Novel Porcine Circoviruses in View of Lessons Learned from Porcine Circovirus Type 2-Epidemiology and Threat to Pigs and Other Species

Efficacy comparison in cap VLPs of PCV2 and PCV3 as swine vaccine vehicle

As one genotype of porcine circovirus (PCV) identified in 2016, PCV3 has brought huge hidden dangers to the global swine industry together with PCV2. Virus-like particles (VLPs) of capsid protein (Cap) of PCV2 serve as an alternative nano-antigen delivery strategy to efficiently induce antiviral immune response against PCV2 and/or other covalently displayed swine pathogens. However, the current understanding is limited on the capability of PCV3 as a nano-vaccine vehicle. Here we systematically compared the characteristics and the immunogenic efficacy of PCV3 Cap (Cap3) and PCV2 Cap (Cap2) in a VLP form. Cap3 VLPs presented higher internalization efficiency into cells and cytokines production compared to those of Cap2. Meanwhile, cross-reactive immunity between Cap3 VLPs and Cap2 VLPs was detected. Furthermore, to evaluate the function of Cap3 VLPs and Cap2 VLPs as vaccine vehicles carrying foreign proteins, the non-structural protein 6 of porcine reproductive and respiratory syndrome virus (PRRSV) was fused to C-terminus of Cap. Cap3-based chimeric particles induced a higher level of nsp6-specific immune response and PRRSV inhibition. Collectively, these self-assembling, Cap-based VLPs offer a compelling platform for enhancing the effectiveness of subunit vaccinations against newly emerging diseases and hold great promise for the development of Cap3-based chimeric subunit vaccines.

A multiplex digital PCR assay for detection and quantitation of porcine circovirus type 2 and type 3

Porcine circovirus (PCV) has become a major pathogen, causing major economic losses in the global pig industry, and PCV type 2 (PCV2) and 3 (PCV3) are distributed worldwide. We designed specific primer and probe sequences targeting PCV2 Cap and PCV3 Rap and developed a multiplex crystal digital PCR (cdPCR) method after optimizing the primer concentration, probe concentration, and annealing temperature. The multiplex cdPCR assay permits precise and differential detection of PCV2 and PCV3, with a limit of detection of 1.39 × 101 and 1.27 × 101 copies/reaction, respectively, and no cross-reaction with other porcine viruses was observed. The intra-assay and interassay coefficients of variation (CVs) were less than 8.75%, indicating good repeatability and reproducibility. To evaluate the practical value of this assay, 40 tissue samples and 70 feed samples were tested for both PCV2 and PCV3 by cdPCR and quantitative PCR (qPCR). Using multiplex cdPCR, the rates of PCV2 infection, PCV3 infection, and coinfection were 28.45%, 1.72%, and 12.93%, respectively, and using multiplex qPCR, they were 25.00%, 0.86%, and 4.31%, respectively This highly specific and sensitive multiplex cdPCR thus allows accurate simultaneous detection of PCV2 and PCV3, and it is particularly well suited for applications that require the detection of small amounts of input nucleic acid or samples with intensive processing and complex matrices.

Epizootiological surveillance of porcine circoviruses in free-ranging wild boars in China

