Taming PRRSV: revisiting the control strategies and vaccine design

Layered Double Hydroxides (LDH) as Delivery Vehicles of a Chimeric Protein Carrying Epitopes from the Porcine Reproductive and Respiratory Syndrome Virus

The Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) causes reproductive failure and respiratory symptoms, leading to huge economic losses for the pig farming industry. Although several vaccines against PRRSV are available in the market; they show an overall low efficacy, and several countries have the need for vaccines covering the local, circulating variants. This project aims at developing a new chimeric antigen targeting specific epitopes from PRRSV and evaluating two test adjuvants to formulate a vaccine candidate. The test antigen was called LTB-PRRSV, which was produced recombinantly in Escherichia coli and consisted of the heat labile enterotoxin B subunit from E. coli (LTB) and four epitopes from PRRSV. LTB-PRRSV was rescued as inclusion bodies and methods for its solubilization, IMAC-based purification, and refolding were standardized, leading to mean yields of 18 mg of pure protein per liter culture. Layered double hydroxides (LDH) have been used as vaccine adjuvants given their biocompatibility, low cost, and positive surface charge that allows an efficient adsorption of negatively charged biomolecules. Therefore, LDH were selected as delivery vehicles of LTB-PRRSV. Pure LTB-PRRSV was adsorbed onto LDH by incubation at different LDH:LTB-PRRSV mass ratios (1:0.25, 1:0.5, 1:1, and 1:2) and at pH 9.5. The best adsorption occurred with a 1:2 mass ratio, and in a sucrose-tween solution. The conjugates obtained had a polydispersity index of 0.26, a hydrodynamic diameter of 192 nm, and a final antigen concentration of 64.2 μg/mL. An immunogenicity assessment was performed by injecting mice with LDH:LTB-PRRSV, Alum/LTB-PRRSV, or LTB-PRRSV in a scheme comprising three immunizations at two-week intervals and two dose levels (1 and 5 μg). LTB-PRRSV was capable of inducing strong humoral responses, which lasted for a longer period when LDH was used as the delivery vehicle/adjuvant. The potential of LDH to serve as an attractive carrier for veterinary vaccines is discussed.

Minor envelope proteins from GP2a to GP4 contribute to the spread pattern and yield of type 2 PRRSV in MARC-145 cells

In China, porcine reproductive and respiratory syndrome virus (PRRSV) vaccines are widely used. These vaccines, which contain inactivated and live attenuated vaccines (LAVs), are produced by MARC-145 cells derived from the monkey kidney cell line. However, some PRRSV strains in MARC-145 cells have a low yield. Here, we used two type 2 PRRSV strains (CH-1R and HuN4) to identify the genes responsible for virus yield in MARC-145 cells. Our findings indicate that the two viruses have different spread patterns, which ultimately determine their yield. By replacing the viral envelope genes with a reverse genetics system, we discovered that the minor envelope proteins, from GP2a to GP4, play a crucial role in determining the spread pattern and yield of type 2 PRRSV in MARC-145 cells. The cell-free transmission pattern of type 2 PRRSV appears to be more efficient than the cell-to-cell transmission pattern. Overall, these findings suggest that GP2a to GP4 contributes to the spread pattern and yield of type 2 PRRSV.

Ubiquitin-specific proteinase 1 stabilizes PRRSV nonstructural protein Nsp1β to promote viral replication by regulating K48 ubiquitination

The porcine reproductive and respiratory syndrome virus (PRRSV) can lead to severe reproductive problems in sows, pneumonia in weaned piglets, and increased mortality, significantly negatively impacting the economy. Post-translational changes are essential for the host-dependent replication and long-term infection of PRRSV. Uncertainty surrounds the function of the ubiquitin network in PRRSV infection. Here, we screened 10 deubiquitinating enzyme inhibitors and found that the ubiquitin-specific proteinase 1 (USP1) inhibitor ML323 significantly inhibited PRRSV replication in vitro. Importantly, we found that USP1 interacts with nonstructural protein 1β (Nsp1β) and deubiquitinates its K48 to increase protein stability, thereby improving PRRSV replication and viral titer. Among them, lysine at position 45 is essential for Nsp1β protein stability. In addition, deficiency of USP1 significantly reduced viral replication. Moreover, ML323 loses antagonism to PRRSV rSD16-K45R. This study reveals the mechanism by which PRRSV recruits the host factor USP1 to promote viral replication, providing a new target for PRRSV defense.IMPORTANCEDeubiquitinating enzymes are critical factors in regulating host innate immunity. The porcine reproductive and respiratory syndrome virus (PRRSV) nonstructural protein 1β (Nsp1β) is essential for producing viral subgenomic mRNA and controlling the host immune system. The host inhibits PRRSV proliferation by ubiquitinating Nsp1β, and conversely, PRRSV recruits the host protein ubiquitin-specific proteinase 1 (USP1) to remove this restriction. Our results demonstrate the binding of USP1 to Nsp1β, revealing a balance of antagonism between PRRSV and the host. Our research identifies a brand-new PRRSV escape mechanism from the immune response.

