Genetic variability of porcine parvovirus isolates revealed by analysis of partial sequences of the structural coding gene VP2

Isolation and characterization of porcine parvovirus in Vietnam

Background and Aim

No study has successfully isolated parvovirus in Vietnam. This study aimed to isolate and characterize parvovirus strains indigenous in Vietnam for vaccine development against porcine parvovirus (PPV).

Materials and Methods

We collected serum and stillbirth samples from six provinces in Vietnam, and PPV-positive samples were identified using a polymerase chain reaction. Parvovirus isolation was attempted using the PK-15 cells maintained in a minimum essential medium supplemented with 5% fetal bovine serum and 1% antibiotics (Penicillin-streptomycin). The cells were incubated at 37°C with 5% CO2. Virulence experiments were conducted on white primiparous sows to evaluate the virulence of the PPV strain through hemagglutination inhibition (HI) titers and fetus lesions.

Results

We analyzed 360 serum and 32 stillbirth (liver and lungs) samples, revealing that 32/392 (8.2% ) of them were PPV-positive, all belonging to PPV1. Thirty-two PPV-positive samples were successfully isolated, with 100% identity as VP2 sequences. The phylogenetic tree revealed a close relationship with the Kresse strain (isolated from Canada in 1996) and the PPV1-0225-L-SD strain (isolated from China in 2022). Two PPV isolates (VC5 from Dongnai and TX7 from Thanhhoa) that exhibited high 50% tissue culture infectious dose titers were selected for the virulence experiment. On day 21, after injection, the HI antibody titers ranged from 10log2 to 12log2. On day 90, 71%-80% of fetuses were mummified.

Conclusion

This study showed that the PPV infection rate in Vietnam was 8.2%. Thirty-two isolates belonged to PPV1. Two PPV strains, VC5 and TX7, were determined to be highly virulent by the results of HI titers after injection into gilts. VC5 and TX7 were determined to be good candidates for further research on PPV vaccines.

Prevalence of viral agents causing swine reproductive failure in Korea and the development of multiplex real-time PCR and RT-PCR assays

This study aimed to investigate the prevalence of viral agents causing reproductive failure in pigs in Korea. In addition, two types of multiplex real-time PCR (mqPCR) were developed for the simultaneous detection of Aujeszky's disease virus (ADV) and porcine parvovirus (PPV) in mqPCR and encephalomyocarditis virus (EMCV) and Japanese encephalitis virus (JEV) in reverse transcription mqPCR (mRT-qPCR). A total of 150 aborted fetus samples collected from 2020 to 2022 were analyzed. Porcine reproductive and respiratory syndrome virus was the most prevalent (49/150 32.7%), followed by porcine circovirus type 2 (31/150, 20.7%), and PPV1 (7/150, 4.7%), whereas ADV, EMCV, and JEV were not detected. The newly developed mqPCR and mRT-qPCR could simultaneously detect and differentiate with high sensitivities and specificities. When applied to aborted fetuses, the newly developed mqPCR for PPV was 33.3% more sensitivities than the previously established diagnostic method. Amino acid analysis of the VP2 sequences of PPV isolates revealed considerable similarity to the highly pathogenic Kresse strain. This study successfully evaluated the prevalence of viral agents causing reproductive failure among swine in Korea, the developed mqPCR and mRT-qPCR methods could be utilized as effective and accurate diagnostic methods for the epidemiological surveillance of ADV, PPV, EMCV, and JEV.

Duration of maternally derived antibodies of porcine parvovirus in growing pigs and presence of antibodies in gilts and sows vaccinated with three different parvovirus vaccines

While gilts and sows are regularly vaccinated against the porcine parvovirus (PPV), little is known on the presence of antibodies in vaccinated sows nor the decline of maternally derived antibodies (MDA) in their offspring. On twelve farms serum samples were taken from 180 gilts and sows vaccinated at least twice with one of three different commercial PPV vaccines. On nine farms, additional 270 serum samples were collected from growing pigs of three different age categories. All 450 samples were examined for PPV antibodies (Abs) by ELISA and haemagglutination inhibition (HI) assay. In total, 65% of all gilts vaccinated twice with either vaccine 1 or vaccine 3 were seronegative by HI assay. In each farm, there were at least three animals with high Ab titres (≥ 1:1280) indicating the presence of PPV in all twelve study farms. However, PPV DNA could not be detected in collected faecal samples. While low to moderately high Ab titres (1:10-1:640) were measured in 98% of twelve-weeks-old pigs, ELISA was only positive in 30% of the same pigs. Though, the statement on the duration of MDA may depend on the applied test, we could confirm an exponential decay of MDA. In addition, we could demonstrate that applied serological tools are insufficient for the confirmation of successful vaccination.