Four species of porcine circoviruses (PCV1-4) have been reported to circulate in Chinese domestic pigs, while the epizootiology of these viruses in free-ranging wild boars in China remains unknown. In this study, tissue and serum samples collected from diseased or apparently healthy wild boars between 2018 and 2020 in 19 regions of China were tested for the prevalence of PCV1-4 infections. Positive rates of PCV1, PCV2, and PCV3 DNA in the tissue samples of Chinese wild boars were 1.6% (4/247), 58.3% (144/247), and 10.9% (27/247) respectively, with none positive for PCV4. Sequence analysis of viral genome showed that the four PCV1 strains distributed in Hunan and Inner Mongolia shared 97.5%-99.6% sequence identity with global distributed reference strains. Comparison of the ORF2 gene sequences showed that 80 PCV2 strains widely distributed in 18 regions shared 79.5%-100% sequence identity with reference strains from domestic pigs and wild boars, and were grouped into PCV2a (7), PCV2b (31) and PCV2d (42). For PCV3, 17 sequenced strains shared 97.2%-100% nucleotide identity at the genomic level and could be divided into PCV3a (3), PCV3b (2) and PCV3c (12) based on the phylogeny of ORF2 gene sequences. Serological data revealed antibody positive rates against PCV1 and PCV2 of 11.4% (19/167) and 53.9% (90/167) respectively. The data obtained in this study improved our understanding about the epidemiological situations of PCVs infection in free-ranging wild boars in China and will be valuable for the prevention and control of diseases caused by PCVs infection.

First report on identification and genomic analysis of a novel porcine circovirus (porcine circovirus 4) in cats

Porcine circovirus type 4 (PCV4) is an emerging circovirus, which has been detected in domestic pigs across various provinces in China and Korea. In this study, we aimed to investigate whether cats are susceptible to PCV4. For this purpose, we collected 116 cat samples from animal hospitals in Sichuan Province, China, between 2021 and 2022. Using a SYBR Green-based real-time PCR assay, we detected PCV4 in 5 out of the 116 clinical samples, indicating a positive rate of 4.31% (5/116) and confirming the presence of PCV4 in cats from Sichuan Province, China. Moreover, we successfully sequenced and analyzed the complete genome of one PCV4 strain (SCGA-Cat) along with 60 reference sequences deposited in the GenBank database. SCGA-Cat exhibited high nucleotide homology (98.2-99.0%) with PCV4 strains from other species, including dogs, pigs, dairy cows, and fur animals. Notably, the SCGA-Cat strain from cats clustered closely with a PCV4 strain derived from a pig collected in Fujian Province, China. To the best of our knowledge, this study represents the first report on the molecular detection of PCV4 in cats worldwide, which prompted us to understand the genetic diversity and cross-species transmission of the ongoing PCV4 cases. However, further investigations are needed to explore the association between PCV4 infection and clinical syndromes in cats.

A Diverse Virome Is Identified in Parasitic Flatworms of Domestic Animals in Xinjiang, China

Parasitic flatworms infect diverse vertebrates and are major threats to animal and even human health; however, little is known about the virome of these lower life forms. Using viral metagenomic sequencing, we characterized the virome of the parasitic flatworms collected from major domestic animals, including Dicrocoelium lanceatum and Taenia hydatigena, Echinococcus granulosus sensu stricto and Echinococcus multilocularis. Seven and three different viruses were discovered from D. lanceatum and T. hydatigena, respectively, and no viral sequences were found in adult tapeworms and protoscoleces of E. granulosus sensu stricto and E. multilocularis. Two out of the five parasitic flatworm species carry viruses, showing a host specificity of these viruses. These viruses belong to the Parvoviridae, Circoviridae, unclassified circular, Rep-encoding single-stranded (CRESS) DNA virus, Rhabdoviridae, Endornaviridae, and unclassified RNA viruses. The presence of multiple highly divergent RNA viruses, especially those that cluster with viruses found in marine animals, implies a deep evolutionary history of parasite-associated viruses. In addition, we found viruses with high identity to common pathogens in dogs, including canine circovirus and canine parvovirus 2. The presence of these viruses in the parasites implies that they may infect parasitic flatworms but does not completely exclude the possibility of contamination from host intestinal contents. Furthermore, we demonstrated that certain viruses, such as CRESS DNA virus may integrate into the genome of their host. Our results expand the knowledge of viral diversity in parasites of important domestic animals, highlighting the need for further investigations of their prevalence among other parasites of key animals. IMPORTANCE Characterizing the virome of parasites is important for unveiling the viral diversity, evolution, and ecology and will help to understand the "Russian doll" pattern among viruses, parasites, and host animals. Our data indicate that diverse viruses are present in specific parasitic flatworms, including viruses that may have an ancient evolutionary history and viruses currently circulating in parasite-infected host animals. These data also raise the question of whether parasitic flatworms acquire and/or carry some viruses that may have transmission potential to animals. In addition, through the study of virus-parasite-host interactions, including the influence of viral infection on the life cycle of the parasite, as well as its fitness and pathogenicity to the host, we could find new strategies to prevent and control parasitic diseases.