Epitope screening and vaccine molecule design of PRRSV GP3 and GP5 protein based on immunoinformatics

Porcine reproductive and respiratory syndrome (PRRS) is a respiratory disease in pigs that causes severe economic losses. Currently, live PRRSV vaccines are commonly used but fail to prevent PRRS outbreaks and reinfection. Inactivated PRRSV vaccines have poor immunogenicity, making PRRSV a significant threat to swine health globally. Therefore, there is an urgent need to develop an effective PRRSV vaccine. This study used immunoinformatics to predict, screen, design and construct a candidate vaccine that fused B-cell epitopes, CTL- and HTL-dominant protective epitopes of PRRSV strain's GP3 and GP5 proteins. The study identified 12 B-cell epitopes, 6 CTL epitopes and 5 HTL epitopes of GP3 and GP5 proteins. The candidate vaccine was constructed with 50S ribosomal protein L7/L1 molecular adjuvant, which has antigenicity, solubility, stability, non-allergenicity and a high affinity for its target receptor, TLR-3. The C-ImmSim immunostimulation results showed significant increases in cellular and humoral responses (B cells and T cells) and production of TGF-β, IL-2, IL-10, IFN-γ and IL-12. The constructed vaccine was stable and immunogenic, and it can effectively induce strong T-cell and B-cell immune responses against PRRSV. Therefore, it is a promising candidate vaccine for controlling and preventing PRRSV outbreaks.

Glycoprotein 5-Derived Peptides Induce a Protective T-Cell Response in Swine against the Porcine Reproductive and Respiratory Syndrome Virus

We analyzed the T-cell responses induced by lineal epitopes of glycoprotein 5 (GP5) from PRRSV to explore the role of this protein in the immunological protection mediated by T-cells. The GP5 peptides were conjugated with a carrier protein for primary immunization and booster doses. Twenty-one-day-old pigs were allocated into four groups (seven pigs per group): control (PBS), vehicle (carrier), PTC1, and PTC2. Cytokine levels were measured at 2 days post-immunization (DPI) from serum samples. Cytotoxic T-lymphocytes (CTLs, CD8+) from peripheral blood were quantified via flow cytometry at 42 DPI. The cytotoxicity was evaluated by co-culturing primed lymphocytes with PRRSV derived from an infectious clone. The PTC2 peptide increased the serum concentrations of pro-inflammatory cytokines (i.e., TNF-α, IL-1β, IL-8) and cytokines that activate the adaptive cellular immunity associated with T-lymphocytes (i.e., IL-4, IL-6, IL-10, and IL-12). The concentration of CTLs (CD8+) was significantly higher in groups immunized with the peptides, which suggests a proliferative response in this cell population. Primed CTLs from immunized pigs showed cytolytic activity in PRRSV-infected cells in vitro. PTC1 and PTC2 peptides induced a protective T-cell-mediated response in pigs immunized against PRRSV, due to the presence of T epitopes in their sequences.

Protection provided by a commercial modified-live porcine reproductive and respiratory syndrome virus (PRRSV) 1 vaccine (PRRSV1-MLV) against a Japanese PRRSV2 field strain

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) vaccines do not provide full cross-protection, mainly due to the virus genetic variability. Despite this, vaccines based on modified-live PRRSV (PRRSV-MLV) reduce the disease impact.

OBJECTIVES

To assess the efficacy of two commercial vaccines-one based on PRRSV1 (PRRSV1-MLV) and another on PRRSV2 (PRRSV2-MLV)-against a Japanese PRRSV2 field strain.

METHODS

Two groups of three-week-old piglets were vaccinated (G1: PRRSV1-MLV; G2: PRRSV2-MLV) and two were kept as non-vaccinated (INF and CTRL). One month later, G1, G2, and INF were challenged with a PRRSV2 field strain.

RESULTS

After the challenge, clinical signs were only observed in INF. Moreover, the highest rectal temperatures and values for the area under the curve (AUC) were observed in INF. Regarding viral detection, both AUC and the proportion of positive samples in blood were higher in INF. In G1, viremic animals never reached 100%. At necropsy (21 d after the challenge), differences for titers among groups were only found in tonsils (G1 < G2 and INF). One animal (belonging to G1) was negative in all tissues. Regarding humoral responses, G1 and G2 seroconverted after vaccination, as detected in the corresponding enzyme-linked immunosorbent assay. Specific neutralizing antibodies (NA) against PRRSV1-MLV were already detected at 14 d after vaccination in G1, showing a significant booster after the challenge, while PRRSV2-MLV NA were detected in G2 at the end of the experiment.

CONCLUSIONS

Despite genetic differences, PRRSV1-MLV has been demonstrated to confer partial protection against a Japanese PRRSV2 strain, at least as good as PRRSV2-MLV.

Complex interplay between PRRSV-1 genetic diversity, coinfections and antimicrobial use influences performance parameters in post-weaning pigs

Porcine reproductive and respiratory syndrome (PRRS) is one of the main diseases of pigs, leading to large economic losses in swine production worldwide. PRRSV high mutation rate and low cross-protection between strains make PRRS control challenging. Through a semi-longitudinal approach, we analysed the relationships among performance parameters, PRRSV-1 genetic diversity, coinfections and antimicrobial use (AMU) in pig nurseries. We collected data over the course of five years in five PRRS-positive nurseries belonging to an Italian multisite operation, for a total of 86 batches and over 200,000 weaners involved. The farm experienced a severe PRRS outbreak in the farrowing unit at the onset of the study, but despite adopting vaccination of all sows, batch-level losses in nurseries in the following years remained constantly high (mean±SE: 11.3 ± 0.5 %). Consistently with previous studies, our phylogenetic analysis of ORF 7 sequences highlighted the peculiarity of strains circulating in Italy. Greater genetic distances between the strain circulating in a weaners' batch and strains from the farrowing unit and the previous batch were associated with increased mortality (p < 0.0001). All the respiratory and enteric coinfections contributed to an increase in losses (all p < 0.026), with secondary infections by Streptococcus suis and enteric bacteria also inducing an increase in AMU (both p < 0.041). Our findings highlight that relying solely on sows' vaccination is insufficient to contain PRRS losses, and the implementation of rigorous biosecurity measures is pivotal to limit PRRSV circulation among pig flows and consequently minimise the risk of exposure to genetically diverse strains that would increase production costs.