Isolation, characterization, and phylogenetic analysis of two new porcine parvovirus 1 isolates from Northern China

Porcine parvovirus (PPV) is a pathogen of infectious reproductive disease, which can cause stillbirth, mummification, embryo death, and infertility (SMEDI) syndrome in pigs. The objective of this study was to gain new insights into the evolution and phylogeny of the PPV1 genome. In this study, we isolated two new PPV1 (HLJ202108-Y and SDLC202109) from northern China and sequenced their whole genomes. The new isolates were found to have three amino acid substitutions (K195R, K562R, and S578P) in nonstructural protein 1. The VP2 amino acid site contained nine nonsynonymous substitutions, including six substitutions of the Kresse strain corresponding to the NADL-2 strain and three substitutions of A414S, S436T, and N555K. Genetic evolution analysis was conducted on 107 reference sequences available in the GenBank database, and 4-5 PPV1 taxa were defined. The new isolates were in the same phylogenetic cluster as strain 27a. The changes in the cluster, specifically marker amino acids, and their potential role in enhancing pathogenicity are discussed in this study. Furthermore, the evolutionary tree map results showed that the strains in China were evolving in two directions: one was becoming increasingly similar to early NADL-2 strains, while the other was evolving toward 27a-like strains. We also compared the proliferation ability of the isolated strains in susceptible cells by analyzing the multistep growth curves. The results showed that the virulence titer of the mutant strain was high. In summary, this study introduced the latest changes in PPV and discussed the virus characteristics that were considered to affect virulence.

Molecular detection of porcine parvovirus 1-associated reproductive failure in southern India

This study used 56 aborted and stillborn fetuses from organized swine farms in Tamil Nadu and Kerala, southern states of India. All samples were screened by using a PCR assay that targets the NS1 gene for PPV. Furthermore, the PCR positive samples were subjected to amplification of the VP2 gene of PPV1 with designed primers and sequenced for further study. The PCR screening of 56 samples found that 14.3% (n = 8) were positive for PPV genome. According to VP2 gene–based PCR for PPV1, 897 bp specific amplicons were detected in all eight of the samples. Two of the eight positive samples (L17 and T5) were sequenced and annotated randomly. The BLAST analysis of contig sequence INDTNCHN-T5 revealed 100% sequence homology with Chinese PPV1genome, whereas sequence from INDTNCHN-L17 revealed 99.43% sequence homology with Spain, Chinese, and German. PPV1 sequences and both the sequences INDTNCHN-T5 and INDTNCHN-L17 were submitted to the GenBank under the accession numbers MW822566 and MW822567 respectively. A phylogenetic analysis of the sequences in this study revealed specific grouping along with PPV1 strains in cluster E. Amino acid analysis of both isolated sequences in addition to the reference sequence from PPV1 showed variations in position 215 (I to T) in both the isolates, variation at position 228 (Q to E) in T5 isolate and variations at position 59 (L to M) and 314 (K to E) in L17 isolate. This study represents the first report of PPV1 cluster E in Tamil Nadu, southern India.

Viral fitness and antigenic determinants of porcine parvovirus at the amino acid level of the capsid protein

Since 2001, strains of porcine parvovirus (PPV), designated 27a-like strains, were observed in Europe, suggesting a predominance of these viruses over older strains. The reasons for the obvious evolutionary advantage are unknown. Here, a series of mutants containing amino acid replacements found in the predominant field strains were generated in a PPV-NADL2 background and their impact on replication efficiency and antibody binding activity was determined. Some amino acid substitutions observed in the 27a-like strains significantly increased viral fitness and decreased neutralization activity of sera raised against commercial vaccines and old virus strains (e.g. NADL2). These mutant viruses and a monoclonal antibody raised against a classical PPV strain defined an 27a-specific neutralizing epitope around amino acid 228 of the capsid protein VP2. Based on the analysis of the mutant viruses, it is hypothesized that the predominant factor for the global spread of the PPV-27a strain substitutions is an increased viral fitness of the 27a-like viruses, possibly supported by a partial immune selection. This is reminiscent to the evolution of canine parvovirus and worldwide replacement of the original virus by the so-called new antigenic types. Importance Porcine parvovirus is one of the most important causes of reproductive failure in swine. Recently, despite the continuous use of vaccines, "new" strains emerged, leading to the hypothesis that the emergence of new amino acid substitutions could be a viral adaptation to the immune response against the commercial vaccines. Our results indicate the amino acid substitutions observed in the 27a-like strains can modify viral fitness and antigenicity. However, an absolute immune escape was not evident.

Effects of three commercial vaccines against porcine parvovirus 1 in pregnant gilts

