Viral load and histological distribution of atypical porcine pestivirus in different tissues of naturally infected piglets

Dynamics of Infection of Atypical Porcine Pestivirus in Commercial Pigs from Birth to Market: A Longitudinal Study

Atypical porcine pestivirus (APPV) was found to be associated with pigs demonstrating congenital tremors (CT), and clinical signs in pigs have been reproduced after experimental challenge. Subsequently, APPV has been identified in both symptomatic and asymptomatic swine of all ages globally. The objective of this research was to perform a longitudinal study following two cohorts of pigs, those born in litters with pigs exhibiting CT and those born in litters without CT, to analyze the virus and antibody dynamics of APPV infection in serum from birth to market. There was a wide range in the percentage of affected pigs (8-75%) within CT-positive litters. After co-mingling with CT-positive litters at weaning, pigs from CT-negative litters developed viremia that was cleared after approximately 2 months, with the majority seroconverting by the end of the study. In contrast, a greater percentage of pigs exhibiting CT remained PCR positive throughout the growing phase, with less than one-third of these animals seroconverting. APPV RNA was present in multiple tissues from pigs in both groups at the time of marketing. This study improved our understanding of the infection dynamics of APPV in swine and the impact that the immune status and timing of infection have on the persistence of APPV in serum and tissues.

Development of a Crystal Digital RT-PCR for the Detection of Atypical Porcine Pestivirus

Atypical porcine pestivirus (APPV), a newly discovered virus, is associated with the type A-II congenital tremor (CT) in neonatal piglets. APPV distributes throughout the world and causes certain economic losses to the swine industry. The specific primers and probe were designed targeting the 5' untranslated region (UTR) of APPV to amplify a 90 bp fragment, and the recombinant standard plasmid was constructed. After optimizing the concentrations of primers and probe, annealing temperature, and reaction cycles, a crystal digital RT-PCR (cdRT-PCR) and real-time quantitative RT-PCR (qRT-PCR) were successfully established. The results showed that the standard curves of the qRT-PCR and the cdRT-PCR had R² values of 0.999 and 0.9998, respectively. Both methods could specifically detect APPV, and no amplification signal was obtained from other swine viruses. The limit of detection (LOD) of the cdRT-PCR was 0.1 copies/µL, and that of the qRT-PCR was 10 copies/µL. The intra-assay and inter-assay coefficients of variation of repeatability and reproducibility were less than 0.90% for the qRT-PCR and less than 5.27% for the cdRT-PCR. The 60 clinical tissue samples were analyzed using both methods, and the positivity rates of APPV were 23.33% by the qRT-PCR and 25% by the cdRT-PCR, with a coincidence rate of 98.33%. The results indicated that the cdRT-PCR and the qRT-PCR developed here are highly specific, sensitive methods for the rapid and accurate detection of APPV.

Detection and investigation of atypical porcine pestivirus in a swine production system

A commercial farrow-to-finish farm was suspicious of atypical porcine pestivirus (APPV) after observing clinical signs of congenital tremors (CT) and splay leg (SL) of newborn pigs. If introduced onto the farrow-to-finish, the two potential routes of introduction could be through replacement gilts or incoming semen doses. Therefore, this study aimed to determine the prevalence of clinical APPV within the sampled population, identify the route of APPV introduction to this system, and determine prevalence of detectable APPV RNA within a population of gilt multiplication farm offspring through an isolation nursery and finisher barn. Farrowing records were analyzed for the presence of CT or SL and corresponding parity of the dam. Overall, prevalence of clinically affected litters within batch farrowing groups ranged from 0 to 31%. Phylogenetic analysis was conducted on a serum sample from a gilt at the isolation nursery, semen dose for the farrow-to-finish farm, and serum of a CT piglet. Results indicated that the virus circulating in clinically affected piglets was most similar to an incoming semen dose (98.9% nucleotide identity). Blood samples were collected at four time points and revealed APPV clinical prevalence was 37.5–77.5% during the nursery phase and 0–26% during the finisher phase. Oral fluids were also collected during the finisher phase and APPV clinical prevalence was 100% for all sampling time points. In summary, introduction of APPV into naïve herds is associated with increased clinical CT and SL cases and is detectable in asymptomatic pigs during the nursery and finisher production phases. This study found that potential screening tests for APPV could include oral fluids or qRT-PCR analysis of semen doses especially when trying to identify prevalence levels on naïve farm.