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

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

Occurrence and Associated Risk Factors of Porcine Reproductive and Respiratory Syndrome Virus and Porcine Circovirus Type 2 Infections in Greece

The objective of the present study was to identify factors associated with the probability of being polymerase chain reaction (PCR) positive and the level of porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2) occurrence in Greek farms. The study included 56 pig farms, with a total population of 22.500 sows, which represent about 40% of the entire capacity of the Greek swine production. A total of 896 blood samples (224 pools*4 samples/pool = 896 samples) from breeding stock, weaners, growers, and finishers were collected from each farm and organized in pools of 4 samples size. Further, data regarding herd health management protocols were collected. The sera were tested for PRRSV and PCV2, using real-time PCR (RT-PCR). The results indicated that both viruses remain a major challenge for the Greek swine industry. Main risk factors involved in the infection process by these viruses were identified. In particular, vaccination programs such as the mass PRRSV vaccination with modified-live virus (MLV) in breeding stock during the last stages of gestation or with killed-virus (KV) during the middle of gestation are more likely to be associated with PRRSV PCR-positivity. Farms with low biosecurity level are associated with higher PRRSV circulation. It has also been revealed that breeding stock is more likely to be associated with PCV2 circulation compared to weaners and growers. In conclusion, our results could be the basis of the development of surveillance protocols for a national monitoring system for PRRSV and PCV2, which could prevent future infection of Greek farms.

Breeding for disease resilience: opportunities to manage polymicrobial challenge and improve commercial performance in the pig industry

Disease resilience, defined as an animal's ability to maintain productive performance in the face of infection, provides opportunities to manage the polymicrobial challenge common in pig production. Disease resilience can deliver a number of benefits, including more sustainable production as well as improved animal health and the potential for reduced antimicrobial use. However, little progress has been made to date in the application of disease resilience in breeding programs due to a number of factors, including (1) confusion around definitions of disease resilience and its component traits disease resistance and tolerance, and (2) the difficulty in characterizing such a complex trait consisting of multiple biological functions and dynamic elements of rates of response and recovery from infection. Accordingly, this review refines the definitions of disease resistance, tolerance, and resilience based on previous studies to help improve the understanding and application of these breeding goals and traits under different scenarios. We also describe and summarize results from a "natural disease challenge model" designed to provide inputs for selection of disease resilience. The next steps for managing polymicrobial challenges faced by the pig industry will include the development of large-scale multi-omics data, new phenotyping technologies, and mathematical and statistical methods adapted to these data. Genome editing to produce pigs resistant to major diseases may complement selection for disease resilience along with continued efforts in the more traditional areas of biosecurity, vaccination and treatment. Altogether genomic approaches provide exciting opportunities for the pig industry to overcome the challenges provided by hard-to-manage diseases as well as new environmental challenges associated with climate change.

Polyplex nanomicelle delivery of self-amplifying RNA vaccine

Self-amplifying RNA (SaRNA) is a burgeoning platform that exploits the replication machinery of alphaviruses such as Venezuelan equine encephalitis (VEE) virus or Sindbis virus (SIN). SaRNA has been used for development of human vaccines, but has not been evaluated for porcine vaccine development. Porcine reproductive and respiratory syndrome virus (PRRSV) causes tremendous economic losses to the worldwide pork industry, but current vaccines trigger delayed neutralizing antibody response and confer only partial protection. Here we first compared two SaRNA systems based on VEE and SIN, and demonstrated that in vitro transcribed VEE-based SaRNA conferred prolonged reporter gene expression and RNA amplification in pig cells with low cytotoxicity, but SIN-based SaRNA imparted evident cytotoxicity and limited gene expression in pig cells. Transfection of VEE-based SaRNA that encodes the major PRRSV antigen dNGP5 (SaRNA-dNGP5) conferred persistent expression for at least 28 days in pig cells. We next complexed SaRNA-dNGP5 with the polyaspartamide block copolymer PEG-PAsp(TEP) to form polyplex nanomicelle with high packaging efficiency and narrow size distribution. The polyplex nanomicelle enabled sustained dNGP5 expression and secretion in vitro. Compared with the commercial PRRS vaccine, nanomicelle delivery of SaRNA-dNGP5 into animal models accelerated the induction of potent neutralizing antibodies with minimal side effects, and elicited stronger IL-4 and IFN-γ responses against homologous and heterologous PRRSV. These properties tackle the problems of current vaccines and implicate the potential of SaRNA-dNGP5 nanomicelle as an effective PRRS vaccine.

Functionally active cyclin-dependent kinase 9 is essential for porcine reproductive and respiratory syndrome virus subgenomic RNA synthesis

Cyclin-dependent kinase 9 (CDK9) is a key regulator of RNA-polymerase II and a candidate therapeutic target for various virus infections such as respiratory syncytial virus, herpes simplex virus, human adenovirus, human cytomegalovirus, hepatitis virus B, and human papillomavirus. We employed CDK9-IN-1, a selective CDK9 inhibitor, to investigate the role of CDK9 in porcine reproductive and respiratory syndrome virus (PRRSV) infection. CDK9-IN-1 dose-dependently reduced PRRSV replication without cytotoxicity in the infected cells. The antiviral activity of CDK9-IN-1 was further confirmed by evaluating the effects of lentivirus-mediated CDK9 knockdown or CDK9 overexpression on PRRSV infection. Briefly, the depletion of CDK9 significantly inhibited viral replication, while the overexpression of CDK9 promoted viral replication. PRRSV infection also enhanced the nuclear export of CDK9 without affecting CDK9 protein expression. Viral replication cycle analyses further revealed that functionally active CDK9 in the cytosol advanced viral subgenomic RNA synthesis. Collectively, our data illustrated that CDK9 was a new host factor that was involved in PRRSV subgenomic RNA synthesis, and CDK9 inhibitor, CDK9-IN-1 was a promising antiviral candidate for PRRSV infection.