Novel Circovirus in Blood from Intravenous Drug Users, Yunnan, China

We identified a novel circovirus (human-associated circovirus 2 [HuCV2]) from the blood of 2 intravenous drug users in China who were infected with HIV-1, hepatitis C virus, or both. HuCV2 is most closely related to porcine circovirus 3. Our findings underscore the risk for HuCV2 and other emerging viruses among this population.

Virome of wild rats (Rattus norvegicus) captured far from pig farms in Jiangsu province of China reveals novel porcine circovirus type 2d (PCV2d) sequences

BACKGROUND

Porcine circovirus type 2 (PCV2) has caused great economic losses in the global pig industry. There have been published records of wild rats acting as the reservoirs of PCV2 (only PCV2a and PCV2b), but almost all of which were related to the PCV2-infected swine herds.

RESULTS

In this study, we carried out the detection, amplification, and characterization of novel PCV2 strains in wild rats that were captured far from pig farms. Nested PCR assay demonstrated that the kidney, heart, lung, liver, pancreas, and large and small intestines of rats were screened positive for PCV2. We subsequently sequenced two full genomes of PCV2 in positive sample pools, designated as js2021-Rt001 and js2021-Rt002. Genome sequence analysis indicated that they had the highest similarity to nucleotide sequences of porcine-origin PCV2 isolates in Vietnam. Phylogenetically, js2021-Rt001 and js2021-Rt002 were a part of the PCV2d genotype cluster, which is a predominant genotype circulating worldwide in recent years. The antibody recognition regions, immunodominant decoy epitope, and heparin sulfate binding motif of the two complete genome sequences coincided with those previously reported.

CONCLUSIONS

Our research reported the genomic characterization of two novel PCV2 strains (js2021-Rt001 and js2021-Rt002) and provided the first supported evidence that PCV2d could naturally infect wild rats in China. However, whether the newly identified strains have potential for circulating in nature in vertical and horizontal transmission or inter-species jumping between rats and pigs needs further research.

Establishment and application of multiplex real-time PCR for simultaneous detection of four viruses associated with porcine reproductive failure

Many pathogens cause reproductive failure in sows suffering a broad spectrum of sequelae, including abortions, stillbirth, mummification, embryonic death, and infertility. Although various detection methods, such as polymerase chain reaction (PCR) and real-time PCR, have been widely used for molecular diagnosis, mainly for a single pathogen. In this study, we developed a multiplex real-time PCR method for the simultaneous detection of porcine circovirus type 2 (PCV2), porcine circovirus type 3 (PCV3), porcine parvovirus (PPV) and pseudorabies virus (PRV) associated with porcine reproductive failure. The R ² values for the standard curve of multiplex real-time PCR of PCV2, PCV3, PPV, and PRV reached to 0.996, 0.997, 0.996, and 0.998, respectively. Importantly, the limit of detection (LoD) of PCV2, PCV3, PPV, and PRV, were 1, 10, 10, 10 copies/reaction, respectively. Meanwhile, specificity test results indicated that multiplex real-time PCR for simultaneous detection is specific for these four target pathogens and does not react with other pathogens, such as classical swine fever virus, porcine reproductive and respiratory syndrome virus, and porcine epidemic diarrhea virus. Besides, this method had good repeatability with coefficients of variation of intra- and inter-assay less than 2%. Finally, this approach was further evaluated by 315 clinical samples for its practicality in the field. The positive rates of PCV2, PCV3, PPV, and PRV were 66.67% (210/315), 8.57% (27/315), 8.89% (28/315), and 4.13% (13/315), respectively. The overall co-infection rates of two or more pathogens were 13.65% (43/315). Therefore, this multiplex real-time PCR provides an accurate and sensitive method for the identification of those four underlying DNA viruses among potential pathogenic agents, allowing it to be applied in diagnostics, surveillance, and epidemiology.