Porcine parvovirosis is a common and important cause of reproductive failure in naïve dams. Even though vaccination is generally effective at preventing disease occurrence, the homology between the vaccine and challenge strains has been recently suggested to play a role in protection. Therefore, the purpose of this study was to evaluate and compare the efficacy of three currently available commercial vaccines against porcine parvovirus genotype 1 (PPV1) in an experimental model using pregnant gilts. Seventy-seven PPV1-negative gilts were included in the trial and randomly allocated to four groups. In group 1, gilts received two doses, three weeks apart, of a PPV1 subunit vaccine (ReproCyc® ParvoFLEX). Following the same scheme, gilts from group 2 received two doses of a PPV1 bivalent vaccine (ERYSENG® PARVO). In group 3, gilts received two doses, four weeks apart, of a PPV1 octavalent vaccine (Porcilis® Ery + Parvo + Lepto). Lastly, gilts from group 4 were left untreated and were used as challenge controls. All gilts were artificially inseminated three weeks after completion of vaccination. Pregnant animals were subsequently challenged around 40 days of gestation with a heterologous PPV1 strain. Foetuses were harvested at around day 90 of gestation and evaluated for their macroscopic appearance (i.e., normal, mummified, or autolytic). Along the study, safety parameters after vaccination, antibody responses against PPV1 and viremia in gilts were also measured. All the foetuses in the challenge control group were mummified, which validated the challenge model, whereas the three evaluated vaccines protected the progeny against PPV1 by preventing the appearance of clinical manifestations associated to parvovirosis. Remarkably, the PPV1 subunit vaccine induced an earlier seroconversion of gilts and was the only vaccine that could prevent viremia after challenge. This vaccine also achieved the largest average litter size accompanied with a high average proportion of clinically healthy foetuses.

Identification of a dominant linear epitope on the VP2 capsid protein of porcine parvovirus and characterization of two monoclonal antibodies with neutralizing abilities

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine, and has caused huge losses throughout the world. The structural viral protein VP2, which is able to self-assemble into empty capsids, known as virus-like particles (VLPs), is crucial to induce PPV-specific neutralizing antibodies and protective immunity. In this study, twelve monoclonal antibodies (mAbs) against PPV were generated. The mAbs were characterized by indirect enzyme-linked immunosorbent assay (ELISA), western blotting (WB) and virus neutralization (VN) assay. Two mAbs were defined to be able to neutralize the standard PPV 7909 strain. Subsequently, peptide scanning was applied to identify linear epitopes. The peptide, ⁸⁹ESGVAGQMV⁹⁷ was defined as a precise linear epitope. Results from structural analysis showed that the epitope was exposed on the virion surface. Multiple sequence alignment analysis indicated that peptide ⁸⁹ESGVAGQMV⁹⁷ was not completely conserved, with a higher amino acid mutation rate at ⁹¹G, ⁹²V and ⁹³A position. Alanine-scanning mutagenesis further revealed that residues ⁸⁹E, ⁹⁰S, ⁹¹G, ⁹²V and ⁹⁴G were the core sites involved in antibody recognition. These findings may facilitate further understanding the function of the VP2 protein and development of diagnostic tools.

Detection and molecular characterization of porcine parvovirus in fetal tissues from sows without reproductive failure in Argentina

Porcine parvovirus (PPV) is one of many pathogens responsible for reproductive failure in pregnant sows. Several studies have reported the appearance of new PPV strains that differ from previous isolates both genetically and antigenically. Thus, the protective effects of commercially inactivated vaccines could not be complete. In South America, the information about PPV is limited. Thus, the aim of the present study was to detect and characterize the PPV strains present in 131 mummies or stillbirths from normal deliveries in sows from a commercial swine farm of Argentina that uses the commercial vaccine. PCR results showed that 17/131 were positive to PPV. Ten of these viruses were isolated and sequenced. All viruses were related to the PPV1 sequence (NADL-2), maintaining the amino acid differences in positions 436 (S–P) and 565 (R–K). This study is the first to report the isolation of PPV in Argentina and the results suggest that PPV can cross the placenta even in vaccinated sows, thus affecting some of the fetuses and being able to cause fetal death in sows without reproductive failure. The results also suggest that vaccination only reduces clinical signs and reproductive disorders and may thus not be a perfect tool to manage PPV infection. This study provides information that needs to be studied in depth to improve strategies to prevent and control PPV infection in swine farms.

Isolation and phylogenetic analysis of a new Porcine parvovirus strain GD2013 in China

Porcine parvovirus (PPV), a causative agent of an infectious reproductive disorder causing stillbirth, mummification, embryonic death and infertility (SMEDI) syndrome in swine, is a threat to both domestic pigs and wild boars regardless of age and gender. Recent studies found that the observed average substitution rate in the PPV genome was close to those of the RNA viruses and new strains showing serological neutralization activities different from that of the vaccine strain NADL-2 have been reported. These observations have increased the need for the development of new commercial vaccine strains. In this study, a new PPV strain, GD2013, was isolated from Guangdong, China, and its entire genome sequenced. A phylogenetic tree based on the complete coding region of the genomes of 32 PPV strains was constructed using the Bayesian Markov Chain Monte Carlo (MCMC) method. The results showed that strain GD2013 fell into the same phylogenetic cluster as the classical vaccine strains NADL-2 and POVCAP, suggesting a close relationship to the vaccine strains. Multiple sequence alignments and amino acid mutation analyses of the PPV VP2 gene revealed a new amino acid polymorphism site at Thr45 on VP2 that could be used to identify low virulence strains as vaccine candidates. Selective pressure analysis of the NS1 and VP2 genes by calculating the mean rates of non-synonymous substitutions (dN) over synonymous substitutions (dS) implied that both of these genes were under negative selection. Therefore, by using phylogenetic and amino acid mutation analyses, a likely candidate strain suitable for evaluation as an attenuated vaccine strain was identified.