Geographic Drivers of Antimicrobial Use and Resistance in Pigs in Khon Kaen Province, Thailand

In Thailand, pig production has increased considerably in the last decades to meet a growing demand for pork. Antimicrobials are used routinely in intensive pig production to treat infections and increase productivity. However, the use of antimicrobials also contributes to the rise of antimicrobial resistance with potential consequences for animal and human health. Here, we quantify the association between antimicrobial use and resistance rates in extensive and intensive farms with a focus on geographic proximity between farm and drugstores. Of the 164 enrolled farms, 79% reported using antimicrobials for disease prevention, treatment, or as a feed additive. Antimicrobial-resistant E. coli were present in 63% of farms. These drugs included critically important antimicrobials, such as quinolones and penicillins. Medium-scale farms with intensive animal production practices showed higher resistance rates than small-scale farms with extensive practices. Farms with drug-resistant Escherichia coli were located closer to drugstores and a had a higher proportion of disease than farms without drug-resistant E. coli. We found no association between the presence of resistance in humans and antimicrobial use in pigs. Our findings call for actions to improve herd health to reduce the need for antimicrobials and systematic training of veterinarians and drugstore owners on judicious use of antimicrobials in animals to mitigate resistance.

A chimeric porcine reproductive and respiratory syndrome virus (PRRSV)-2 vaccine is safe under international guidelines and effective both in experimental and field conditions

Vaccination is currently the most effective strategy to control porcine reproductive and respiratory syndrome (PRRS). New-generation PRRS vaccines are required to be safe and broadly cross-protective. We have recently created the chimeric PRRS virus K418DM which proved to be a good vaccine candidate under field conditions. In the present study, we designed safety and efficacy tests under experimental and field conditions for further evaluation of K418DM1.1, a plaque-purified K418DM. In the homologous challenge study, K418DM1.1 induced high serum virus neutralization (SVN) antibody titers (i.e., 4.2 log₂ ± 1.7) at 21 days post-challenge (dpc) and provided protection as demonstrated by the significantly lower levels of viremia at 3 and 7 dpc and significantly lower microscopic lung lesion scores compared to the unvaccinated group. K418DM1.1 was also protective in the heterologous challenge study, with vaccinated pigs showing significantly lower levels of viremia at 14 dpc compared to the unvaccinated pigs. A field study was performed to evaluate the efficacy of K418DM1.1 against heterologous exposure and vaccinated pigs presented significantly lower viremia than unvaccinated pigs. According to the safety test for the examination of virulence reversion, no infectivity was observed in tissue homogenate filtrate both in the vaccinated and comingled groups. Thus, the risk of virulence, as well as transmission, appeared negligible. These overall results indicate that K418DM1.1 is a good vaccine candidate based on its safety and protective efficacy.

A Review on the Use of Antimicrobial Peptides to Combat Porcine Viruses

Viral infectious diseases pose a serious threat to animal husbandry, especially in the pig industry. With the rapid, continuous variation of viruses, a series of therapeutic measures, including vaccines, have quickly lost their efficacy, leading to great losses for animal husbandry. Therefore, it is urgent to find new drugs with more stable and effective antiviral activity. Recently, it has been reported that antimicrobial peptides (AMPs) have great potential for development and application in animal husbandry because of their significant antibacterial and antiviral activity, and the antiviral ability of AMPs has become a research hotspot. This article aims to review the research situation of AMPs used to combat viruses in swine production of animal husbandry, clarify the mechanism of action of AMPs on viruses and raise some questions, and explore the future potential of AMPs in animal husbandry.

Why, when and how should exposure be considered at the within-host scale? A modelling contribution to PRRSv infection

Understanding the impact of pathogen exposure on the within-host dynamics and its outcome in terms of infectiousness is a key issue to better understand and control the infection spread. Most experimental and modelling studies tackling this issue looked at the impact of the exposure dose on the infection probability and pathogen load, very few on the within-host immune response. Our aim was to explore the impact on the within-host response not only of the exposure dose, but also of its duration and peak, for contrasted virulence levels. We used an integrative modelling approach of the within-host dynamics at the between-cell level. We focused on the porcine reproductive and respiratory syndrome virus, a major concern for the swine industry. We quantified the impact of exposure and virulence on the viral dynamics and immune response by global sensitivity analyses and descriptive statistics. We found that the area under the viral curve, an indicator of the infection severity, was fully determined by the exposure intensity. The infection duration increased with the strain virulence and, for a given strain, exhibited a positive linear correlation with the exposure intensity logarithm and the exposure duration. Taking into account the exposure intensity is hence necessary. Besides, representing the exposure due to contacts by a single punctual dose would tend to underestimate the infection duration. As the infection severity and duration both contribute to the pig infectiousness, a prolonged exposure of the adequate intensity would be recommended in an immuno-epidemiological context.

Investigating the relationship between vaginal microbiota and host genetics and their impact on immune response and farrowing traits in commercial gilts

