Recent progress on porcine circovirus type 3

Molecular detection and genetic characteristics of porcine circovirus 3 and porcine circovirus 4 in central China

Porcine circoviruses (PCVs) are a significant cause of concern for swine health, with four genotypes currently recognized. Two of these, PCV3 and PCV4, have been detected in pigs across all age groups, in both healthy and diseased animals. These viruses have been associated with various clinical manifestations, including porcine dermatitis and nephropathy syndrome (PDNS) and respiratory and enteric signs. In this study, we detected PCV3 and PCV4 in central China between January 2022 and February 2023. We tested fecal swabs and tissue samples from growing-finishing and suckling pigs with or without respiratory and systemic manifestations and found the prevalence of PCV3 to be 15.15% (15/99) and that of PCV3/PCV4 coinfection to be 4.04% (4/99). This relatively low prevalence might be attributed to the fact that most of the clinical samples were collected from pigs exhibiting respiratory signs, with only a few samples having been obtained from pigs with diarrhea. In some cases, PCV2 was also detected, and the coinfection rates of PCV2/3, PCV2/4, and PCV2/3/4 were 6.06% (6/99), 5.05% (5/99), and 3.03% (3/99), respectively. The complete genomic sequences of four PCV3 and two PCV4 isolates were determined. All four of the PCV3 isolates were of subtype PCV3b, and the two PCV4 isolates were of subtype PCV4b. Two mutations (A24V and R27K) were found in antibody recognition domains of PCV3, suggesting that they might be associated with immune escape. This study provides valuable insights into the molecular epidemiology and evolution of PCV3 and PCV4 that will be useful in future investigations of genotyping, immunogenicity, and immune evasion strategies.

Construction of recombinant pseudorabies virus expressing PCV2 Cap, PCV3 Cap, and IL-4: investigation of their biological characteristics and immunogenicity

Background

Porcine circovirus type 2 (PCV2) is a globally prevalent and recurrent pathogen that primarily causes slow growth and immunosuppression in pigs. Porcine circovirus type 3 (PCV3), a recently discovered virus, commonly leads to reproductive disorders in pigs and has been extensively disseminated worldwide. Infection with a single PCV subtype alone does not induce severe porcine circovirus-associated diseases (PCVD), whereas concurrent co-infection with PCV2 and PCV3 exacerbates the clinical manifestations. Pseudorabies (PR), a highly contagious disease in pigs, pose a significant threat to the swine industry in China.

Methods

In this study, recombinant strains named rPRV-2Cap/3Cap and rPRV-2Cap/3Cap/IL4 was constructed by using a variant strain XJ of pseudorabies virus (PRV) as the parental strain, with the TK/gE/gI genes deleted and simultaneous expression of PCV2 Cap, PCV3 Cap, and IL-4. The two recombinant strains obtained by CRISPR/Cas gE gene editing technology and homologous recombination technology has genetic stability in baby hamster Syrian kidney-21 (BHK-21) cells and is safe to mice.

Results

rPRV-2Cap/3Cap and rPRV-2Cap/3Cap/IL4 exhibited good safety and immunogenicity in mice, inducing high levels of antibodies, demonstrated 100% protection against the PRV challenge in mice, reduced viral loads and mitigated pathological changes in the heart, lungs, spleen, and lymph nodes during PCV2 challenge. Moreover, the recombinant viruses with the addition of IL-4 as a molecular adjuvant outperformed the non-addition group in most indicators.

Conclusion

rPRV-2Cap/3Cap and rPRV-2Cap/3Cap/IL4 hold promise as recombinant vaccines for the simultaneous prevention of PCV2, PCV3, and PRV, while IL-4, as a vaccine molecular adjuvant, effectively enhances the immune response of the vaccine.

Prevalence of porcine viral respiratory diseases in India

The pig industry is growing rapidly in India and contributes a major share of growth in the livestock sector. Over the last few years, there is a gradual increase in the adoption of pigs for production by economically weaker sections of the country. However, this production is affected by many respiratory diseases which are responsible for significant economic loss. The occurrence and impact of these diseases are still under-documented. The four important pathogens including porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza A viruses (SIV) and classical swine fever virus (CSFV) are documented here. These diseases are highly devastating in nature and frequent outbreaks have been reported from different parts of the country. The rapid and specific diagnosis, effective prevention and control measures are required for the eradication of these diseases which is urgently required for the growth of the pig industry. This review highlights the prevalence, epidemiology, diagnostics and information gaps on important respiratory viral pathogens of pigs reported from different parts of India. This review also emphasizes the importance of these viral diseases and the urgent need to develop vaccines and effective measures for the eradication of these diseases.

Diverse single-stranded DNA viruses from viral metagenomics on a cynopterus bat in China

Bats serve as reservoirs for many emerging viruses. Cressdnaviruses can infect a wide range of animals, including agricultural species, such as pigs, in which porcine circoviruses cause severe gastroenteritis. New cressdnaviruses have also attracted considerable attention recently, due to their involvement with infectious diseases. However, little is known about their host range and many cressdnaviruses remain poorly characterized. We identified and characterized 11 contigs consisting of previously unknown cressdnaviruses from a rectal swab sample of a Cynopterus bat collected in Yunnan Province, China, in 2011. Full genomes of two cressdnaviruses (OQ267680, 2069 nt; OQ351951, 2382 nt), and a nearly complete genome for a third (OQ267683, 2361 nt) were obtained. Phylogenetic analyses and the characteristics of these viral genomes suggest a high degree of ssDNA virus diversity. These results shed light on cressdnavirus diversity and the probable role of Cynopterus bats as their hosts.

Chromogenic in situ hybridization technique for detecting porcine circovirus 3 in lung and lymphoid tissues

Background and Aim

Porcine circovirus 3 (PCV3) was recently reported in Malaysian commercial pig population in 2020 by conventional polymerase chain reaction (PCR), revealing a molecular prevalence of 17.02% in the sampled domestic pig population. This study aims to describe a chromogenic in situ hybridization (ISH) technique using digoxigenin (DIG)-labeled cloned PCV3 open reading frame 1 (ORF1) fragment DNA to detect and localize the PCV3 antigen in formalin-fixed, paraffin-embedded lung, and lymphoid tissue specimens.

Materials and Methods

Since PCV3 was mainly detected in lung and lymphoid tissues, we obtained tissue specimens from these organs from the previous Malaysian PCV3 study. Digoxigenin-labeled ISH probes were designed to target a 69 bp region of PCV3 ORF1 spanning from the nucleotide positions (282-350).

Results

Light microscopy analysis revealed that chromogenic staining of PCV3 antigens was visualized within the cytoplasm of pneumocytes and lymphocytes, indicating positive ISH results. The results of molecular detection of PCV3 using PCR and ISH showed a high agreement of 90.91%, including for the negative PCV3 status for all samples.

Conclusion

This study reports a chromogenic ISH technique using DIG-labeled probes targeting PCV3 ORF1 to detect PCV3 antigens in lung and lymphoid tissues. Despite the limited availability of PCV3 antibodies, ISH remains relevant for investigating PCV3 replication and pathogenesis and can be used complementarily with PCR for evaluating the localization of antigens in infected tissues.