Porcine parvovirus VP1/VP2 on a time series epitope mapping: exploring the effects of high hydrostatic pressure on the immune recognition of antigens

Porcine parvovirus (PPV) is a DNA virus that causes reproductive failure in gilts and sows, resulting in embryonic and fetal losses worldwide. Epitope mapping of PPV is important for developing new vaccines. In this study, we used spot synthesis analysis for epitope mapping of the capsid proteins of PPV (NADL-2 strain) and correlated the findings with predictive data from immunoinformatics. The virus was exposed to three conditions prior to inoculation in pigs: native (untreated), high hydrostatic pressure (350 MPa for 1 h) at room temperature and high hydrostatic pressure (350 MPa for 1 h) at − 18 °C, and was compared with a commercial vaccine produced using inactivated PPV. The screening of serum samples detected 44 positive spots corresponding to 20 antigenic sites. Each type of inoculated antigen elicited a distinct epitope set. In silico prediction located linear and discontinuous epitopes in B cells that coincided with several epitopes detected in spot synthesis of sera from pigs that received different preparations of inoculum. The conditions tested elicited antibodies against the VP1/VP2 antigen that differed in relation to the response time and the profile of structurally available regions that were recognized.

A comparative molecular characterization of AMDV strains isolated from cases of clinical and subclinical infection

The Aleutian mink disease virus (AMDV) is one of the most serious threats to modern mink breeding. The disease can have various courses, from progressive to subclinical infections. The objective of the study was to provide a comparative molecular characterization of isolates of AMDV from farms with a clinical and subclinical course of the disease. The qPCR analysis showed a difference of two orders of magnitude between the number of copies of the viral DNA on the farm with the clinical course of the disease (10⁵) and the farm with the subclinical course (10³). The sequencing results confirm a high level of homogeneity within each farm and variation between them. The phylogenetic analysis indicates that the variants belonging to different farms are closely related and occupy different branches of the same clade. The in silico analysis of the effect of differences in the sequence encoding the VP2 protein between the farms revealed no effect of the polymorphism on its functionality. The close phylogenetic relationship between the isolates from the two farms, the synonymous nature of most of the polymorphisms and the potentially minor effect on the functionality of the protein indicate that the differences in the clinical picture may be due not only to polymorphisms in the nucleotide and amino acid sequences, but also to the stage of infection on the farm and the degree of stabilization of the pathogen-host relationship.

Biology of Porcine Parvovirus (Ungulate parvovirus 1)

Porcine parvovirus (PPV) is among the most important infectious agents causing infertility in pigs. Until recently, it was thought that the virus had low genetic variance, and that prevention of its harmful effect on pig fertility could be well-controlled by vaccination. However, at the beginning of the third millennium, field observations raised concerns about the effectiveness of the available vaccines against newly emerging strains. Subsequent investigations radically changed our view on the evolution and immunology of PPV, revealing that the virus is much more diverse than it was earlier anticipated, and that some of the “new” highly virulent isolates cannot be neutralized effectively by antisera raised against “old” PPV vaccine strains. These findings revitalized PPV research that led to significant advancements in the understanding of early and late viral processes during PPV infection. Our review summarizes the recent results of PPV research and aims to give a comprehensive update on the present understanding of PPV biology.

Perspectives on the Evolution of Porcine Parvovirus

Porcine parvovirus (PPV) is one of the main causes of porcine reproductive failure. It is important for swine industries to understand the recent trends in PPV evolution. Previous data show that PPV has two genetic lineages originating in Germany. In this study, two more genetic lineages were defined, one of which was distinctly Asian. Additionally, amino acid substitutions in European strains and Asian strains showed distinct differences in several regions of the VP2 gene. The VP1 gene of the recent PPV isolate (T142_South Korea) was identical to that of Kresse strain isolated in the USA in 1985, indicating that modern PPV strains now resemble the original strains (Kresse and NADL-2). In this study, we compared strains isolated in the 20th century to recent isolates and confirmed the trend that modern strains are becoming more similar to previous strains.

Identification of three PPV1 VP2 protein-specific B cell linear epitopes using monoclonal antibodies against baculovirus-expressed recombinant VP2 protein