Inguinal lymph node sample collected by minimally invasive sampler helps to accurately diagnose ASF in dead pigs without necropsy

African swine fever (ASF) is a highly contagious hemorrhagic and transboundary animal disease, and it threatens global food security. A full necropsy to harvest the sample matrices for diagnosis in the farm may lead to contamination of the premises and directly threaten to the herds. In the present study, we compared the ASFV loads of the common samples that can be collected without necropsy. The unmatched nasal, throat, rectal samples were randomly taken using cotton swabs, and inguinal lymph node samples were collected by the minimally invasive samplers from the dead pigs of an ASF field outbreak farm. The ASFV loads of the samples were detected by qPCR and the results suggested that the overall ASFV nucleic acids levels of inguinal lymph node samples were higher than the swabs. What's more, sets of matched nasal swabs, rectal swabs, throat swabs, inguinal lymph nodes, serums, spleens and lungs samples were collected from 15 dead ASFV naturally infected pigs. Similarly, the results showed that inguinal lymph node samples, together with serum, spleen and lungs samples, contained more ASFV nucleic acids than the swabs. Our findings demonstrated that the inguinal lymph node collected by minimally invasive sampler is an ideal tissue for diagnosing ASFV infection in dead pigs without necropsy.

DNAJC14-Independent Replication of the Atypical Porcine Pestivirus

Atypical porcine pestiviruses (APPV; Pestivirus K) are a recently discovered, very divergent species of the genus Pestivirus within the family Flaviviridae. The presence of APPV in piglet-producing farms is associated with the occurrence of so-called “shaking piglets,” suffering from mild to severe congenital tremor type A-II. Previous studies showed that the cellular protein DNAJC14 is an essential cofactor of the NS2 autoprotease of all classical pestiviruses. Consequently, genetically engineered DNAJC14 knockout cell lines were resistant to all tested noncytopathogenic (non-cp) pestiviruses. Surprisingly, we found that the non-cp APPV can replicate in these cells in the absence of DNAJC14, suggesting a divergent mechanism of polyprotein processing. A complete laboratory system for the study of APPV was established to learn more about the replication of this unusual virus. The inactivation of the APPV NS2 autoprotease using reverse genetics resulted in nonreplicative genomes. To further investigate whether a regulation of the NS2-3 cleavage is also existing in APPV, we constructed synthetic viral genomes with deletions and duplications leading to the NS2 independent release of mature NS3. As observed with other pestiviruses, the increase of mature NS3 resulted in elevated viral RNA replication levels and increased protein expression. Our data suggest that APPV exhibit a divergent mechanism for the regulation of the NS2 autoprotease activity most likely utilizing a different cellular protein for the adjustment of replication levels.

IMPORTANCE DNAJC14 is an essential cofactor of the pestiviral NS2 autoprotease, limiting replication to tolerable levels as a prerequisite for the noncytopathogenic biotype of pestiviruses. Surprisingly, we found that the atypical porcine pestivirus (APPV) is able to replicate in the absence of DNAJC14. We further investigated the NS2-3 processing of APPV using a molecular clone, monoclonal antibodies, and DNAJC14 knockout cells. We identified two potential active site residues of the NS2 autoprotease and could demonstrate that the release of NS3 by the NS2 autoprotease is essential for APPV replication. Defective interfering genomes and viral genomes with duplicated NS3 sequences that produce mature NS3 independent of the NS2 autoprotease activity showed increased replication and antigen expression. It seems likely that an alternative cellular cofactor controls NS2-3 cleavage and thus replication of APPV. The replication-optimized synthetic APPV genomes might be suitable live vaccine candidates, whose establishment and testing warrant further research.

Genetic characterization of atypical porcine pestivirus from neonatal piglets with congenital tremor in Hubei province, China

Background

Atypical porcine pestivirus (APPV) is a single-stranded RNA virus with high genetic variation that causes congenital tremor (CT) in newborn piglets, belonging to the genus Pestivirus of the family Flaviviridae. Increasing cases of APPV infection in China in the past few years would pose severe challenges to the development of pig production. In view of the high genetic variability of APPV, the genetic characteristics of APPV in Hubei province was determined.

Methods

52 tissue samples from 8 CT-affected newborn piglets were collected at two different periods in the same pig farm in Hubei province. Viral nucleic acid was extracted to detect pathogens that can cause CT in piglets or other common clinical pathogens by RT-PCR. Haematoxylin and eosin (HE) staining, immunohistochemical (IHC) analysis, and qRT-PCR were performed to observe histopathological changes and histological distribution, and detect the viral load of APPV in CT-affected piglets. The full-length genome of APPV was obtained and sequence analysis was conducted to determine the phylogenetic relationship.