Molecular detection and genetic characterization of porcine circovirus 4 (PCV4) in Thailand during 2019-2020

Porcine circovirus 4 (PCV4) is considered a novel PCV, firstly found in China in 2019 and later discovered in Korea. This present study investigated the prevalence and genetic characteristics of PCV4 from high pig-density areas in Thailand during 2019-2020. From 734 samples, three samples (0.4%) from aborted fetuses and porcine respiratory disease complex (PRDC) cases were found positive for PCV4, two of the PCV4-positive samples were coinfected with both PCV2 and PRRSV, and the other PCV4-positive sample was found coinfected with PCV2. In situ hybridization (ISH) revealed the presence of PCV4 in the bronchial epithelial cells and in lymphocytes and histiocyte-like cells in the lymphoid follicles of the PRDC-affected pig. The complete Thai PCV4 genome had over 98% nucleotide identity with other PCV4 strains and was closely related to the Korean and Chinese PCV4b strains. Importantly, the amino acid residue at position 212 of the Cap gene is recommended for differentiating PCV4a (²¹²L) from PCV4b (²¹²M) based on currently available PCV4 genome sequences. These findings provide important clues for the pathogenesis, epidemiology, and genetic characteristics of PCV4 in Thailand.

Porcine circoviruses in wild boars in Nagano Prefecture, Japan

We investigated the prevalence of porcine circovirus 2 (PCV2) and 3 (PCV3) in wild boars captured or found dead in Nagano Prefecture in 2020. Based on PCR testing, 21 of 254 (8.3%) wild boars were PCV2-positive and 43 of 256 (16.8%) wild boars were PCV3-positive, 5 of 253 (2.0%) wild boars were both PCV2-positive and PCV3-positive. The frequency of detecting PCV3 in wild boars was significantly higher in adults than in juveniles (P=0.014). The PCV2-positive wild boars were found in all districts except for the North Alps and Hokushin, while PCV3-positive wild boars were found in all districts except for Suwa. This is the first report of PCV2 and PCV3 detected in wild boars in Japan.

Prevalence and phylogenetic analysis of porcine circovirus type 2 (PCV2) and type 3 (PCV3) in the Southwest of China during 2020-2022

Introduction

Porcine circovirus type 2 (PCV2) is considered one of the viruses with substantial economic impact on swine industry in the word. Recently, porcine circovirus type 3 (PCV3) has been found to be associated with porcine dermatitis and nephropathy syndrome (PDNS)-like disease. And the two viruses were prone to co-infect clinically.

Methods

To further investigate the prevalence and genetic diversity of the two viruses, 257 pig samples from 23 different pig farms in southwest China with suspected PCVAD at different growth stages were analyzed by real-time PCR between 2020 and 2022 to determine the presence of PCV2 and PCV3.

Results

Results showed high prevalence of PCV2 and PCV3: 26.46% samples were PCV2 positive and 33.46% samples were PCV3 positive. The coinfection rate was doubled from 2020 (5.75%) to 2022 (10.45%). Subsequently, the whole genome sequences of 13 PCV2 and 18 PCV3 strains were obtained in this study. Of these, 1 strain was PCV2a, 5 strains were PCV2b and 7 strains were PCV2d, indicating that PCV2d was the predominant PCV2 genotype prevalent in the Southwest of China.

Discussion

In addition, the phylogenetic analysis of PCV3 showed high nucleotide homology (>98%) between the sequences obtained in this study and reference sequences. And 3 mutations (A24V, R27K and E128D) were found in PCV3 antibody recognition domains, which might be related to the mechanism of viral immune escape. Thus, this study will enhance our understanding of the molecular epidemiology and evolution of PCV2 and PCV3, which are conducive to the further study of the genotyping, immunogenicity and immune evasion of PCVs.

Expression and immunogenicity analysis of the capsid proteins of porcine circovirus types 2 to 4

Porcine circovirus (PCV) comprises four types, PCV1, PCV2, PCV3, and PCV4, which belong to the Circovirus genus of the family Circoviridae. PCV1 is nonpathogenic, whereas PCV2, PCV3, and PCV4 can infect pigs and cause disease. However, due to a lack of experimental evidence, whether vaccines based on PCV capsid (Cap) can induce cross-reactivity against PCVs remains controversial. In this study, recombinant truncated capsids (rCaps) of PCV2, PCV3, and PCV4 were highly and efficiently expressed and purified, followed by the development and evaluation of antibodies against PCVs. The results showed that monovalent and trivalent antigens based on the recombinant Caps had adequate immunogenicity to stimulate specific antibodies against the corresponding protein and virus. Furthermore, antisera prepared from the recombinant Caps also cross-reacted with different PCVs. Therefore, recombinant proteins can be used as candidate antigens to develop vaccines and ELISA diagnostic kits. In addition, the antibodies prepared in this study are promising candidates for the simultaneous prevention and treatment of PCVs in the clinic.

Advances in Crosstalk between Porcine Circoviruses and Host

Porcine circoviruses (PCVs), including PCV1 to PCV4, are non-enveloped DNA viruses with a diameter of about 20 nm, belonging to the genus Circovirus in the family Circoviridae. PCV2 is an important causative agent of porcine circovirus disease or porcine circovirus-associated disease (PCVD/PCVAD), which is highly prevalent in pigs and seriously affects the swine industry globally. Furthermore, PCV2 mainly causes subclinical symptoms and immunosuppression, and PCV3 and PCV4 were detected in healthy pigs, sick pigs, and other animals. Although the pathogenicity of PCV3 and PCV4 in the field is still controversial, the infection rates of PCV3 and PCV4 in pigs are increasing. Moreover, PCV3 and PCV4 rescued from infected clones were pathogenic in vivo. It is worth noting that the interaction between virus and host is crucial to the infection and pathogenicity of the virus. This review discusses the latest research progress on the molecular mechanism of PCVs–host interaction, which may provide a scientific basis for disease prevention and control.

Porcine Circovirus Type 4 Strains Circulating in China Are Relatively Stable and Have Higher Homology with Mink Circovirus than Other Porcine Circovirus Types

Porcine circovirus type 4 (PCV4) is a newly identified porcine circovirus (PCV) belonging to the Circovirus genus Circoviridae family. Although several groups have conducted epidemiological investigations on PCV4 and found that PCV4 also exists widely in pigs, there are few reports on the origin and evolution of PCV4. In this study, the genetic relationship between PCV4, mink circovirus (MiCV), bat circovirus (BtCV), PCV1, PCV2, and PCV3 was analyzed, and the consistency of viral proteins in three-dimensional (3D) structure and epitopes was predicted. We found that the genome and protein structure of PCV4 was relatively stable among current circulating PCV4 strains. Furthermore, PCV4 was more similar to MiCV in terms of its genome, protein structure, and epitope levels than other PCVs and BtCVs, suggesting that PCV4 may be derived from MiCV or have a common origin with MiCV, or mink may be an intermediate host of PCV4, which may pose a great threat to other animals and/or even human beings. Therefore, it is necessary to continuously monitor the infection and variation of PCV4, analyze the host spectrum of PCV4, and establish the prevention and treatment methods of PCV4 infection in advance.

