigate the presence of porcine circovirus-like agents in dogs and cats in China.