Our objectives were to evaluate the interaction between host genetics and vaginal microbiota and their relationships with antibody (Ab) response to porcine reproductive and respiratory syndrome virus (PRRSV) vaccination and farrowing performance in commercial gilts. The farrowing performance traits were number born alive, number weaning (NW), total number born, number born dead, stillborn, mummies and preweaning mortality (PWM). The vaginal microbiota was collected on days 4 (D4) and 52 (D52) after vaccination for PRRSV. Blood samples were collected on D52 for Ab measurement. Actinobacteria, Bacterioidetes, Firmicutes, Proteobacteria and Tenericutes were the most abundant Phyla identified in the vaginal microbiota. Heritability ranged from ~0 to 0.60 (Fusobacterium) on D4 and from ~0 to 0.63 (Terrisporobacter) on D52, with 43 operational taxonomic units (OTUs) presenting moderate to high heritability. One major QTL on chromosome 12 was identified for 5 OTUs (Clostridiales, Acinetobacter, Ruminococcaceae, Campylobacter and Anaerococcus), among other 19 QTL. The microbiability for Ab response to PRRSV vaccination was low for both days (<0.07). For farrowing performance, microbiability varied from <0.001 to 0.15 (NW on D4). For NW and PWM, the microbiability was greater than the heritability estimates. Actinobacillus, Streptococcus, Campylobacter, Anaerococcus, Mollicutes, Peptostreptococcus, Treponema and Fusobacterium showed different abundance between low and high Ab responders. Finally, canonical discriminant analyses revealed that vaginal microbiota was able to classify gilts in high and low Ab responders to PRRSV vaccination with a misclassification rate of <0.02. Although the microbiota explained limited variation in Ab response and farrowing performance traits, there is still potential to explore the use of vaginal microbiota to explain variation in traits such as NW and PWM. In addition, these results revealed that there is a partial control of host genetic over vaginal microbiota, suggesting a possibility for genetic selection on the vaginal microbiota.

Predicting vaccine effectiveness in livestock populations: A theoretical framework applied to PRRS virus infections in pigs

Vaccines remain one of the main tools to control infectious diseases in domestic livestock. Although a plethora of veterinary vaccines are on the market and routinely applied to protect animals against infection with particular pathogens, the disease in question often continues to persist, sometimes at high prevalence. The limited effectiveness of certain vaccines in the field leaves open questions regarding the required properties that an effective vaccine should have, as well as the most efficient vaccination strategy for achieving the intended goal of vaccination programmes. To date a systematic approach for studying the combined effects of different types of vaccines and vaccination strategies is lacking. In this paper, we develop a theoretical framework for modelling the epidemiological consequences of vaccination with imperfect vaccines of various types, administered using different strategies to herds with different replacement rates and heterogeneity in vaccine responsiveness. Applying the model to the Porcine Reproductive and Respiratory Syndrome (PRRS), which despite routine vaccination remains one of the most significant endemic swine diseases worldwide, we then examine the influence of these diverse factors alone and in combination, on within-herd virus transmission. We derive threshold conditions for preventing infection invasion in the case of imperfect vaccines inducing limited sterilizing immunity. The model developed in this study has practical implications for the development of vaccines and vaccination programmes in livestock populations not only for PRRS, but also for other viral infections primarily transmitted by direct contact.

How to prevent viremia rebound? Evidence from a PRRSv data-supported model of immune response

Background

Understanding what determines the between-host variability in infection dynamics is a key issue to better control the infection spread. In particular, pathogen clearance is desirable over rebounds for the health of the infected individual and its contact group. In this context, the Porcine Respiratory and Reproductive Syndrome virus (PRRSv) is of particular interest. Numerous studies have shown that pigs similarly infected with this highly ubiquitous virus elicit diverse response profiles. Whilst some manage to clear the virus within a few weeks, others experience prolonged infection with a rebound. Despite much speculation, the underlying mechanisms responsible for this undesirable rebound phenomenon remain unclear.

Results

We aimed at identifying immune mechanisms that can reproduce and explain the rebound patterns observed in PRRSv infection using a mathematical modelling approach of the within-host dynamics. As diverse mechanisms were found to influence PRRSv infection, we established a model that details the major mechanisms and their regulations at the between-cell scale. We developed an ABC-like optimisation method to fit our model to an extensive set of experimental data, consisting of non-rebounder and rebounder viremia profiles. We compared, between both profiles, the estimated parameter values, the resulting immune dynamics and the efficacies of the underlying immune mechanisms. Exploring the influence of these mechanisms, we showed that rebound was promoted by high apoptosis, high cell infection and low cytolysis by Cytotoxic T Lymphocytes, while increasing neutralisation was very efficient to prevent rebounds.

Conclusions

Our paper provides an original model of the immune response and an appropriate systematic fitting method, whose interest extends beyond PRRS infection. It gives the first mechanistic explanation for emergence of rebounds during PRRSv infection. Moreover, results suggest that vaccines or genetic selection promoting strong neutralising and cytolytic responses, ideally associated with low apoptotic activity and cell permissiveness, would prevent rebound.

Electronic supplementary material

The online version of this article (10.1186/s12918-018-0666-7) contains supplementary material, which is available to authorized users.

Cross-sectional study on risk factors for Porcine Reproductive and Respiratory Syndrome virus sow herd instability in German breeding herds

BACKGROUND

Porcine Reproductive and Respiratory Syndrome virus (PRRSV) stability, besides meeting production targets, is also a requirement for several control options of PRRS in pig breeding farms. This study aimed to investigate the frequency of, and risk factors associated with, PRRSV instability in pig breeding farms in northern Germany. In 120 sow herds, a questionnaire on production and management parameters was filled, and blood samples from 30 suckling pigs from 10 different litters were taken and examined using real-time polymerase chain reaction (PCR).

RESULTS

PRRSV was detected in 32 herds (27%), thus classified as PRRSV-unstable. According to multivariable logistic regression analysis, a suckling period ≤ 21 days, a low distance between the cadaver collection site and the actual sow barn, ≥ 2 pig herds in a 1000 m radius, presence of external employees, a time interval between purchase of gilts of ≤ 9 weeks and a 1- or 2-weekly farrowing rhythm were associated with a higher risk of PRRSV instability.

CONCLUSIONS

External and internal biosecurity as well as management factors were associated with PRRSV instability, which could be targeted by farmers and veterinarians to help them to achieve PRRSV PCR-negative status and in the control of PRRS.