Porcine Circoviruses and Herpesviruses Are Prevalent in an Austrian Game Population

During the annual hunt in a privately owned Austrian game population in fall 2019 and 2020, 64 red deer (Cervus elaphus), 5 fallow deer (Dama dama), 6 mouflon (Ovis gmelini musimon), and 95 wild boars (Sus scrofa) were shot and sampled for PCR testing. Pools of spleen, lung, and tonsillar swabs were screened for specific nucleic acids of porcine circoviruses. Wild ruminants were additionally tested for herpesviruses and pestiviruses, and wild boars were screened for pseudorabies virus (PrV) and porcine lymphotropic herpesviruses (PLHV-1-3). PCV2 was detectable in 5% (3 of 64) of red deer and 75% (71 of 95) of wild boar samples. In addition, 24 wild boar samples (25%) but none of the ruminants tested positive for PCV3 specific nucleic acids. Herpesviruses were detected in 15 (20%) ruminant samples. Sequence analyses showed the closest relationships to fallow deer herpesvirus and elk gammaherpesvirus. In wild boars, PLHV-1 was detectable in 10 (11%), PLHV-2 in 44 (46%), and PLHV-3 in 66 (69%) of animals, including 36 double and 3 triple infections. No pestiviruses were detectable in any ruminant samples, and all wild boar samples were negative in PrV-PCR. Our data demonstrate a high prevalence of PCV2 and PLHVs in an Austrian game population, confirm the presence of PCV3 in Austrian wild boars, and indicate a low risk of spillover of notifiable animal diseases into the domestic animal population.

Exploratory Study of the Frequency of Detection and Tissue Distribution of Porcine Circovirus 3 (PCV-3) in Pig Fetuses at Different Gestational Ages

Porcine circovirus 3 (PCV-3) has been associated with several pig diseases. Despite the pathogenicity of this virus has not been completely clarified, reproductive disorders are consistently associated with its infection. The aim of the present work was to analyze the presence of PCV-3 DNA in tissues from pig fetuses from different gestational timepoints. The fetuses were obtained either from farms with no reproductive problems (NRP, n = 249; all of them from the last third of gestation) or from a slaughterhouse (S, n = 51; 49 of the second-third of gestation and 2 from the third one). Tissues collected included brain, heart, lung, kidney, and/or spleen. Overall, the frequency of detection of PCV-3 was significantly higher in fetuses from the last third of the gestation (69/251, 27.5%) when compared to those from the second-third (5/49, 10.2%), although the viral loads were not significantly different. Moreover, the frequency of detection in NRP fetuses (69/249, 27.7%) was significantly higher than in S ones (5/51, 9.8%). Furthermore, PCV-3 DNA was detected in all tissue types analyzed. In conclusion, the present study demonstrates a higher frequency of PCV-3 DNA detection in fetuses from late periods of the gestation and highlights wide organ distributions of the virus in pig fetuses.

Comprehensive Analysis of Codon Usage Patterns in Chinese Porcine Circoviruses Based on Their Major Protein-Coding Sequences

Porcine circoviruses (PCVs) are distributed in swine herds worldwide and represent a threat to the health of domestic pigs and the profits of the swine industry. Currently, four PCV species, including PCV-1, PCV-2, PCV-3 and PCV-4, have been identified in China. Considering the ubiquitous characteristic of PCVs, the new emerged PCV-4 and the large scale of swine breeding in China, an overall analysis on codon usage bias for Chinese PCV sequences was performed by using the major proteins coding sequences (ORF1 and ORF2) to better understand the relationship of these viruses with their host. The data from genome nucleotide frequency composition and relative synonymous codon usage (RSCU) analysis revealed an overrepresentation of AT pair and the existence of a certain codon usage bias in all PCVs. However, the values of an effective number of codons (ENC) revealed that the bias was of low magnitude. Principal component analysis, ENC-plot, parity rule two analysis and correlation analysis suggested that natural selection and mutation pressure were both involved in the shaping of the codon usage patterns of PCVs. However, a neutrality plot revealed a stronger effect of natural selection than mutation pressure on codon usage patterns. Good host adaptation was also shown by the codon adaptation index analysis for all these viruses. Interestingly, obtained data suggest that PCV-4 might be more adapted to its host compared to other PCVs. The present study obtained insights into the codon usage pattern of PCVs based on ORF1 and ORF2, which further helps the understanding the molecular evolution of these swine viruses.

Recent Progress on Epidemiology and Pathobiology of Porcine Circovirus 3

The recently discovered porcine circovirus 3 (PCV3) belongs to the Circovirus genus of the Circoviridae family together with the other three PCVs, PCV1, PCV2, and PCV4. As reported, PCV3 can infect pig, wild boar, and several other intermediate hosts, resulting in single or multiple infections in the affected animal. The PCV3 infection can lead to respiratory diseases, digestive disorders, reproductive disorders, multisystemic inflammation, and immune responses. Up to now, PCV3 infection, as well as the disease caused by PCV3, has been reported in many swine farms worldwide with high positive rates, which indicates that the virus may be another important pathogen in the swine industry. Therefore, we reviewed the current progress on epidemiology and pathobiology of PCV3, which may provide the latest knowledge of the virus and PCV3-related diseases.

Single Multiple Cross Displacement Amplification for Rapid and Real-Time Detection of Porcine Circovirus 3

Since 2016, a novel porcine circovirus, PCV3, has been infecting pigs, causing significant economic losses to the pig industry. In recent years, the infection rate of PCV3 has been increasing, and thus rapid and accurate detection methods for PCV3 are essential. In this study, we established a novel probe-based single multiple cross displacement amplification (P-S-MCDA) method for PCV3. The method was termed as P-S-MCDA. The P-S-MCDA uses seven primers to amplify the capsid gene, and the assay can be performed at 60°C for 30 min, greatly shortening the reaction time. The results of P-S-MCDA can not only be monitored in real time through fluorescence signals but also be determined by observing the fluorescence of the reaction tubes using a smartphone-based cassette. This method demonstrated good specificity and the same sensitivity as qPCR, with a minimum detection limit of 10 copies. In 139 clinical samples, the coincidence rate with qPCR was 100%. The P-S-MCDA can be widely applied in PCV3 detection in laboratories or in rural areas.

First detection and phylogenetic analysis of porcine circovirus 3 in female donkeys with reproductive disorders

Background

PCV3 is a pathogen associated with porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, and cardiac and multiorgan inflammation, which was newly identified in 2016 in sows in USA. Recently, PCV3 has also been identified from several non-porcine species like (cattle, dog, wild boar, deer, mice and ticks). However, PCV3 infection in donkey is not well established. Since 2019, 300 blood samples were collected from female donkey, which was characterized by abortion and sterility, in Liaocheng city of China.

Results

In the present study, an investigation of PCV3 in donkey blood samples was undertaken employing by real time PCR. Positive rates of PCV3 in donkeys reach to 21.0 %. In addition, one full-length PCV3 genome sequence was obtained, and it had a highest identity with porcine circovirus 3 PCV3/CN/Nanjing2017 strain and is clustered to PCV3a genotype based on ORF2 sequences.

Conclusions

This is the first report of detection of PCV3 from female donkeys presenting reproductive failure in large-scale donkey farms, China. In addition, the PCV3 strain identified in this study shared the closest relationship with those from porcine, suggesting that PCV3 may be transmitted from pigs to donkeys. Totally, PCV3 infection in donkey should be concerned although the association between it and reproductive failure are not better understood.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03013-6.