First Molecular Detection and Genetic Analysis of a Novel Porcine Circovirus (Porcine Circovirus 4) in Dogs in the World

A novel circovirus species was identified in farmed pigs and designated porcine circovirus 4 (PCV4); it has recently been proved to be pathogenic to piglets. However, little is known about its cross-species transmission, and there is no evidence of PCV4 in dogs. A total of 217 fecal samples were collected from diarrheal dogs in Henan Province, China, and tested for the presence of PCV4 using a real-time PCR assay. Among the 217 samples, the total positivity rate for PCV4 was 5.99% (13/217 samples), with rates of 7.44% and 4.17% in 2020 and 2021, respectively. PCV4 was detected in dogs in 6 of 10 cities, demonstrating that PCV4 could be detected in dogs in Henan Province, China. One PCV4 strain (HN-Dog) was sequenced in this study and shared high levels of identity (97.9% to 99.6%) with reference strains at the genome level. Phylogenetic analysis based on complete genome sequences of HN-Dog and 42 reference strains showed that the HN-Dog strain was closely related to 3 PCV4 reference strains (from pig, raccoon dog, and fox) but differed genetically from other viruses in the genus Circovirus. Three genotypes, i.e., PCV4a, PCV4b, and PCV4c, were confirmed by phylogenetic analysis of complete genome sequences of 42 PCV4 strains, and one amino acid variation in Rep protein (V239L) and three amino acid variations in Cap protein (N27S, R28G, and M212L) were considered conserved genotype-specific molecular markers. In conclusion, the present study is the first to report the discovery of the PCV4 genome in dogs, and the association between PCV4 infection and diarrhea warrants further study. IMPORTANCE This study is the first to report the presence of PCV4 in dogs worldwide, and the first complete genome sequence was obtained from a dog affected with diarrhea. Three genotypes of PCV4 strains (PCV4a, PCV4b, and PCV4c) were determined, as supported by specific amino acid markers (V239L for open reading frame 1 [ORF1] and N27S R28G and M212L for ORF2). These findings help us understand the current status of intestinal infections in pet dogs in Henan Province, China, and also prompted us to accelerate research on the pathogenesis, epidemiology, and cross-species transmission of PCV4.

Vaccination Failures in Pigs-The Impact of Chosen Factors on the Immunisation Efficacy

Infectious diseases that often lead to economic losses still pose a severe problem in the pig production sector. Because of increasing restrictions on antibiotic usage, vaccines may become one of the major approaches to controlling infectious diseases; much research has proved that they could be very efficient. Nevertheless, during their life, pigs are exposed to various factors that can interfere with vaccination efficacy. Therefore, in the present paper, we reviewed the influence of chosen factors on the pig immunisation process, such as stress, faecal microbiota, host genetics, the presence of MDAs, infections with immunosuppressive pathogens, and treatment with antibiotics and mycotoxins. Many of them turned out to have an adverse impact on vaccine efficacy.