Porcine parvovirus type 1 (PPV1) is a major causative agent of embryonic and fetal death in swine. The PPV1 VP2 protein is closely associated with viral immunogenicity for eliciting neutralizing antibodies, but its antigenic structures have been largely unknown. We generated three monoclonal antibodies (MAbs) against baculovirus-expressed recombinant PPV1 VP2 protein. A PEPSCAN analysis identified the minimal B cell linear epitopes of PPV1 VP2 based on these MAbs. Three core epitopes, (228)QQITDA(233), (284)RSLGLPPK(291), and (344)FEYSNGGPFLTPI(356), were defined and mapped onto three-dimensional models of the PPV1 virion and VP2 monomer. The epitope (228)QQITDA(233) is exposed on the virion surface, and the other two are located inside the protein. An alignment of the PPV1 VP2 amino acid sequences showed that (284)RSLGLPPK(291) and (344)FEYSNGGPFLTPI(356) are absolutely conserved, whereas (228)QQITDA(233) has a single substitution at residue 233 in some (S → A or T). We developed a VP2 epitope-based indirect enzyme-linked immunosorbent assay (iELISA) to test for anti-PPV1 antibodies. In a comparative analysis with an immunoperoxidase monolayer assay using 135 guinea pig sera, the VP2-epitope-based iELISA had a concordance rate of 85.19 %, sensitivity of 83.33 %, and specificity of 85.47 %. MAb 8H6 was used to monitor VP2 during the PPV1 replication cycle in vitro with an indirect immunofluorescence assay, which indicated that newly encapsulated virions are released from the nucleus at 24 h postinfection and the PPV1 replication cycle takes less than 24 h. This study provides valuable information clarifying the antigenic structure of PPV1 VP2 and lays the foundations for PPV1 serodiagnosis and antigen detection.

A TaqMan qPCR for quantitation of Ungulate protoparvovirus 1 validated in several matrices

Ungulate protoparvovirus 1 (UPV1) is one of the major causes of reproductive disorders in swine. Recently, the rapid viral evolution of UPV1 and its viral persistence in several tissues has been described. Based upon this, a real-time qPCR method using upgraded primers targeting VP1 and applying the TaqMan technology was developed in this study for UPV1, and it was validated in feces, serum and tissue. Within the results, the limit of detection of the qPCR was 100copies of the viral genome per reaction of serum and feces and 1000copies of the viral genome per reaction of the grinded tissue (pre-inoculated matrices with diluted serially viruses). No cross reactivity was observed with other viruses associated with reproductive disorders. The assay was specific and reproducible, presenting low intra- and inter-assay variation (0.93% and 1.06%, respectively). In 50 clinical samples, the method was found to be more sensitive than immunofluorescence and a SYBR Green PCR. In conclusion, this qPCR represents an upgraded and useful tool to quantify UPV1 in different sample matrices for diagnostic and research purposes.

An overview of live attenuated recombinant pseudorabies viruses for use as novel vaccines

Pseudorabies virus (PRV) is a double-stranded, DNA-based swine virus with a genome approximating 150 kb in size. PRV has many nonessential genes which can be replaced with genes encoding heterologous antigens but without deleterious effects on virus propagation. Recombinant PRVs expressing both native and foreign antigens are able to stimulate immune responses. In this paper, we review the current status of live attenuated recombinant PRVs and live PRV-based vector vaccines with potential for controlling viral infections in animals.

Polymorphism and differentiation indices preferred for genetic analysis among cattle, goats, and sheep

Most studies of polymorphism and differentiation are based on a single gene; few have treated differences among numerous genes. We have analyzed the parameters of conservation, polymorphism, and genetic differentiation to find the preferred index of 40 genes, with 1,614 sequences from cattle, goats, and sheep. Clustering and principal component factor (PCF) analyses revealed three PCFs: PCF1, representing polymorphism and differentiation; PCF2, representing nucleotide usage; and PCF3, representing gene flow (G ST ) and gene differentiation (δ ST ). We found significantly higher correlations (p < 0.001) with other indices for sequence conservation (r of -0.66 to -0.98), θη (0.52-0.95), and γ ST (0.54-0.96). This shows that they are preferable for estimating conservation, polymorphism, and differentiation of genes, respectively. The codon bias index was significantly different between the autosome and sex chromosomes (p < 0.01).

A sensitive and specific nanoparticle-assisted PCR assay for rapid detection of porcine parvovirus

A novel nanoparticle-assisted polymerase chain reaction (nanoPCR) assay to detect porcine parvovirus (PPV) is described here. Primers for this assay were designed based on the conserved region of the nonstructural protein 1 (NS1) gene of PPV, which encodes one of the nonstructural proteins. The sensitivity of the PPV nanoPCR assay was measured by using diluted recombinant plasmids in which the PPV NS1 gene had been inserted. The detection limit was 5.6 × 10(1) copies μl(-1) for the PPV nanoPCR assay vs 5.6 × 10(3) copies μl(-1) for conventional PCR assay. The results showed that the sensitivity of PPV nanoPCR assay was 100 times higher than that of conventional PCR assay. The PPV nanoPCR assay produced 142-bp product as expected when amplifying PPV DNA, while produced nothing when amplifying the DNA or cDNA of the following viruses: swine encephalomyocarditis virus, classical swine fever virus, porcine pseudorabies virus, porcine reproductive and respiratory syndrome virus, porcine teschovirus and porcine circovirus type II. PPV was detected in 108 of 109 clinical swine samples from Heilongjiang, Jilin and Henan provinces using the nanoPCR assay, and the results were confirmed by sequencing.