Genetic Diversity of Porcine Circovirus Subtypes from Aborted Sow Fetuses in Vietnam

Porcine circovirus type 3 (PCV3) is an emerging circovirus that is highly distributed among swine worldwide and associated with porcine dermatitis and nephropathy syndrome, reproductive failure, and multisystemic inflammation. Here, we investigated and characterized PCV3 from aborted fetuses in Vietnam. We found that the whole genomes of PCV3 collected in these Vietnamese pig farms share 98.4-99.45% sequence identity with reference PCV3 sequences. Several distinct mutation were identified in both the Rep protein and Cap protein of these strains. These strains were clustered into two distinct subtypes (3a1 and 3b). This study contributes to a better understanding of the molecular characteristics and genetic diversity of PCV3 in Vietnam.

Porcine circovirus 2 capsid protein produced in N. benthamiana forms virus-like particles that elicit production of virus-neutralizing antibodies in guinea pigs

Porcine circovirus type 2 (PCV2) is a non-enveloped, icosahedral virus of the Circoviridae family, with a small, circular, single-stranded DNA genome. PCV2 infections cause substantial economic losses in the pig industry worldwide. Currently, commercially produced PCV2 vaccines are expensive, whereas plant-based expression systems can produce recombinant proteins at low cost for use as vaccines. In this study, recombinant PCV2 capsid protein (rCap) was transiently expressed in Nicotiana benthamiana and purified by metal affinity chromatography, with a yield of 102 mg from 1 kg plant leaves. Electron microscopy confirmed that purified rCap self-assembled into virus-like particles (VLPs) at neutral pH. It was shown to provoke a strong immune response in guinea pigs. The results indicate that plant systems can enable production of large amounts of proteins to serve as candidates for subunit vaccines.

Emergence of porcine circovirus-like viruses associated with porcine diarrheal disease in China

Background

The circular replication‐associated protein (Rep)‐encoding single‐stranded (CRESS) DNA virus emergence in diverse host has been associated with severe disease. Porcine circovirus‐like virus (Po‐Circo‐like [PCL] virus) is a CRESS DNA virus, the prevalence and pathogenicity of which are rarely studied.

Methods

We obtained two blood samples, four faecal samples, and two intestinal samples from a pig farm suffered from diarrheal disease in the delivery room in September 2020 and attempted to isolate and identify a causative pathogen. Subsequently, only PCL virus was positive, and qRT‐PCR was designed to detect the loading titre of PCL virus. We then initiated a heightened surveillance program on the pathogenicity and epidemiology of PCL virus.

Results

Six PCL virus strains, with severe diarrhoea and haemorrhagic enteritis, have been found in six different pig farms in Guangdong province, China. A multiple sequence alignment of these PCL viruses and bovine circovirus‐like virus/CH showed a similarity of 92.5‐94.8% for the Rep protein, indicating these PCL viruses are highly homologous to Bo‐Circo‐like virus associated with calf diarrhoea. There were striking similarities between the PCL virus and bovine circovirus‐like virus outbreaks in aetiological settings and Genomic sequence. We found that 11.2% (20/178) of diarrhoea samples and 13.3% (6/45) of pig farms were positive for PCL virus, suggesting that PCL virus may have spread widely in Pig farms. Moreover, this article underscores the risk of PCL virus spilling over and adapting to new species.

Conclusions

Porcine circovirus‐like virus was found to be associated with porcine diarrheal disease in China.

The seroprevalence of the newly identified porcine circovirus type 4 in China investigated by an enzymed-linked immunosorbent assay

Porcine circovirus 4 (PCV4) is the fourth porcine circovirus newly identified in China, and it could be detected in diseased and healthy pigs. To date, the prevalence of PCV4 DNA in pig herds has been investigated in many provinces from both China and Korea, with positive rates varied from 3.28% to 25.4% in samples from different regions. However, up to now no serological data have been reported to evaluate the prevalence of PCV4 in pig herds. In this study, an indirect anti-PCV4 IgG enzyme-linked immunosorbent assay (ELISA) based on replicase protein (Rep) was developed and utilized to investigate the seroprevalence of PCV4 in pig herds of China. A total of 1790 swine serum samples from 17 provinces of China were tested including samples confirmed positive for PCV4 DNA. There was no cross-reactivity of this ELISA with PCV1, PCV2 and PCV3. PCV4 Rep antibodies have been detected in serum samples from 16 out of 17 provinces in China. The PCV4 overall seroprevalence was 43.97%, with the highest of 67.8% been detected in sows, followed by fattening and suckling pigs with positive rates of 35.0% and 14.56%, respectively, and the lowest of 12.61% been detected in nursery pigs. Moreover, from the present data, the earliest positive sample could be retrieved to at least 2008. The present study provides an overall seroprevalence of PCV4 in China, and is helpful to understand the prevalence of PCV4 in the pig herds since it was discovered.

Porcine circovirus type 3 capsid protein induces NF-κB activation and upregulates pro-inflammatory cytokine expression in HEK-293T cells

Porcine circovirus type 3 (PCV3) has been widely detected throughout the world since it was first discovered on pig farms in 2015. PCV3 is closely associated with cardiac and multisystem inflammation, respiratory disease, congenital tremors, myocarditis, diarrhea, encephalitis and neurologic disease, and periarteritis. However, there have been few reports on the relationship between PCV3 and inflammatory pathways. The NF-κB signaling pathway plays an important role in the defense against viral infection. Here, we demonstrate that the capsid protein (Cap) of PCV3 plays a key role in the activation of NF-κB signaling in HEK-293T cells. Furthermore, PCV3 Cap promotes the mRNA expression of the pro-inflammatory cytokines IL6 and TNFα. In addition, PCV3 Cap promotes RIG-I and MDA5 mRNA expression in RIG-like receptor (RLR) signaling and MyD88 mRNA expression in Toll-like receptor (TLR) signaling but does not influence TRIF mRNA expression in TLR signaling. These results show that PCV3 Cap activates NF-κB signaling, possibly through the RLR and the TLR signaling pathways. This work illustrates that PCV3 Cap activates NF-κB signaling and thus may provide a basis for the pathogenesis of PCV3 and the innate immunity of the host.

Identification and whole-genome characterization of a novel Porcine Circovirus 3 subtype b strain from swine populations in Vietnam

Porcine circovirus 3 (PCV3) is a novel circovirus detected in pigs suffering from porcine dermatitis and nephropathy syndrome (PDNS), reproductive failure, and multisystemic infection. In this study, we identified PCV3 infection in aborted fetuses and reported the full-length genome sequence of a PCV3 strain identified from southern Vietnam. The complete genome of this PCV3 strain is 2000 nucleotides in length. We found that it shares 98.5-99.25% sequence identity with other reference sequences and that it clusters with the PCV3b subtype. Several specific mutated sites were found to be unique to this Vietnamese PCV3b strain, including I14M in the Rep protein and K139R, I150F, and P169T in the Cap protein. The sequence data that have been made publically available as part of this study will help investigators to better understand the molecular characteristics, genetic diversity, and evolutionary history of PCV3. Careful and in-depth investigations into the epidemiology, pathogenicity, and the evolution of this novel virus is a matter of urgent economic and agricultural interest in Vietnam.