Results

Histopathological observation and histological distribution analysis showed that the histological lesions and distribution of APPV were mainly in central nervous system (CNS) tissues and immune tissues. Viral load analysis revealed that the viral copy number was higher in the cerebellum, submaxillary lymph nodes, tonsil, and serum than in other tissues. Phylogenetic analysis showed that CH-HB2020 and CH-HB2021 belonged to Clade I.3, and is most closely related to APPV_CH-GX2016. Sequence alignment based on APPV encoding sequences (CDS) showed that the nucleotide identities of CH-HB2020 or CH-HB2021 with Clade I, Clade II, and Clade III strains were 83.5–98.6%, 83.1–83.5%, and 81.1–81.4%, respectively, while the amino acid identities were 91.9–99.2%, 91.2–95.3%, and 90.77–91.4%, respectively. No recombination event was observed in CH-HB2020 or CH-HB2021 strains.

Conclusions

These findings enhance our understanding of the pathogenesis of APPV and may provide potential molecular evidence for its prevalence and transmission.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01780-8.

Fc-Mediated E2-Dimer Subunit Vaccines of Atypical Porcine Pestivirus Induce Efficient Humoral and Cellular Immune Responses in Piglets

Congenital tremor (CT) type A-II in piglets is caused by an emerging atypical porcine pestivirus (APPV), which is prevalent in swine herds and a serious threat to the pig production industry. This study aimed to construct APPV E2 subunit vaccines fused with Fc fragments and evaluate their immunogenicity in piglets. Here, APPV E2Fc and E2ΔFc fusion proteins expressed in Drosophila Schneider 2 (S2) cells were demonstrated to form stable dimers in SDS-PAGE and western blotting assays. Functional analysis revealed that aE2Fc and aE2ΔFc fusion proteins could bind to FcγRI on antigen-presenting cells (APCs), with the affinity of aE2Fc to FcγRI being higher than that of aE2ΔFc. Moreover, subunit vaccines based on aE2, aE2Fc, and aE2ΔFc fusion proteins were prepared, and their immunogenicity was evaluated in piglets. The results showed that the Fc fusion proteins emulsified with the ISA 201VG adjuvant elicited stronger humoral and cellular immune responses than the IMS 1313VG adjuvant. These findings suggest that APPV E2 subunit vaccines fused with Fc fragments may be a promising vaccine candidate against APPV.

Detection and localization of atypical porcine pestivirus in the testicles of naturally infected, congenital tremor affected piglets

Atypical porcine pestivirus (APPV) belongs to the genus Pestivirus within the family Flaviviridae. Recently, APPV has been identified as the causative agent of congenital tremor (CT) type AII. The disease is a neurological disorder that affects newborn piglets and is characterized by generalized trembling of the animals and often splay legs. CT is well known worldwide, and the virus seems to be highly prevalent in major swine producing areas. However, little is known about the epidemiology of the infection, transmission and spread of the virus between herds. Here, we show the high prevalence of APPV in processing fluid samples collected from Hungarian pig herds which led us to investigate the cellular targets of the virus in the testicles of newborn piglets affected by CT. By the development of an RNA in situ hybridization assay and the use of immunohistochemistry on consecutive slides, we identified the target cells of APPV in the testicle: interstitial Leydig cells, peritubular myoid cells and smooth muscle cells of medium-sized arteries. Previous studies have shown that APPV can be found in the semen of sexually mature boars suggesting the role of infected boars and their semen in the transmission of the virus similar to many other members of the Flaviviridae family. As in our case, the virus has not been identified in cells beyond the Sertoli cell barrier, further studies on infected adult boars' testicles and other reproductive glands are needed to analyze the possible changes in the cell tropism of APPV that might contribute to its prolonged extraction by the semen beyond the period of viraemia.

Vaccination with virus-like particles of atypical porcine pestivirus inhibits virus replication in tissues of BALB/c mice

Congenital tremor (CT) type A-II in piglets is a worldwide disease caused by an emerging atypical porcine pestivirus (APPV). Preparation and evaluation of vaccines in laboratory animals is an important preliminary step toward prevention and control of the disease. Here, virus-like particles (VLPs) of APPV were prepared and VLPs vaccine was evaluated in BALB/c mice. Purified E^rns and E2 proteins expressed in E. coli were allowed to self-assemble into VLPs, which had the appearance of hollow spherical particles with a diameter of about 100 nm by transmission electron microscopy (TEM). The VLPs induced strong antibody responses and reduced the viral load in tissues of BALB/c mice. The data from animal challenge experiments, RT-PCR, and immunohistochemical analysis demonstrated that BALB/c mice are an appropriate laboratory model for APPV. These results suggest the feasibility of using VLPs as a vaccine for the prevention and control of APPV and provide useful information for further study of APPV in laboratory animals.