SIGNIFICANCE AND IMPACT OF THE STUDY

Nanoparticle-assisted polymerase chain reaction (nanoPCR) assay is an improved PCR. NanoPCR is highly sensitive and specific because the nanofluids formed in the nanobuffer have high thermal conductivity, which reduces the time required to reach the target temperature. It is more sensitive than conventional PCR, and it could detect the cases earlier than conventional PCR. This report describes the first application of the highly efficient nanoPCR technology for the detection of porcine parvovirus (PPV). The PPV nanoPCR assay will be useful for the detection and study of PPV and will also be applicable to improve the detection of other viruses.

Population dynamics and in vitro antibody pressure of porcine parvovirus indicate a decrease in variability

To estimate the impact of porcine parvovirus (PPV) vaccines on the emergence of new phenotypes, the population dynamic history of the virus was calculated using the Bayesian Markov chain Monte Carlo method with a Bayesian skyline coalescent model. Additionally, an in vitro model was performed with consecutive passages of the ‘Challenge’ strain (a virulent field strain) and NADL2 strain (a vaccine strain) in a PK-15 cell line supplemented with polyclonal antibodies raised against the vaccine strain. A decrease in genetic diversity was observed in the presence of antibodies in vitro or after vaccination (as estimated by the in silico model). We hypothesized that the antibodies induced a selective pressure that may reduce the incidence of neutral selection, which should play a major role in the emergence of new mutations. In this scenario, vaccine failures and non-vaccinated populations (e.g. wild boars) may have an important impact in the emergence of new phenotypes.

Interferon induction and suppression in swine testicle cells by porcine parvovirus and its proteins

Porcine parvovirus (PPV) is a major causative agent of reproductive failure in swine, which currently affects the swine industry worldwide. Although PPV was identified several years ago, little is known about how it overcomes host innate immunity. In this study, we used quantitative real-time PCR and a luciferase reporter assay to determine whether PPV infection induces type I interferon (IFN-α and IFN-β) and whether PPV infection blocks dsRNA-induced IFN-β promoter activation in cell cultures. The results indicate that PPV does not induce type I interferon and that the NS2 protein of PPV could blocks dsRNA-induced IFN-β promoter activation.

Characterization of the capsid protein VP2 gene of a virulent strain NE/09 of porcine parvovirus isolated in China

A virulent strain NE/09 of porcine parvovirus (PPV) was isolated from mummified swine fetus samples in China and its capsid protein VP2 gene was analyzed. The data showed that the VP2 gene encompassed an open reading frame of 1713 nucleotides with a composition of 684 adenine (39.9%), 387 cytosine (22.6%), 268 guanine (15.7%), and 374 thymine (21.8%). The VP2 gene encoded a 570aa-long protein which could be recognized by anti-PPV-VP2 monoclonal antibody 3C9. Sequence analysis revealed that the VP2 gene of PPV-NE/09 had close sequence similarity with the VP2 gene of other PPV strains. However, a 27 nucleotide region that encodes a glycine-rich domain at the N-terminal region of VP2 was deleted in the PPV-NE/09 strain. The PPV-NE/09 virions were purified by 1.39 g/ml CsCl density centrifugation and subjected to SDS-PAGE analysis. The results showed that only two major polypeptides VP1 and VP2 with molecular weights of 83 and 64kDa, respectively were observed, whereas VP3 with a molecular weight of 60 kDa was not present, in contrast to the PPV-NADL-2 strain. According to the phylogenetic relationship of the VP2 gene of PPV-NE/09, the isolate was a new mutant strain of PPV prevailing in China.

High rate of viral evolution in the capsid protein of porcine parvovirus

In recent years, it has been shown that some parvoviruses exhibit high substitution rates, close to those of RNA viruses. In order to monitor and determine new mutations in porcine parvovirus (PPV), recent PPV field isolates from Austria, Brazil, Germany and Switzerland were sequenced and analysed. These samples, together with sequences retrieved from GenBank, were included in three datasets, consisting of the complete NS1 and VP1 genes and a partial VP1 gene. For each dataset, the nucleotide substitution rate and the molecular clock were determined. Analysis of the PPV field isolates revealed that a recently described amino acid substitution, S436T, appeared to be common in the VP2 protein in the Austrian, Brazilian and German virus populations. Furthermore, new amino acid substitutions were identified, located mainly in the viral capsid loops. By inferring the evolutionary dynamics of the PPV sequences, nucleotide substitution rates of approximately 10−5 substitutions per site per year for the non-structural protein gene and 10−4 substitutions per site per year for the capsid protein gene (for both viral protein datasets) were found. The latter rate is similar to those commonly found in RNA viruses. An association of the phylogenetic tree with the molecular clock analysis revealed that the mutations on which the divergence for both capsid proteins was based occurred in the past 30 years. Based on these findings, it was concluded that PPV variants are continuously evolving and that vaccines, which are based mainly on strains isolated about 30 years ago, should perhaps be updated.

Phylogenetic analysis of porcine parvoviruses from swine samples in China

BACKGROUND

Porcine parvovirus (PPV) usually causes reproductive failure in sows. The objective of the present study was to analyze the phylogenetic distribution and perform molecular characterization of PPVs isolated in China, as well as to identify two field strains, LZ and JY. The data used in this study contained the available sequences for NS1 and VP2 from GenBank, as well as the two aforementioned Chinese strains.