The first dog-origin porcine circovirus type 4 complete genomic sequence have high homology with that of pig-derived strains

Introduction: Porcine circovirus 4 (PCV4) was discovered in 2019 and then proved to be pathogenic to piglets. Nevertheless, few studies were currently available about PCV4 infection in species other than pigs and there is no information about the prevalence of PCV4 in dogs. Methods: To fill this gap, 264 dog samples were collected from animal hospitals in the Southwest of China from 2021 to 2022 and screened for PCV4. Moreover, the complete genome of one PCV4 strain (SCABTC-Dog2022) were obtained successfully and shared a high identity (97.9-99.0%) with other PCV4 strains derived from pigs, dairy cows, raccoon dogs and foxes. The SCABTC-Dog2022 were analyzed together with 51 reference sequences. Results and Discussion: The detected results showed a low percentage of PCV-4 DNA (1.14%, 3/264), indicating that PCV4 could be identified in dogs in southwest China. Phylogenetic tree showed that SCABTC-Dog2022 strain derived from dog were clustered in a closed relative and geographically coherent branch with other PCV4 strains collected from four provinces (Sichuan, Fujian, Hunan and Inner Mongolia) of China. To our knowledge, it is the first detection of PCV4 in dogs globally. The association between PCV4 status and clinical syndromes in dogs deserves additional investigations.

Development of a TaqMan-Probe-Based Multiplex Real-Time PCR for the Simultaneous Detection of Porcine Circovirus 2, 3, and 4 in East China from 2020 to 2022

Porcine circovirus disease (PCVD) caused by porcine circovirus (PCV) is an important swine disease that is characterized by porcine dermatitis, nephropathy syndrome, and reproductive disorders in sows. However, disease caused by PCV2, PCV3, or PCV4 is hard to distinguish, so a rapid and sensitive detection method is urgently needed to differentiate these three types. In this study, four pairs of specific primers and the corresponding probes for PCV 2, -3, and -4, and porcine endogenous gene β-Actin as the positive internal reference index, were designed to establish a TaqMan multiplex real-time PCR (qPCR) assay for the simultaneous differential diagnosis of different types of viruses. The results showed that this assay has good specificity and no cross-reactivity with other important porcine viral pathogens. Furthermore, it has high sensitivity, with a detection limit of 101 copies/μL, and good reproducibility, with intra- and inter-group coefficients of variation below 2%. Subsequently, 535 clinical samples of suspected sow reproductive disorders collected from Shandong, Zhejiang, Anhui, and Jiangsu provinces from 2020 to 2022 were analyzed using the established assay. The results showed that the individual positive rates of PCV2, PCV3, and PCV4 were 31.03%, 30.09%, and 30.84%, respectively; the mixed infection rates of PCV2 and PCV3, PCV2 and PCV4, and PCV3 and PCV4 were 31.03%, 30.09%, and 30.84%, respectively; the mixed infection rate of PCV2, PCV3, and PCV4 was 28.22%. This indicated that this assay provides a convenient tool for the rapid detection and differentiation of PCV2, PCV3, and PCV4 in pig farms in East China. Our findings highlight that there are different types of porcine circovirus infection in pig farms in East China, which makes pig disease prevention and control difficult.

Detection and Complete Genome Analysis of Porcine Circovirus 2 (PCV2) and an Unclassified CRESS DNA Virus from Diarrheic Pigs in the Dominican Republic: First Evidence for Predominance of PCV2d from the Caribbean Region

We report here high rates (47.5%, 48/101) of detection of porcine circovirus 2 (PCV2) in diarrheic pigs from three pig farms in the Dominican Republic. Seventeen of the PCV2 positive samples, representing the three pig farms, different age groups and sampling periods (2020–2021), were amplified for the complete PCV2 genome. Based on analysis of open reading frame 2 and complete genome sequences, the 17 PCV2 strains were assigned to the PCV2d genotype. Significant differences were observed in PCV2 detection rates between the vaccinated (20% (10/50)) and unvaccinated (62.5% (10/16) and 80% (28/35)) farms, corroborating previous observations that PCV2a-based vaccines confer protection against heterologous PCV2 genotypes. The present study is the first to report detection and molecular characterization of PCV2 from the Dominican Republic, warranting large-scale molecular epidemiological studies on PCV2 in pig farms and backyard systems across the country. For the first time, PCV2d was identified as the predominant PCV2 genotype in a study from the Caribbean region, suggesting that a genotype shift from PCV2b to PCV2d might be happening in the Caribbean region, which mirrored the current PCV2 genotype scenario in many other parts of the world. Besides PCV2, we also identified a pigeon circovirus-like virus, and a circular Replication-associated protein (Rep)-encoding single-stranded (CRESS) DNA virus, which was characterized for the complete genome. The CRESS DNA virus shared a similar genomic organization and was related to unclassified CRESSV2 DNA viruses (belonging to the Order Cirlivirales) from porcine feces in Hungary, indicating that related unclassified CRESS DNA viruses are circulating among pigs in different geographical regions, warranting further studies on the epidemiology and biology of these novel viruses.