Retrospective surveillance of porcine circovirus 4 in pigs in Inner Mongolia, China, from 2016 to 2018

A novel circovirus designated "porcine circovirus type 4" (PCV4) was recently reported in pigs with severe clinical disease in Hunan Province, China. Relatively little is known about the molecular epidemiology of this recently discovered virus. In order to assess the prevalence of PCV4 infection in pigs and to analyze its genomic characteristics, 1683 clinical samples were collected in Inner Mongolia, China, from 2016 to 2018. The overall infection rate of PCV4 was 1.6% (27/1683) at the sample level and 21.6% (11/51) at the farm level, with rates ranging from 3.2% (1/31) to 20.0% (6/30) on different PCV4-positive pig farms. In addition, the PCV4 infection rates at both the sample and farm level increased from 2016 to 2018. This also showed that PCV4 was present in pigs in 2016 in China and therefore did not arrive later than this date. Additionally, our findings showed that PCV4 infections had no association with PCV2 or PCV3 infections. We sequenced the complete genomes of three PCV4 strains and found that the PCV4 strains had a high degree of genetic stability but shared less than 80% sequence identity with other circoviruses. We identified six amino acid mutations in the Rep protein and seven in the Cap protein. Phylogenetic analysis based on Cap and Rep sequences confirmed that the PCV4 strains grouped in an independent branch. Our findings provide important information about the prevalence and genetic characteristics of PCV4 strains.

Molecular Investigation of Porcine Circovirus Type 3 Infection in Pigs in Namibia

Porcine circovirus type 3 (PCV-3) infection is widely distributed in domestic pig populations in America, Europe, and Asia. However, no data is currently available about its presence and distribution in Africa. This study investigated the presence of PCV-3 in pigs (n = 122) in Namibia, by means of biomolecular methods. The pig samples collected (n = 122) were representative of the swine industry in Namibia, covering the major pig production facilities in the country. All of the samples tested were negative for PCV-3, and this indicated that the virus was either not present in the country or was circulating at low levels. Further studies are needed to better understand the distribution, if any, of PCV-3 in Namibia.

Field infection of a gilt and its litter demonstrates vertical transmission and effect on reproductive failure caused by porcine circovirus type 3 (PCV3)

BACKGROUND

PCV3 is a member of the Circovirus family, associated with disease and mortality in pigs. It is not clear whether PCV3 putatively causes clinical symptoms and disease. In the present case, we reported a gilt infected with PCV3 associated with reproductive failures, vertical transmission, tissue lesions, viral replication by in situ hybridization, and the hypothesis that some strains of PCV3 clade one are associated with reproductive failures at the field level.

CASE PRESENTATION

In May 2019, a pig farm in Colombia reported increased reproductive failures, and the presence of PCV3 in gilts and sows was established in a single form or coinfections, mainly with PCV2 and PPV7. Ten sows with a single infection with PCV3 were found, and one gilt with a pre-farrowing serum viral load above 10³ was studied. This gilt was followed up during the pre-farrowing, farrowing period and on her litter for 6 weeks. During dystocic farrowing, a mummy and ten piglets were released, including two weak-born piglets. The highest viral loads for PCV3 were found in the mummy and the placenta. In the weak-born piglets, there were viral loads both in serum and in tissues, mainly in the mesenteric ganglia and lung. Replication of PCV3 in these tissues was demonstrated by in situ hybridizations. PCV3 was also found in the precolostrum sera of piglets and colostrum, showing vertical transmission. The viral load in piglets decreased gradually until week six of life. The viral genome's complete sequencing was made from the mummy, and its analysis classified it as PCV3 clade one.

CONCLUSIONS

This report confirms that PCV3 can cause disease at the field level, and putatively, in this case, we find the generation of reproductive failures. The ability of PCV3 to cause disease as a putative pathogen may be associated with the viral load present in the pig and the strain that is affecting the farm. For this case, we found that viral loads above 10³ (4.93 log genomic copies / mL) in the gilt were associated with clinical manifestation and that some PCV3 strains belonging to clade one are more associated with the reproductive presentation.

Genetic diversity of porcine circovirus 3 strains and the first detection of two different PCV3 strains coinfecting the same host in Minas Gerais, Brazil

Porcine circovirus 3 (PCV3) is a recently emerged circovirus discovered in 2016 that has drawn the attention of the swine industry worldwide. In this study, we evaluated the genetic diversity of PCV3 strains on pig farms. A total of 261 samples from sows, weaning pigs, growing pigs, and stillborn/mummified fetuses were analyzed by quantitative real-time PCR. The results revealed that at least two main lineages of PCV3 are circulating in Brazil. For the first time, it was possible to detect the presence of two different PCV3 strains in the same host.

Development of cost-effective quantitative PCR method for parallel detection of porcine circovirus2 and porcine parvovirus in perspective of North-eastern India

Pig farming performs as an intricate part in the socio-economic situation in the north-eastern region of India. This region contributes 38% (3.95 million) of total pigs in India. In spite of this, the region unables to flourish as an enterprise as per the expectation due to a low productivity rate. Porcine infectious pathogens like porcine cirovirus2 (PCV2) and porcine parvovirus (PPV) have a direct economic impact on pig farming through slow growth rate, abortion, and mortality and ultimately maximize the production cost by increasing the usage of antibiotic or antiviral drugs. The veterinary diagnostic infrastructure is a fundamental aspect of the development of livestock status by rapid and effective detection of pathogens. Quantitative PCR (qPCR) is a precise and fast-track technique used for the routine diagnostic method. Hence, we developed a highly precise and comparatively cost-effective SYBR Green reporter dye-based qPCR assay for parallel identification of PCV2 and PPV. In the present assay, the correlation coefficient (R²) value was 0.99, and 10 copies of the gene/μl were the least limit of detection (LOD) concerning both viruses. Melt curve analysis of this study represented PCV2-specific melt curve (Tm) at 81.2 °C and PPV-specific melt curve (Tm) at 73.5 °C. Therefore, the assay easily differentiates the true positive amplicons of PCV2 and PPV through specific Tm values. Among the 50 field samples, 26 (52%) samples were PCV2 positive, 18 (36%) samples PPV positive, and 11 (22%) samples were co-infected of both the viruses. This method is cost-effective, precise, and sensitive to diagnose the concurrent or individual infection of the PCV2 and PPV in the pig. Hence, considering the impact of pig farming in the north-eastern part of the country, the present assay gives an unprecedented achievement in disease diagnosis.

Detection and genetic characterization of porcine circovirus 4 (PCV4) in Guangxi, China

Porcine circovirus type 4 (PCV4), a novel circovirus, was identified in pigs with serious symptoms, including porcine dermatitis and nephropathy syndrome (PDNS)-like signs, in China in 2019. This study investigated the prevalence and genome diversity of PCV4 in pigs from Guangxi Province, China, between 2015 and 2019. Thirteen of 257 (5.1%) samples were positive for PCV4, 9 of these (69.2%) PCV4-positive samples were coinfected with PCV2 or PCV3, and one PCV4-positive sample was coinfected with both PCV2 and PCV3. Three complete PCV4 genomes shared 36.9-73.8% nucleotide similarity with other representative circovirus genomes. Phylogenetic analysis indicated that PCV4 was most closely related to bat-associated circovirus and mink circovirus. In summary, this is the first epidemiological investigation and evolutionary analysis of PCV4 in Guangxi Province, China, and the results provide insight into the molecular epidemiology of PCV4.