RESULTS

Phylogenetic analysis shows that the PPV sequences are divided into four groups. The early Chinese PPV isolates are Group I viruses, and nearly all of the later Chinese PPV isolates are Group II viruses. LZ belongs to group II, whereas the JY strain is a Group III virus. This is the first report on the isolation of a Group III virus in China. The detection of selective pressures on the PPV genome shows that the NS1 and VP2 genes are under purifying selection and positive selection, respectively. Moreover, the amino acids in the VP2 capsid are highly variable because of the positive selection.

CONCLUSIONS

Our study provides new molecular data on PPV strains in China, and emphasizes the importance of etiological studies of PPV in pigs.

Genetic elements in the VP region of porcine parvovirus are critical to replication efficiency in cell culture

Factors controlling porcine parvovirus (PPV) replication efficiency are poorly characterized. Two prototype strains of PPV, NADL-2 and Kresse, differ greatly in pathogenic capacity both in vivo and in vitro, yet their genomic sequence is nearly identical (13 single-nucleotide substitutions and a 127-nucleotide noncoding repeated sequence). We have created a series of chimeras of these strains to identify the genetic elements involved in replication efficiency in the host porcine cell line. While the capsid proteins ultimately determine viral replication fitness, interaction between the NS1 protein and the VP gene occurs and involves interaction with the noncoding repeated sequence.

A multiplex RT-PCR for rapid and simultaneous detection of porcine teschovirus, classical swine fever virus, and porcine reproductive and respiratory syndrome virus in clinical specimens

A multiplex RT-PCR (mRT-PCR) assay was developed and evaluated for its ability to detect multiple viral infections of swine simultaneously. One pair of primers was selected carefully for each of the following three RNA viruses: porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), and porcine teschovirus (PTV). Each target produced a specific amplicon with a size of 451bp (PRRSV), 343bp (CSFV), or 163bp (PTV). The sensitivity of the mRT-PCR using purified plasmid constructs containing the specific viral target fragments was 2.02 x 10², 2.90 x 10³, and 6.16 x 10³ copies for PRRSV, CSFV, and PTV, respectively. Among 69 clinical samples from Heilongjiang, Jilin, and Henan provinces, co-infection by PRRSV and CSFV was 4.4%, co-infection by PRRSV and PTV was 11.6%, co-infection by PTV and CSFV was 13.0%, and co-infection by the three viruses was 8.7%. In conclusion, the mRT-PCR should be useful for routine molecular diagnosis and epidemiology.

Reproductive failure in wild boars associated to porcine parvovirus infection and in vivo and in vitro characterization of the causal isolate

Investigations were made to identify the causal agent of an acute outbreak of abortions in a domesticated herd of wild boar. Only porcine parvovirus (PPV) was isolated from samples of organs from the still-born sucklings and mummified aborted fetuses. The isolated virus hemagglutinated erythrocytes of guinea pig, murine, rat, and chicken. Identity of the virus, designated the BQ strain, was confirmed by the production of a specific cytopathic effect on susceptible cells and by the results from ELISA, PCR, immunofluorescence assay, and electron microscopy. PPV BQ strain was adapted to growth in a swine testicular cell line. When inoculated into healthy sows, PPV BQ caused the same reproductive disorder observed in the affected herd.

Real-time PCR to detect and analyze virulent PPV loads in artificially challenged sows and their fetuses

To establish a real-time polymerase chain reaction with SYBR Green for detection and quantification of porcine parvovirus (PPV) in porcine tissues, two primers specific for the non-structural protein 1 gene were designed. The detection limit of this assay was 3-23 gene copies/reaction, equivalent to 0.001 TCID(50)/ml. The assay was linear over a 10(6) dilution range of template concentrations. Other porcine pathogens involved in reproductive disorders (porcine circovirus 2, porcine reproductive and respiratory virus, pseudorabies virus, classical swine fever virus) were negative by this assay. This assay could detect PPV titres at least 10(5) smaller than the hemagglutination assay. To better understand the pathogenesis of PPV, the levels of viral DNA in various tissues of artificially challenged sows and their fetuses were quantified with this method. The virus was found mainly in the heart, lung, spleen, kidney, and endometrium of sows, and mainly in the heart, spleen, lung, and testis of fetuses. This study provides a new tool for the study of PPV infection and distribution in sows and their fetuses.

A multiplex PCR for rapid and simultaneous detection of porcine circovirus type 2, porcine parvovirus, porcine pseudorabies virus, and porcine reproductive and respiratory syndrome virus in clinical specimens

A multiplex PCR (mPCR) assay was developed and evaluated for its ability to simultaneously detect multiple viral infections of swine. Specific primers were designed for each of the following four DNA or RNA viruses: porcine circovirus type 2 (PCV2), porcine parvovirus (PPV), pseudorabies virus (PRV), and porcine reproductive and respiratory syndrome virus (PRRSV). Each target produced a specific amplicon with a size of 353 bp (PCV2), 271 bp (PPV), 194 bp (PRV), or 434 bp (PRRSV). The assay was sensitive and specific in detecting each target agent in composite cell cultures and clinical specimens. Results from mPCR were confirmed by PCR for individual viruses and by virus isolation. In conclusion, the mPCR has the potential to be useful for routine molecular diagnosis and epidemiology.