Cross-species transmission of an emerging porcine circovirus (PCV4): First molecular detection and retrospective investigation in dairy cows

Porcine circovirus 4 (PCV4), a novel porcine circovirus identified in pigs, has recently been proved to be pathogenic to piglets. However, little is known about its cross-species transmission, and demonstration of PCV4 in dairy cows is lacking. To explore whether the PCV4 genome exists in dairy cows, 1170 fecal samples were collected from dairy farms in 7 cities in Henan Province of China during 2012-2021, and screened by qPCR for the presence of PCVs (PCV2-PCV4). The detection results showed that the positive rate of PCV4 in dairy cows was 2.22 % (26/1170), but all fecal samples were negative for PCV2 and PCV3. Three full-length and five partial genomes of PCV4 strains were acquired, of which two PCV4 strains (NY2012-DC and XC2013-DC) were achieved from 2012 and 2013, indicating that PCV4 has been circulating in dairy cows in Henan Province of China for at least 10 years. The three PCV4 strains sequenced in this study shared high identity (97.5-99.5 %) with reference strains at the genome level. In phylogenetic analysis, three genotypes (PCV4a, PCV4b and PCV4c) were temporarily confirmed by analyzing 44 strains, and one amino acid variation in Rep (V239L) and three amino acid variations in Cap (N27S, R28G and M212L) were considered as a conserved genotype specific molecular marker. Analyzed from three perspectives (cross-time, cross-species and transboundary), the high nucleotide homology of PCV4 strains indicated the PCV4 evolutionary rate might be slow. Overall, this study was the first to report the detection of PCV4 in dairy cows and conducted a long-term retrospective investigation of PCV4 in Henan Province of China, which has important implications for understanding the genetic diversity and cross-species transmission of the ongoing PCV4 cases.

Live attenuated Salmonella enterica serovar Choleraesuis vector delivering a virus-like particles induces a protective immune response against porcine circovirus type 2 in mice

The virus-like particles (VLPs) of porcine circovirus type 2 (PCV2) is an attractive vaccine candidate that retains the natural conformation of the virion but lacks the viral genome to replicate, thus balancing safety and immunogenicity. However, the assembly of VLPs requires cumbersome subsequent processes, hindering the development of related vaccines. In addition, as a subunit antigen, VLPs are defective in inducing cellular and mucosal immune responses. In this study, the capsid (Cap) protein of PCV2 was synthesized and self-assembled into VLPs in the recombinant attenuated S. Choleraesuis vector, rSC0016(pS-Cap). Furthermore, rSC0016(pS-Cap) induced a Cap-specific Th1-dominant immune response, mucosal immune responses, and neutralizing antibodies against PCV2. Finally, the virus genome copies in mice immunized with the rSC0016(pS-Cap) were significantly lower than those of the empty vector control group after challenge with PCV2. In conclusion, our study demonstrates the potential of using S. Choleraesuis vectors to delivery VLPs, providing new ideas for the development of PCV2 vaccines.