Lack of Evidence on the Susceptibility of Ticks and Wild Rodent Species to PCV3 Infection

Porcine circovirus 3 (PCV3) is an emerging virus, first detected in 2016 and widespread in the swine industry. Although not considered a primary pathogen, PCV3 is potentially linked to several clinical conditions that threaten swine farming. Wild boars are considered the main reservoir species for PCV3 infection in the wild, but recent detection in roe deer, chamois and associated ticks has complicated our understanding of its epidemiology. Much emphasis has been placed on ticks, as competent vectors, and wild rodents, which typically feed immature tick stages. The aim of this study was to clarify whether wild rodent species and associated ticks are susceptible to PCV3 infection and involved in its spread. Wild small mammals' serum samples and hosted ticks were, therefore, collected from areas where no wild boars were present and tested by PCR, targeting the PCV3 rep gene. A total of 90 yellow-necked mice (Apodemus flavicollis), two wood mice (A. sylvaticus), 26 bank voles (Myodes glareolus) and 262 Ixodes spp. ticks were investigated. PCV3 DNA was not detected in serum or in tick samples. These findings support the hypothesis that the investigated species do not have an actual role as PCV3 reservoirs. Further studies would be necessary to state whether these species, or others that we did not test, are involved in PCV3 infection spread-in particular when susceptible species share the same habitat.

Complete genome characterization of porcine circovirus 3 recovered from wild boars in Southern Brazil

In the present study, the complete nucleotide sequence of porcine circovirus 3 (PCV3) recovered from wild boars lymph nodes is described. The full genome was named PCV3-wb/Br/RS and comprises 2,000 nucleotides with two open reading frames (ORFs) with a stem-loop motif in intergenic region. The ORFs are oriented in opposite directions and encode the putative capsid (Cap) and replicase (Rep) proteins. Based on amino acid motif analysis, PCV3-wb/Br/RS as well as most of the sequences from wild boars are classified as PCV3b. Phylogenetic analysis including 97 PCV3 sequences available in databases showed that the PCV3-wb/Br/RS genome is more closely related to genomes recovered in Spain, China, Germany and Denmark. Phylogenetic inferences among PCV3-wb/Br/RS and other circoviruses confirmed that these seem to have a most recent common ancestor with bat-associated circoviruses. In addition, PCV3 infection was investigated by real-time PCR in a cohort of 80 wild boars in Southern Brazil. A total of 29 animals (36.3%) were PCV3-positive leading the conclusion that PCV3 is circulating in the wild boar population in Southern Brazil. The role played by PCV3-like infections in wild boars and the risk these could pose to commercial swine production within that region remains to be further investigated.

Prevalence, pathogenesis, and evolution of porcine circovirus type 3 in China from 2016 to 2019

Porcine circovirus type 3 (PCV3) infection causes substantial economic losses in pig populations since its discovery in 2016. However, PCV3 molecular epidemiology remains need further study. In order to assess the prevalence of PCV3 infection in China, 4094 clinical samples from 271 pig farms in 10 provinces of China were evaluated by PCR. It was shown that the overall prevalence of PCV3 infection was 29.3 % (1200/4094) and 74.2 % (201/271) at sample and farm levels respectively, suggesting that PCV3 infection is prevalent in China. Furthermore, a statistical analysis showed PCV3 might exacerbate PCV2 and PRRSV infection rate and have a potential association with pig clinical disease. In addition, we sequenced the entire genome of 57 PCV3 strains; homology analysis showed that PCV3 strains have more than 96 % similarities at the nucleotide level, and PCV3 shares less than 60 % similarities with other circoviruses. By comparing the total 673 PCV3 strains from the NCBI GenBank, we found the major of amino acid mutations are located in predicted epitope regions and the mutations ratio changed during PCV3 evolution. Phylogenetic analysis revealed that all isolates belonged to PCV3a and PCV3b, and increasing PCV3a and decreasing PCV3b trends were observed during PCV3 evolution. Overall, this study provides important insights for understanding PCV3 prevalence, pathogenesis, and evolution and will guide future efforts to develop effective preventive and control measures.

A Novel Technique for Constructing Infectious Cloning of Type 3 Porcine Circovirus

Porcine circovirus type 3 (PCV3), which currently lacks effective preventive measures, has caused tremendous economic losses to the pig husbandry. Obtaining the strain of PCV3 is the key to preparing related vaccines and developing corresponding antiviral drugs. In this study, according to the linear sequence of PCV3 DNA published on GenBank, the sequence was rearranged with SnapGene gene-editing software, and after rearrangement, the HindIII restriction endonuclease site was added to the end of the linear DNA, so that both ends have the same restriction endonuclease site. On this basis, the rearranged linear DNA is obtained by gene synthesis, PCR amplification, DNA purification, etc., and is digested and connected in vitro to obtain cyclized DNA. PCV3 infectious clones were obtained by transfecting 3D4/21 cell lines. The obtained PCV3 was identified by PCR, Western blotting, and indirect immunofluorescence tests. The results showed that this study successfully obtained the strain of PCV3 in vitro. To further evaluate the pathogenicity of the obtained PCV3 infectious clones, this study established an animal model of Kunming mice infected with PCV3. The results of RT-PCR, Western blotting and immunohistochemistry showed that PCV3 can infect myocardium and alveoli of Kunming mice, but no PCV3 was detected in other tissues. The above studies indicate that PCV3 circular DNA can be used to construct PCV3 infectious clones. This research will provide a new method for the construction of circular DNA viruses and lay the foundation for the research and pathogenesis of PCV3 vaccine.

Wide Range of the Prevalence and Viral Loads of Porcine Circovirus Type 3 (PCV3) in Different Clinical Materials from 21 Polish Pig Farms

Porcine circovirus type 3 (PCV3) was described in different clinical cases and healthy pigs. However, little is known about its circulation in pig farms. In order to assess PCV3 prevalence in 21 Polish farms, serum, feces, and oral fluid samples were examined by quantitative real-time PCR. In total, 1451 pairs of serum and feces from the same animals, as well as 327 samples of oral fluids were analyzed. The results showed that PCV3 is more commonly detected in oral fluids (37.3% positives) than in serum (9.7% positives) or feces (15.0% positives) samples. The viral loads detected in these materials ranged from 10^2.5–10^7.2 genome equivalent copies/mL. Although in most farms PCV3 was detected post weaning, in nine farms, the virus was also found in groups of suckling piglets, and in six of them viremia was detected. In four farms with reproductive failure, fetal materials were also obtained. PCV3 was detected in 36.0% of fetuses or stillborn piglets (9/25) with viral loads of 10^3.1–10^10.4 genome equivalent copies/mL. In summary, the virus circulation may show different patterns, and congenital or early infection is not uncommon. Precise quantification of PCV3 loads in clinical materials seems to be necessary for the study and diagnosis of the infection.

