1. INTRODUCTION
Porcine circovirus (PCV), containing a small single-stranded, nonenveloped, closed circular DNA genome, has been reported as one of the smallest viruses and belongs to the genus Circovirus under the family Circoviridae (Tischer et al., 1982). Its genome contains two major open reading frames (ORF1 and ORF2), which encode a replication-associated protein (Rep) and a structural protein (capsid protein, Cap), respectively. Specifically, Cap contains multiple cell epitopes that are associated with virus neutralization (Cao et al., 2018; Mayr et al., 1968).
Three major genotypes of PCV have been reported. Porcine circovirus type 1 (PCV1) was first identified as a contaminant in a pig kidney cell culture (PK-15) and is considered nonpathogenic (Cao et al., 2018). However, PCV2 and PCV3 are the causative agents of multiple clinical diseases in swine and result in substantial economic losses for the pig industry worldwide (Meng, 2013; Palinski et al., 2017; Sun et al., 2019; Wang et al., 2019). In a recent report, porcine circovirus type 4 (PCV4), a novel and genetically divergent porcine circovirus, was first revealed in Hunan, China. PCV4 is suspected to be associated with severe clinical disease involving respiratory signs, enteric signs and porcine dermatitis and nephropathy syndrome (PDNS) (Zhang et al., 2019).
Similar to the PCV genome, the PCV4 genome is also circular single-stranded DNA and contains two major open reading frames. Genomic and phylogenetic analyses revealed that PCV4 has the closest relationship to mink circovirus (MiCV), which is associated with enteric disease (Zhang et al., 2019). However, knowledge about the infection rate and pathogenicity of this virus is limited. Consequently, we became interested in understanding the seroprevalence of PCV4, preferably using a molecular approach that facilitates any necessary genetic analyses.