Estimating Parameters Related to the Lifespan of Passively Transferred and Vaccine-Induced Porcine Reproductive and Respiratory Syndrome Virus Type I Antibodies by Modeling Field Data

The outputs of epidemiological models are strongly related to the structure of the model and input parameters. The latter are defined by fitting theoretical concepts to actual data derived from field or experimental studies. However, some parameters may remain difficult to estimate and are subject to uncertainty or sensitivity analyses to determine their variation range and their global impact on model outcomes. As such, the evaluation of immunity duration is often a puzzling issue requiring long-term follow-up data that are, most of time, not available. The present analysis aims at characterizing the kinetics of antibodies against Porcine Reproductive and Respiratory Syndrome virus (PRRSv) from longitudinal data sets. The first data set consisted in the serological follow-up of 22 vaccinated gilts during 21 weeks post-vaccination (PV). The second one gathered the maternally derived antibodies (MDAs) kinetics in piglets from three different farms up to 14 weeks of age. The peak of the PV serological response against PRRSv was reached 6.9 weeks PV on average with an average duration of antibodies persistence of 26.5 weeks. In the monitored cohort of piglets, the duration of passive immunity was found relatively short, with an average duration of 4.8 weeks. The level of PRRSv-MDAs was found correlated with the dams' antibody titer at birth, and the antibody persistence was strongly related to the initial MDAs titers in piglets. These results evidenced the importance of PRRSv vaccination schedule in sows, to optimize the delivery of antibodies to suckling piglets. These estimates of the duration of active and passive immunity could be further used as input parameters of epidemiological models to analyze their impact on the persistence of PRRSv within farms.

Reproductive effects of arteriviruses: equine arteritis virus and porcine reproductive and respiratory syndrome virus infections

Equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) are the most economically important members of the family Arteriviridae. EAV and PRRSV cause reproductive and respiratory disease in equids and swine, respectively and constitute a significant economic burden to equine and swine industries around the world. Furthermore, they both cause abortion in pregnant animals and establish persistent infection in their natural hosts, which fosters viral shedding in semen leading to sexual transmission. The primary focus of this article is to provide an update on the effects of these two viruses on the reproductive tract of their natural hosts and provide a comparative analysis of clinical signs, virus-host interactions, mechanisms of viral pathogenesis and viral persistence.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): porcine reproductive and respiratory syndrome (PRRS)

Porcine reproductive and respiratory syndrome (PRRS) has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of PRRS to be listed, Article 9 for the categorisation of PRRS according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to PRRS. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, PRRS can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria as in Sections 4 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (d) and (e) of Article 9(1). The animal species to be listed for PRRS according to Article 8(3) criteria are domestic pigs and wild boar.

Comparison of Immune Responses in Pigs Infected with Chinese Highly Pathogenic PRRS Virus Strain HV and North American Strain NADC-20

Introduction:

Chinese HP-PRRSV characterized by high morbidity and mortality of all ages of pigs emerged since 2006 in China. The immune response of HP-PRRSV was never compared with conventional low pathogenic PRRSV strain.

Objective:

In this study, we compared the immune responses elicited by a Chinese HP-PRRSV strain HV and a North American RRSV strain NADC20 infections.

Result:

Pigs infected with NADC-20 showed significantly higher Ab titers than HV-PRRSV infected pigs at 9 DPI. Infection with HV-PRRSV induced a significantly higher levels of TNF-α and IL-10 in both sera and lung tissues and higher IFN-α and IFN-γ in the serum. Flow cytometry analysis showed that HV-PRRSV infected pigs generated significantly higher frequencies of NK cells in the peripheral blood and Th/memory, CTLs, and T-reg cells in the lung as compared with NADC-20 infected pigs.

Conclusion:

This study demonstrates that different immunity profiles were elicited by HV-PRRSV and NADC-20, and these differences may contribute to the distinct pathogenesis of HV-PRRSV and NADC-20.

Overexpression of Histone Deacetylase 6 Enhances Resistance to Porcine Reproductive and Respiratory Syndrome Virus in Pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically relevant viral pathogens in pigs and causes substantial losses in the pig industry worldwide each year. At present, PRRSV vaccines do not effectively prevent and control this disease. Consequently, it is necessary to develop new antiviral strategies to compensate for the inefficacy of the available vaccines. Histone deacetylase 6 (HDAC6) is an important member of the histone deacetylase family that is responsible for regulating many important biological processes. Studies have shown that HDAC6 has anti-viral activities during the viral life cycle. However, whether HDAC6 overexpression enhances resistance to PRRSV in pigs remains unknown. In this study, we used a somatic cell cloning method to produce transgenic (TG) pigs that constitutively overexpress porcine HDAC6. These TG pigs showed germ line transmission with continued overexpression of HDAC6. In vitro, virus-challenged porcine alveolar macrophages (PAMs) overexpressed HDAC6, which suppressed viral gene expression and PRRSV production. In vivo, resistance to PRRSV in TG pigs was evaluated by direct or cohabitation mediated infection with a highly pathogenic PRRSV (HP-PRRSV) strain. Compared with non-TG (NTG) siblings, TG pigs showed a significantly lower viral load in the lungs and an extended survival time after infection with HP-PRRSV via intramuscular injection. In the cohabitation study, NTG pigs housed with challenged NTG pigs exhibited significantly worse clinical symptoms than the other three in-contact groups. These results collectively suggest that HDAC6 overexpression enhances resistance to PRRSV infection both in vitro and in vivo. Our findings suggest the potential involvement of HDAC6 in the response to PRRSV, which will facilitate the development of novel therapies for PRRSV.