Plasma Virome Reveals Blooms and Transmission of Anellovirus in Intravenous Drug Users with HIV-1, HCV, and/or HBV Infections

Intravenous drug users (IDUs) are a high-risk group for HIV-1, hepatitis C virus (HCV), and hepatitis B virus (HBV) infections, which are the leading causes of death in IDUs. However, the plasma virome of IDUs and how it is influenced by above viral infections remain unclear. Using viral metagenomics, we determined the plasma virome of IDUs and its association with HIV-1, HCV, and/or HBV infections. Compared with healthy individuals, IDUs especially those with major viral infections had higher viral abundance and diversity. Anelloviridae dominated plasma virome. Coinfections of multiple anelloviruses were common, and anelloviruses from the same genus tended to coexist together. In this study, 4,487 anellovirus ORF1 sequences were identified, including 1,620 (36.1%) with less than 69% identity to any known sequences, which tripled the current number. Compared with healthy controls (HC), more anellovirus sequences were observed in neg-IDUs, and HIV-1, HCV, and/or HBV infections further expanded the sequence number in IDUs, which was characterized by the emergence of novel divergent taxons and blooms of resident anelloviruses. Pegivirus was mainly identified in infected IDUs. Five main pegivirus transmission clusters (TCs) were identified by phylogenetic analysis, suggesting a transmission link. Similar anellovirus profiles were observed in IDUs within the same TC, suggesting transmission of anellome among IDUs. Our data suggested that IDUs suffered higher plasma viral burden especially anelloviruses, which was associated with HIV-1, HCV, and/or HBV infections. Blooms in abundance and unprecedented diversity of anellovirus highlighted active evolution and replication of this virus in blood circulation, and an uncharacterized role it may engage with the host.

IMPORTANCE Virome is associated with immune status and determines or influences disease progression through both pathogenic and resident viruses. Increased viral burden in IDUs especially those with major viral infections indicated the suboptimal immune status and high infection risks of these population. Blooms in abundance and unprecedented diversity of anellovirus highlighted its active evolution and replication in the blood circulation, and sensitive response to other viral infections. In addition, transmission cluster analysis revealed the transmission link of pegivirus among IDUs, and the individuals with transmission links shared similar anellome profiles. In-depth monitoring of the plasma virome in high-risk populations is not only needed for surveillance for emerging viruses and transmission networks of major and neglected bloodborne viruses, but also important for a better understanding of commensal viruses and their role it may engage with immune system.

Current knowledge on epidemiology and evolution of novel porcine circovirus 4

Porcine circovirus type 4 (PCV4) is a newly emerging virus, with both PCV4 genomic DNA and specific antibodies detected in swine herds in several provinces in China and South Korea. Although the virus was first identified in 2019 in Hunan, China, retrospective research suggests that serum samples collected as early as 2008 were positive for PCV4 antibody. Infections with only PCV4 or co-infections with other pathogens have been associated with several clinical manifestations, but its pathogenesis remains to be determined. The purpose of this review was the following: (1) to characterize PCV4 epidemiology by assessing evolutionary dynamics and genetic diversity of PCV4 strains circulating in swine herds; (2) to reconstruct a computerized 3D model to analyze PCV4 Cap properties; (3) and to summarize the current evidence of PCV4-associated clinical-pathological manifestations. The origin of PCV4 is apparently distinct from other PCV, based on analysis of phylogenetic trees. Of note, PCV4 shares an ancient common ancestor with mink circoviruses. Furthermore, the amino acid residue at position 27 of the PCV4 Cap is a key benchmark to distinguish PCV4a (²⁷S) from PCV4b (²⁷ N), based on PCV4 strains currently available, and variation of this residue may alter Cap antigenicity. In addition, the capsid surface of PCV4 has characteristics of increased polar residues, compared to PCV2, which raises the possibility that PCV4 may target negatively charged host receptors to promote virus infection. Further studies are required, including virus isolation and culture, and more detailed characterization of molecular epidemiology and genetic diversity of PCV4 in swine herds.