Fine mapping of linear B cell epitopes on capsid protein of porcine circovirus 3

Porcine circovirus type 3 (PCV3) is an emerging swine pathogen associated with acute porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, and multisystemic inflammation. Current evidence shows that PCV3 is spread worldwide, and its high incidence may pose a threat to the global pig industry. Capsid (Cap) protein is the sole structural protein which plays an important role in inducing protective immunity against PCV3 infection. In this study, monoclonal antibodies (mAbs) against Cap protein of PCV3 were produced by the hybridoma technique. Subsequently, 12 serial overlapping peptides (P1 to P12) spanning the entire region of Cap were synthesized to determine the B cell epitope regions using the mAbs. Results from dot-blot and peptide ELISA identified that P3, P9, and P10 were the major B cell antigenic regions. Fine mapping by shorter N- and C-terminal truncated peptides confirmed that the motifs ⁵⁷NKPWH⁶¹, ¹⁴⁰KHSRYFT¹⁴⁶, and ¹⁶¹QSLFFF¹⁶⁶ were linear B cell epitopes, which were highly conserved among different PCV3 strains. Interestingly, we found that the motif ¹⁴⁰KHSRYFT¹⁴⁶ was highly conserved in all reported types of PCVs (i.e., PCV1, PCV2, PCV3, and PCV4), except for the substitution (Y → K → R) of the first residue. This is the first research to identify B cell epitopes of PCV3 Cap, and these findings may lead to a better understanding of the antibody-antigen interaction and provide some guidance for PCV3 vaccine design.Key points• The recombinant Cap protein of PCV3 was expressed and purified in soluble form. • PCV3 Cap-specific mAbs prepared in this study had no cross-reactivity with PCV1/PCV2 Cap. • This is the first report of three conserved linear B cell epitopes on PCV3 Cap. • The minimal residues of the epitopes were 57-61 aa, 140-146 aa, and 161-166 aa.

Genetic Characterization of Porcine Circovirus 3 Strains Circulating in Sardinian Pigs and Wild Boars

Porcine circovirus 3 (PCV3) is a recently discovered member of the Circoviridae family. So far, its presence has been reported in North America, Asia, South America, and Europe. In this study, blood and tissue samples from 189 Sardinian suids (34 domestic pigs, 115 feral free ranging pigs, and 39 wild boars) were used to genetically characterize the PCV3 strains from Sardinia. PCV3 infection in the animals was confirmed by real time PCR. The detection rate in the three groups analyzed was l7.64% in domestic pigs, 77.39% in free ranging pigs, and 61.54% in wild boars. Moreover, our results showed that co-infection of PCV3 with other viruses is quite a common occurrence. Molecular characterization of Sardinian PCV3 strains was performed by sequencing 6 complete genomes and 12 complete cap genes. Our results revealed that there is a high similarity between our strains and those identified in different countries, confirming the genetic stability of PCV3 regardless of geographical origin. Haplotype network analysis revealed the presence of 6 whole genomes or 12 unique ORF2 haplotypes and a nonsynonymous mutation in ORF2 that leads to an R14K amino acid substitution. Phylogenetic analysis of whole genome and ORF2 was also conducted. The Sardinian strains were allocated in three different clusters of phylogenetic trees of both complete genome and ORF2. With this study, we have provided a snapshot of PCV3 circulation in Sardinia. Our findings might help to achieve a deeper understanding of this emerging porcine virus.

Phylogenetic analysis of porcine circovirus type 2 (PCV2) between 2015 and 2018 in Henan Province, China

Background

Porcine circovirus type 2 (PCV2) is the pathogen of porcine circovirus associated diseases (PCVAD) and one of the main pathogens in the global pig industry, which has brought huge economic losses to the pig industry. In recent years, there has been limited research on the prevalence of PCV2 in Henan Province. This study investigated the genotype and evolution of PCV2 in this area.

Results

We collected 117 clinical samples from different regions of Henan Province from 2015 to 2018. Here, we found that the PCV2 infection rate of PCV2 was 62.4%. Thirty-seven positive clinical samples were selected to amplify the complete genome of PCV2 and were sequenced. Based on the phylogenetic analysis of PCV2 ORF2 and complete genome, it was found that the 37 newly detected strains belonged to PCV2a (3 of 37), PCV2b (21 of 37) and PCV2d (13 of 37), indicating the predominant prevalence of PCV2b and PCV2d strains. In addition, we compared the amino acid sequences and found several amino acid mutation sites among different genotypes. Furthermore, the results of selective pressure analysis showed that there were 5 positive selection sites.

Conclusions

This study indicated the genetic diversity, molecular epidemiology and evolution of PCV2 genotypes in Henan Province during 2015–2018.

ITRAQ-based quantitative proteomics reveals the first proteome profiles of piglets infected with porcine circovirus type 3

Porcine circovirus type 3 (PCV3) infection induces porcine dermatitis and nephropathy syndrome, reproductive failure, and multisystemic inflammatory lesions in piglets and sows. To better understand the host responses to PCV3 infection, isobaric tags for relative and absolute quantification (iTRAQ) labeling combined with LC-MS/MS analysis was used for quantitative determination of differentially regulated cellular proteins in the lungs of specific-pathogen-free piglets after 4 weeks of PCV3 infection. Totally, 3429 proteins were detected in three independent mass spectrometry analyses, of which 242 differential cellular proteins were significantly regulated, consisting of 100 upregulated proteins and 142 downregulated proteins in PCV3-infected group relative to control group. Bioinformatics analysis revealed that these higher or lower abundant proteins involved primarily metabolic processes, innate immune response, MHC-I and MHC-II components, and phagosome pathways. Ten genes encoding differentially regulated proteins were selected for investigation via real-time RT-PCR. The expression levels of six representative proteins, OAS1, Mx1, ISG15, IFIT3, SOD2, and HSP60, were further confirmed by Western blotting and immunohistochemistry. This study attempted for the first time to investigate the protein profile of PCV3-infected piglets using iTRAQ technology; our findings provide valuable information to better understand the mechanisms underlying the host responses to PCV3 infection in piglets. SIGNIFICANCE: Our study identified differentially abundant proteins related to a variety of potential signaling pathways in the lungs of PCV3-infected piglets. These findings provide valuable information to better understand the mechanisms of host responses to PCV3 infection.

First detection and genomic characterization of porcine circovirus 3 in mosquitoes from pig farms in China

The porcine circovirus type 3 (PCV3) becomes an important causative agent of swine disease since its discovery in 2016. PCV3 infection exhibits a wide range of clinical syndromes causing substantial economic losses in swine industry. Previous studies have reported the detection of numerous known viruses including circovirus in mosquitoes. However, the transmission of PCV3 in field-caught mosquitoes remains largely unknown. This study aims to detect PCV3 infection in mosquitoes and analyze its genomic characteristics. Here, we performed a PCR to detect the PCV3 in 269 mosquito samples collected from pig farms located in Heilongjiang, Jilin, and Yunnan provinces. The proportion of PCV3-positive mosquitoes was 32.0 % (86/269), ranging from 21.4%-42.5% at farm level, which may imply that mosquito serves as a route of transmission for PCV3. To determine the possible origin of PCV3 in mosquitoes, 80 pig serum samples were collected from the pig farms where mosquito sampling was also performed. The proportion of PCV3-positive farms ranged from 15.0%-30.0 % in which infection of positive pigs positively correlated with mosquitoes carrying the virus. Additionally, we sequenced the entire genome of 6 strains of PCV3 in mosquitoes and 2 strains of PCV3 in pigs. Sequence analysis indicated a 100 % nucleotide similarity between mosquito and pig viral isolates that were all collected from similar farms. Phylogenetic analysis showed that PCV3 could be divided into two clades, PCV3a and PCV3b, and the PCV3 strains isolated in mosquitoes were distributed on the two clades. Our results demonstrate that mosquitoes may serve as a potential transmission vector in the life-cycle of PCV3, revealing possible transmission routes of PCV3.