Positive immunomodulatory effects of heterologous DNA vaccine- modified live vaccine, prime-boost immunization, against the highly-pathogenic PRRSV infection

Porcine reproductive and respiratory syndrome virus (PRRSV) infection is one of the most important swine pathogens, and causes a major economic impact worldwide. Recently, a new variant type 2 PRRSV, highly pathogenic PRRSV (HP-PRRSV) has emerged and continued to circulate in Southeast Asia region. Currently, commercially available PRRSV vaccines, modified live PRRS vaccines (MLV) are not able to provide complete protection against HP-PRRSV and been reported to induce negative immunomodulatory effects. Interestingly, a novel DNA vaccine was developed and successfully used to improve PRRSV-specific immune responses following MLV vaccination. To investigate the efficacy of a heterologous DNA-MLV prime-boost immunization against the HP-PRRSV infection, an experimental vaccinated-challenged study was conducted. Two-week-old, PRRSV-seronegative, crossbred pigs (5-8 pigs/group) were allocated into 5 groups. At day -14 (D-14), the treatment group (DNA-MLV) was immunized with a DNA vaccine encoding PRRSV-truncated nucleocapsid protein (pORF7t), followed by a commercial modified live type 2 PRRS vaccine (MLV) at D0. The other groups included the group that received PBS at D-14 followed by MLV at D0 (MLV), pORF7t at D-14 (DNA), PBS at D0 (PBS) and the negative control group. At D42, all groups, except the negative control group, were challenged with HP-PRRSV (strain 10PL1). The results demonstrated that pigs that received MLV, regardless of the DNA priming, exhibited less clinical signs and faster viral clearance. Following HP-PRRSV challenge, the DNA-MLV group exhibited improved PRRSV-specific immunity, as observed by increased neutralizing antibody titers and PRRSV-specific IFN-γ production, and reduced IL-10 and PRRSV-specific Treg productions. However, neither the prime-boost immunization nor the MLV was able to induce complete clinical protection against HP-PRRSV infection. In conclusion, improved immunological responses, but not clinical protection, were achieved by DNA-MLV prime-boost immunization. This study highlights the potential use of heterologous prime-boost vaccination regimen, where DNA can be incorporated with other vaccine candidates, for improving anti-PRRSV immunity that may eventually lead induction of complete PRRSV protection.

Spatial analysis and characteristics of pig farming in Thailand

Background

In Thailand, pig production intensified significantly during the last decade, with many economic, epidemiological and environmental implications. Strategies toward more sustainable future developments are currently investigated, and these could be informed by a detailed assessment of the main trends in the pig sector, and on how different production systems are geographically distributed. This study had two main objectives. First, we aimed to describe the main trends and geographic patterns of pig production systems in Thailand in terms of pig type (native, breeding, and fattening pigs), farm scales (smallholder and large-scale farming systems) and type of farming systems (farrow-to-finish, nursery, and finishing systems) based on a very detailed 2010 census. Second, we aimed to study the statistical spatial association between these different types of pig farming distribution and a set of spatial variables describing access to feed and markets.

Results

Over the last decades, pig population gradually increased, with a continuously increasing number of pigs per holder, suggesting a continuing intensification of the sector. The different pig-production systems showed very contrasted geographical distributions. The spatial distribution of large-scale pig farms corresponds with that of commercial pig breeds, and spatial analysis conducted using Random Forest distribution models indicated that these were concentrated in lowland urban or peri-urban areas, close to means of transportation, facilitating supply to major markets such as provincial capitals and the Bangkok Metropolitan region. Conversely the smallholders were distributed throughout the country, with higher densities located in highland, remote, and rural areas, where they supply local rural markets. A limitation of the study was that pig farming systems were defined from the number of animals per farm, resulting in their possible misclassification, but this should have a limited impact on the main patterns revealed by the analysis.

Conclusions

The very contrasted distribution of different pig production systems present opportunities for future regionalization of pig production. More specifically, the detailed geographical analysis of the different production systems will be used to spatially-inform planning decisions for pig farming accounting for the specific health, environment and economical implications of the different pig production systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0849-7) contains supplementary material, which is available to authorized users.

Enriched Housing Reduces Disease Susceptibility to Co-Infection with Porcine Reproductive and Respiratory Virus (PRRSV) and Actinobacillus pleuropneumoniae (A. pleuropneumoniae) in Young Pigs

Until today, anti-microbial drugs have been the therapy of choice to combat bacterial diseases. Resistance against antibiotics is of growing concern in man and animals. Stress, caused by demanding environmental conditions, can reduce immune protection in the host, influencing the onset and outcome of infectious diseases. Therefore psychoneuro-immunological intervention may prove to be a successful approach to diminish the impact of diseases and antibiotics use. This study was designed to investigate the effect of social and environmental enrichment on the impact of disease, referred to as "disease susceptibility", in pigs using a co-infection model of PRRSV and A. pleuropneumoniae. Twenty-eight pigs were raised in four pens under barren conditions and twenty-eight other pigs were raised in four pens under enriched conditions. In the enriched pens a combination of established social and environmental enrichment factors were introduced. Two pens of the barren (BH) and two pens of the enriched housed (EH) pigs were infected with PRRSV followed by A. pleuropneumoniae, the other two pens in each housing treatment served as control groups. We tested if differences in disease susceptibility in terms of pathological and clinical outcome were related to the different housing regimes and if this was reflected in differences in behavioural and immunological states of the animals. Enriched housed pigs showed a faster clearance of viral PRRSV RNA in blood serum (p = 0.014) and histologically 2.8 fold less interstitial pneumonia signs in the lungs (p = 0.014). More barren housed than enriched housed pigs developed lesions in the lungs (OR = 19.2, p = 0.048) and the lesions in the barren housed pigs showed a higher total pathologic tissue damage score (p<0.001) than those in enriched housed pigs. EH pigs showed less stress-related behaviour and differed immunologically and clinically from BH pigs. We conclude that enriched housing management reduces disease susceptibility to co-infection of PRRSV and A. pleuropneumoniae in pigs. Enrichment positively influences behavioural state, immunological response and clinical outcome in pigs.