High genetic diversity of the VP2 gene of a canine parvovirus strain detected in a domestic cat

This study reports the detection of co-infection by multiple CPV variants and the high genetic complexity of a CPV-2 strain detected in a domestic cat. The CPV variants selected by cloning the VP2 gene were sequenced, and genetic diversity and selection pressure were investigated. Comparison of the nucleotide sequences has evidenced 10 different viral populations, and, in the same animal, more CPV variants coexist. Our analysis excludes the possibility that the recombination events took place during infection and that negative selection acted on the VP2 gene. These findings confirm that CPV-2 shows high genetic heterogeneity resembling the quasispecies found in RNA viruses.

VP1 sequences of German porcine parvovirus isolates define two genetic lineages

In order to evaluate the genetic variability of Porcine parvovirus (PPV), the complete capsid protein sequences (VP1/VP2) from seven recent field isolates from Germany, one isolate from the UK and one German vaccine strain were sequenced and analysed, along with two American (NADL-2 and Kresse), three Asian and 22 Brazilian partial PPV sequences retrieved from GenBank. The analysis revealed a high degree of diversity: 1·2–2·6 % at the nucleotide level and 1·2–6·8 % at the amino acid level. Phylogenetic analysis defined two German clusters: one formed by four German isolates and the English, Asian and American sequences; and the second, distinct cluster formed by the other three of the seven German isolates examined. The latter cluster was still observed when the 22 partial sequences (853 nt of the 3′ terminus of the VP2 gene) from the Brazilian isolates were included in the analyses, indicating that the VP2 sequence determines the phylogeny.

Tissue distribution of two field isolates and two vaccine strains of porcine parvovirus in foetal organs after experimental infection of pregnant sows as determined by real-time PCR

The aim of this study was to investigate the tissue distribution of two different field isolates and two vaccine strains of porcine parvoviruses (PPV) in infected piglets after transplacental infection. The viral load in 10 different foetal organs was determined by real-time polymerase chain reaction assays with SYBR Green targeting the viral VP2 gene and the genomic c-myc gene in 12 foetuses. The viral load in foetal tissues differed greatly among the different parvoviruses. Between one virulent field isolate compared with the other field isolate and the vaccine strains, the detected viral copy number differed in an order of magnitude of 10(9). The virulent isolate contained PPV in all 10 organs with viral loads varying between 10(11) and 10(15) per 10(6) cells. Concerning the other field isolate and the two vaccine strains, if PPV was detected, in most of the cases the highest viral load was found in foetal kidneys with a maximum viral load of 10(3) per 10(6) cells. Additionally, PPV was found in the heart of one foetus, in the liver and duodenum of one foetus and in the thymus of one foetus with viral loads varying between 10(2.1) and 10(3.5) per 10(6) cells. In completely mummified foetuses with no discriminable organs of foetuses infected with the vaccine strains and the less virulent isolate, PPV was present in very low amounts or even below the detection limit.

Parvovirus variation for disease: a difference with RNA viruses?

The Parvoviridae, a family of viruses with single-stranded DNA genomes widely spread from invertebrates to mammal and human hosts, display a remarkable evolutionary capacity uncommon in DNA genomes. Parvovirus populations show high genetic heterogeneity and large population sizes resembling the quasispecies found in RNA viruses. These viruses multiply in proliferating cells, causing acute, persistent or latent infections relying in the immunocompetence and developmental stage of the hosts. Some parvovirus populations in natural settings, such as carnivore autonomous parvoviruses or primate adeno associated virus, show a high degree of genetic heterogeneity. However, other parvoviruses such as the pathogenic B19 human erythrovirus or the porcine parvovirus, show little genetic variation, indicating different virus-host relationships. The Parvoviridae evolutionary potential in mammal infections has been modeled in the experimental system formed by the immunodeficient scid mouse infected by the minute virus of mice (MVM) under distinct immune and adaptive pressures. The sequence of viral genomes (close to 10(5) nucleotides) in emerging MVM pathogenic populations present in the organs of 26 mice showed consensus sequences not representing the complex distribution of viral clones and a high genetic heterogeneity (average mutation frequency 8.3 x 10(-4) substitutions/nt accumulated over 2-3 months). Specific amino acid changes, selected at a rate up to 1% in the capsid and in the NS2 nonstructural protein, endowed these viruses with new tropism and increased fitness. Further molecular analysis supported the notion that, in addition to immune pressures, the affinity of molecular interactions with cellular targets, as the Crml nuclear export receptor or the primary capsid receptor, as well as the adaptation to tissues enriched in proliferating cells, are major selective factors in the rapid parvovirus evolutionary dynamics.