The circular, Rep-encoding Single-stranded (ss) DNA (CRESS-DNA) viruses are wide spread in various habitats and show high diversity (Shulman and Davidson, 2017; Kazlauskas et al., 2019). In recent years, an increasing number of CRESS viruses have been reported with the help of the high-throughput sequencing technologies. PCL virus (porcine circovirus-like virus) is a new type of CRESS virus that belongs to the proposed Kirkoviridae family (Liu et al., 2021). PCL virus was first discovered in pig stool by American scholar Shan in 2011 and was reported in China in 2014 (Shan et al., 2011; Zhang et al., 2014; Sun et al., 2021). Existing epidemiological investigations of PCL virus have shown that PCL virus mainly infects piglets, causing symptoms such as diarrhea, hemorrhagic enteritis. Yang et al. (2021) detected PCL virus in a dead sow with porcine epidemic diarrhea virus (PEDV) in Anhui Province. In addition to PEDV, pigs infected with PCL virus are also co-infected with viruses such as porcine circovirus type 2 (PCV-2), porcine parvovirus (PPV; Sun et al., 2021; Yang et al., 2021; Zeng et al., 2022). In this study, PCL virus was detected for the first time and in pregnant sows in Hunan Province, which greatly enriched the epidemiological information of PCL virus. It is noteworthy that pigs infected with PCL virus had diarrhea symptoms, but some sows showed abortion. Further investigation is needed to determine the relationship between the virus infection, diarrhea and abortion.

Circoviridae is divided into two genera of Circovirus and Cyclovirus, and can infect a variety of animals (Li et al., 2011; Phan et al., 2014). In general, circoviruses are small, single-stranded, linear, non-enveloped DNA viruses with a genome approximately 1.7–2.1 kb in length (Breitbart et al., 2017). Their genomes contain two major open reading frames (ORFs) that encode the replication-associated protein (Rep) and the sole structural protein (capsid protein, Cap). PCL virus has a similar genome to PCVs, as a circular genome and a stem-loop. However, all the amino acid sequences of Rep proteins of the viruses belonging to proposed family Kirkoviridae indicate significant genetic distance with those of the viruses within family Circoviridae (Shan et al., 2011; Guo et al., 2018; Sun et al., 2021). In fact, PCL viruses have little nucleotide identity with PCV. In recent years, more PCL virus have been reported one after another. Nowadays PCL virus has three predicted types of the stem loop:GGGCAATTCTGCCC, GGGCAAGTCTGCCC, and GGGCAAATCTGCCC. Surprisingly, there was a substitution in the stem loop of the PCL virus (Cheung, 2007; Liu et al., 2021; Sun et al., 2021). In 2018, Guo et al. (2018) discovered a novel CRESS DNA virus from a calf with severe hemorrhagic enteritis. However the replication proteins (Rep) of Bo-Circo-like virus CH showed extremely high identity with Po-Circo-like virus (Liu et al., 2021). This finding indicated that the PCL virus is different from PCV and is similar to Bo-Circo-like. Rep proteins have been proposed for use as the basis for megataxonomic classification of CRESS DNA viruses (Liu et al., 2014; Koonin et al., 2020). Fewer amino acid mutations were observed on the Rep protein of these viruses (Liu et al., 2021). The current study, phylogenetic analysis also based on the replication proteins (Rep), divides PCL virus into two subpopulations PCLa and PCLb (Cheung, 2007). In the past few years, research has focused on the genetic characteristics and evolution of PCL viruses. No cell lines have been identified for isolation of the virus. Unfortunately, we attempted to use PK15 cell lines to isolate the virus, which was not successful. So research on the pathogenesis of PCL virus is currently stagnant. This study was the first to report the detection of the PCL virus in Hunan provinces of China. However, its clinical significance, epidemiology and disease prevention deserve further in-depth study.

A novel CRESS DNA virus was first detected in pig intestinal tissue in the United States (Shan et al., 2011). The virus called the PCL virus, is similar to PCVs, as it has a circular genome. One difference is that the PCL virus does not express a typical capsid protein (Cap). At present, PCL virus has been reported in the United States and China. PCL virus infection was reported in pig farms in six provinces of China. In this study, we studied of the genomic characteristics of PCL virus in Hunan Province, China. According to the amino acid sequence analysis of Rep protein, the PCL virus strains in this study were closely related to 21 and 22 strains in the United States and other virus strains in China. Fewer amino acid mutations on the Rep protein of these viruses were observed. However, it is very interesting that Bo-Circo-like virus also has high identity with the strains in this study (Figure 2, 3; Supplementary Table 2). In summary, we detected PCL virus for the first time in Hunan Province, China, and sequenced the whole genomes of four strains. Once again demonstrated the high similarity between Bo-circo-like virus and PCL virus, further indicating that PCL virus possible transmitted to non-porcine hosts. Further studies based on more information of molecular epidemiology will help better understanding origin, evolution and transmission patterns of PCL virus in pigs in China.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

The animal study was reviewed and approved by South China Agricultural University Experimental Animal Welfare Ethics Committee.

JM and YS: conceived and designed the experiments. CJ, MZ, and XL: performed the experiments. CJ, MZ, and YW: sample collection. CJ and MZ: analyzed the data and contributed to the writing. All authors contributed to the article and approved the submitted version.

This work was financially supported by the Guangdong Major Project of Basic and Applied Basic Research (no. 2020B0301030007) and Natural Science Foundation of Guangdong Province (nos. 2022A1515012473 and 2020A1515010295).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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