Identification of Nuclear Localization Signals in the ORF2 Protein of Porcine Circovirus Type 3

Porcine circovirus type 3 (PCV3) contains two major open reading frames (ORFs) and the ORF2 gene encodes the major structural capsid protein. In this study, nuclear localization of ORF2 was demonstrated by fluorescence observation and subcellular fractionation assays in ORF2-transfected PK-15 cells. The subcellular localization of truncated ORF2 indicated that the 38 N-terminal amino acids were responsible for the nuclear localization of ORF2. The truncated and site-directed mutagenesis of this domain were constructed, and the results demonstrated that the basic amino acid residues at positions 8-32 were essential for the strict nuclear localization. The basic motifs ₈RRR-R-RRR₁₆ and ₁₆RRRHRRR₂₂ were further shown to be the key functional nucleolar localization signals that guide PCV3 ORF2 into nucleoli. Furthermore, sequence analysis showed that the amino acids of PCV3 nuclear localization signals were highly conserved. Overall, this study provides insight into the biological and functional characteristics of the PCV3 ORF2 protein.

Genotyping based on complete coding sequences of porcine circovirus type 3 is stable and reliable

Porcine circovirus type 3 (PCV3) is a newly identified virus, which is associated with PDNS-like clinical signs, reproductive failure, cardiac and multiorgan inflammation. However, the genotype of PCV3 is still controversial. Here, we reconstructed the phylogenies of 194 complete coding sequences of PCV3 using five different phylogenetic methods. The results showed five trees reconstructed using different methods displayed similar phylogenies, indicating genotyping based on complete coding sequences of PCV3 is stable and accurate.

Reservoirs of Porcine Circoviruses: A Mini Review

Porcine circovirus (PCV) is one of the smallest known DNA viruses in mammals. At present, PCVs are divided into three species, PCV1, PCV2, and PCV3. PCV1 and PCV2 were found in the 1970s and the 1990s, respectively, whereas PCV3 was discovered recently in 2016. PCV1 does not cause diseases in pigs. However, PCV3, similar to PCV2, is reported to be associated with several swine diseases, including porcine dermatitis and nephropathy syndrome (PDNS) and reproductive failure. PCVs are very common in domestic pigs as well as wild boars. However, PCVs have been occasionally isolated from non-porcine animals, including ruminants (such as cattle, goats, wild chamois, and roe deers), rodents (such as NMRI mice, BALB/c mice, Black C57 mice, ICR mice, Mus musculus, and Rattus rattus), canines (such as dogs, minks, foxes, and raccoon dogs), insects (such as flies, mosquitoes, and ticks), and shellfish. Moreover, PCVs are frequently reported in biological products, including human vaccines, animal vaccines, porcine-derived commercial pepsin products, and many cell lines. PCVs are also abundant in the environment, including water samples and air samples. Interestingly, PCV1 and/or PCV2 antibody or antigen has also been detected in sera, stool samples and respiratory swab samples of human, revealing zoonotic potential of PCVs. Thus, PCVs inhabit many types of reservoirs. In this review, we summarize the reservoirs of PCVs, and this information would be helpful in understanding the natural circulating status and possible cross-species transmission of PCVs.

Novel polymerase spiral reaction assay for the visible molecular detection of porcine circovirus type 3

Background

Porcine circovirus type 3 (PCV3) is a newly emerging circovirus that might be associated with porcine dermatitis and nephropathy syndrome, reproductive failure, and cardiac and multisystemic inflammation. To aid the prevention and control of the infectious disease caused by PCV3, we developed a novel isothermal amplification assay using polymerase spiral reaction (PSR), which allows the visual detection of preserved strains and clinical samples.

Results

This assay precisely amplified the PCV3 genome with the use of a water bath at 62 °C for 50 min. The detection limit was found to be 1.13 × 10² copies/μL by gel electrophoresis or with the use of a visible dye (an indicator comprising phenol red and cresol red). No cross-reaction with other porcine infectious viruses was observed. The detection results for 23 PCV3-positive samples by PSR were in accordance with loop-mediated isothermal amplification (LAMP) assay. The detection rate of the PSR assay for PCV3 positivity of clinical samples was 68/97, which was higher than LAMP assay (67/97).

Conclusions

These results indicated that the PSR assay provides an accurate and rapid method for the detection of PCV3 with high sensitivity and specificity. It is particularly suited for use in a simple laboratory setting without a thermal cycler or gel electrophoresis equipment.

Evolution and Genetic Diversity of Porcine Circovirus 3 in China

The identification of a new circovirus (Porcine Circovirus 3, PCV3) has raised concern because its impact on swine health is not fully known. In Fujian Province in eastern China, even its circulating status and genetic characteristics are unclear. Here, we tested 127 tissue samples from swine from Fujian Province that presented respiratory symptoms. All of the PCV3 positive samples were negative for many other pathogens involved in respiratory diseases like PCV2, PRRSV, and CSFV, suggesting that PCV3 is potentially pathogenic. From phylogenetic analysis, PCV3 strains are divided into two main clades and five sub-clades; PCV3a-1, PCV3a-2, PCV3a-3, PCV3b-1, and PCV3b-2. Our identified strains belong to genotypes PCV3a-1, PCV3a-2, PCV3a-3, and PCV3b-2, indicating a high degree of genetic diversity of PCV3 in Fujian province until 2019. Interestingly, we found the time of the most recent common ancestor (tMRCA) of PCV3 was dated to the 1950s, and PCV3 has a similar evolutionary rate as PCV2 (the main epidemic genotypes PCV2b and PCV2d). In addition, positive selection sites N56D/S and S77T/N on the capsid gene are located on the PCV3 antigen epitope, indicating that PCV3 is gradually adaptive in swine. In summary, our results provide important insights into the epidemiology of PCV3.

Porcine circovirus type 2 induces a strong cytopathic effect in the serum-free culture cell line CPK-NS

When the adherent stable serum-free porcine kidney cell line CPK-NS were inoculated with porcine circovirus type 2 (PCV2) and passaged, viral titre concentration-dependent cell detachment was observed. The copy number of viral genes in supernatants of the infected CPK-NS cells decreased as cell detachment progressed. Furthermore, cell detachment was completely inhibited via neutralisation of the virus using antisera collected from PCV2-infected specific pathogen-free pigs. These results indicated that detachment of CPK-NS cells is a cytopathic effect (CPE) caused via infection with PCV2. Only a single round of cell passaging was required to observe clear a CPE when the inoculated viral titre was significantly high [≥10^4.5 median tissue culture infectious dose (TCID₅₀)/mL]. Our study confirms that PCV2, which is normally non-cytopathogenic, is capable of inducing a distinct CPE in CPK-NS cells. Application of CPK-NS cells for detection of viruses may contribute towards the diagnosis and control of PCV2-mediated infectious diseases.