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.