Distribution and persistence of atypical porcine pestivirus in experimentally inoculated pigs

Atypical porcine pestivirus (APPV) is a cause of congenital tremors (CTs) in piglets and has been found in swine populations around the globe. Although systemic distribution of the virus has been reported, there is limited information regarding viral localization at the cellular level and distribution at the tissue level. We collected multiple tissues from 2-d-old piglets (n = 36) born to sows inoculated at 45 or 62 d of gestation with APPV via 3 simultaneous routes: intravenous, intranasal, and directly in amniotic vesicles. In addition, 2 boars from APPV-inoculated sows with CT were raised and euthanized when 11 mo old. In situ hybridization performed on tissue samples from piglets demonstrated a broad and systemic distribution of viral RNA including endothelial cells, fibroblasts, and smooth muscle. Labeling in tissues was more pronounced in piglet tissues compared to boars, with the notable exception of diffuse labeling of the cerebellum in boars. Presence of APPV in boar tissues well after resolution of clinical signs suggests persistence of APPV similar to other pestiviruses.

Evolution and genetic diversity of atypical porcine pestivirus (APPV) from piglets with congenital tremor in Guangxi Province, Southern China

Atypical porcine pestivirus (APPV) was identified and associated with congenital tremor (CT) type A‐II in new born piglets and has been reported in many countries. In China, the first APPV identification in swine herds was reported in Guangdong province in 2016. To investigate the genetic characteristics of APPV in Guangxi province, 53 tissue samples from neonatal piglets with CT were collected and detected from October 2017 to May 2019. Five APPV strains which were named as GX04/2017, GX01‐2018, GX02‐2018, GX01‐2019 and GX02‐2019 were obtained. Sequence analysis revealed that all six APPV strains from Guangxi province, including five strains from this study and one from a previous report, shared 83.3%‐97.5% nucleotide identity of complete genome and 91.7%‐99.1% amino acid identity of the open reading frame (ORF), and shared 77.7%‐97.7% nucleotide identity of complete genome and 90.6%‐99.3% amino acid identity of ORF with reference strains. Phylogenetic analysis indicated that all APPV strains could be divided into three clades based on the complete genome, N^pro, E^rns and E2 gene sequences, respectively; and the APPV strains from Guangxi province distributed in two clades (clades I and II). No sign of recombination was observed from Guangxi strains. Evolution analysis performed on the complete genome of 58 APPV strains showed that America, Europe and Asia strains during 2006–2019 evolved at a mean rate of 1.37 × 10^–4 substitutions/site/year, and the most recent common ancestor (tMRCA) of them was estimated as 1,700.5 years ago. The findings of this study indicated that there existed a high degree of genetic diversity of APPV from Guangxi province, Southern China, which provided important information on the epidemiological features and evolutionary relationships of APPV.

Pestivirus K (Atypical Porcine Pestivirus): Update on the Virus, Viral Infection, and the Association with Congenital Tremor in Newborn Piglets

The atypical porcine pestivirus (APPV) belongs to the species Pestivirus K of the genus Pestivirus and the family Flaviviridae, and it has been associated with congenital tremor (CT) type A-II in newborn piglets. Although APPV was discovered in 2015, evidence shows that APPV has circulated in pig herds for many years, at least since 1986. Due to the frequently reported outbreaks of CT on different continents, the importance of this virus for global pig production is notable. Since 2015, several studies have been conducted to clarify the association between APPV and CT. However, some findings regarding APPV infection and the measures taken to control and prevent the spread of this virus need to be contextualized to understand the infection better. This review attempts to highlight advances in the understanding of APPV associated with type A-II CT, such as etiology, epidemiology, diagnosis, and control and prevention measures, and also describes the pathophysiology of the infection and its consequences for pig production. Further research still needs to be conducted to elucidate the host's immune response to APPV infection, the control and prevention of this infection, and the possible development of vaccines.

Development of the reverse genetics system for emerging atypical porcine pestivirus using in vitro and intracellular transcription systems

Atypical porcine pestivirus (APPV) is a novel pestivirus causing congenital tremor (CT) type AII in piglets and exhibiting a broad geographical distribution. Lack of an operating system for the viral genome is one of bottlenecks which restrict further research on pathogenesis and gene functions of APPV. Reverse genetics system (RGS) is a feasible solution to this bottleneck problem, but, to-date, no RGSs have been developed for APPV. Here, for the first time, recombinant APPV CH-GD2017 were rescued using in vitro and intracellular transcription systems and the virons were observed via transmission electron microscopy. As the process of in vitro transcription is time-consuming and inefficient, a full-length cDNA clone in an intracellular transcription was further constructed using an RNA polymerase II system. Then, the rescued virus was identified via RT-PCR detection, indirect immunofluorescent assay, and transmission electron microscopy. Development of the RGS for APPV will provide an important tool for further research on this newly emerging pathogen.