Inactivated porcine reproductive and respiratory syndrome virus vaccine adjuvanted with Montanide™ Gel 01 ST elicits virus-specific cross-protective inter-genotypic response in piglets

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BEI-inactivated PRRSV candidate vaccine was developed using local Kazakh viral strains.

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Immune response and clinical disease were compared with a commercial PRRSV vaccine.

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Compared to commercial vaccine our vaccine induced better cross-protective response.

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Use of a potent adjuvant and local PRRSV strains in the vaccine formulation is beneficial.

The efficacy of a novel BEI-inactivated porcine reproductive and respiratory syndrome virus (PRRSV) candidate vaccine in pigs, developed at RIBSP Republic of Kazakhstan and delivered with an adjuvant Montanide™ Gel 01 ST (D/KV/ADJ) was compared with a commercial killed PRRSV vaccine (NVDC-JXA1, C/KV/ADJ) used widely in swine herds of the Republic of Kazakhstan. Clinical parameters (body temperature and respiratory disease scores), virological and immunological profiles [ELISA and virus neutralizing (VN) antibody titers], macroscopic lung lesions and viral load in the lungs (quantitative real-time PCR and cell culture assay) were assessed in vaccinated and both genotype 1 and 2 PRRSV challenged pigs. Our results showed that the commercial vaccine failed to protect pigs adequately against the clinical disease, viremia and lung lesions caused by the challenged field isolates, Kazakh strains of PRRSV type 1 and type 2 genotypes. In contrast, clinical protection, absence of viremia and lung lesions in D/KV/ADJ vaccinated pigs was associated with generation of VN antibodies in both homologous vaccine strain LKZ/2010 (PRRSV type 2) and a heterogeneous type 1 PRRSV strain (CM/08) challenged pigs. Thus, our data indicated the induction of cross-protective VN antibodies by D/KV/ADJ vaccine, and importantly demonstrated that an inactivated PRRSV vaccine could also induce cross-protective response across the viral genotype.

Virus replicon particles expressing porcine reproductive and respiratory syndrome virus proteins elicit immune priming but do not confer protection from viremia in pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of one of the most devastating and economically significant viral disease of pigs worldwide. The vaccines currently available on the market elicit only limited protection. Recombinant vesicular stomatitis virus (VSV) replicon particles (VRP) have been used successfully to induce protection against influenza A virus (IAV) in chickens and bluetongue virus in sheep. In this study, VSV VRP expressing the PRRSV envelope proteins GP5, M, GP4, GP3, GP2 and the nucleocapsid protein N, individually or in combination, were generated and evaluated as a potential vector vaccine against PRRSV infection. High level expression of the recombinant PRRSV proteins was demonstrated in cell culture. However, none of the PRRSV antigens expressed from VRP, with the exception of the N protein, did induce any detectable antibody response in pigs before challenge infection with PRRSV. After challenge however, the antibody responses against GP5, GP4 and GP3 appeared in average 2 weeks earlier than in pigs vaccinated with the empty control VRP. No reduction of viremia was observed in the vaccinated group compared with the control group. When pigs were co-vaccinated with VRP expressing IAV antigens and VRP expressing PRRSV glycoproteins, only antibody responses to the IAV antigens were detectable. These data show that the VSV replicon vector can induce immune responses to heterologous proteins in pigs, but that the PRRSV envelope proteins expressed from VSV VRP are poorly immunogenic. Nevertheless, they prime the immune system for significantly earlier B-cell responses following PRRSV challenge infection.

Transdermal delivery of plasmid encoding truncated nucleocapsid protein enhanced PRRSV-specific immune responses

BACKGROUND

Porcine Reproductive and Respiratory Syndrome virus (PRRSV) induces several immunomodulatory mechanisms that resulted in delayed and ineffective anti-viral immune responses. Recently, it has been shown that intradermal immunization of plasmid encoding truncated nucleocapsid protein (pORF7t) could reduce PRRSV-induced immunomodulatory activities and enhances anti-PRRSV immunity in vaccinated pigs. However, intradermal immunization may not be practical for farm setting. Currently, there are several transdermal delivery systems available in the market, although they were not originally designed for plasmid delivery.

OBJECTIVES

To investigate the potential use of a transdermal delivery system for delivering of pORF7t and its immunological outcomes.

METHOD

The immunomodulatory effects induced by transdermal delivery of pORF7t were compared with intradermal immunization in an experimental pig model. In addition, immunomodulatory effects of the DNA vaccine were determined in the fattening pigs kept in a PRRSV-positive farm environment, and in the experimental pigs receiving heterologous prime-boost, pORF7t-modified live vaccine (MLV) immunization.

RESULT

The patterns of PRRSV-specific cellular responses induced by transdermal and intradermal immunizations of pORF7t were similar. Interestingly, the pigs transdermally immunized with pORF7t exhibited higher number of PRRSV-specific CD8(+)IFN-γ(+) cells. Pigs immunized with pORF7t and kept at PRRSV-positive environment exhibited enhanced PRRSV-specific IFN-γ(+) production, reduced numbers of regulatory T lymphocytes (Tregs) and lower lung scores at the end of the finishing period. In the heterologous prime-boost experiment, priming with pORF7t prior to MLV vaccination resulted in significantly higher numbers of CD3(+)IFN-γ(+) subpopulations, lower numbers of PRRSV-specific CD3(+)IL-10(+) cells and Tregs, and rapid antibody responses in immunized pigs.

CONCLUSION

Transdermal immunization with pORF7t reduced PRRRSV-induced immunomodulatory effects and enhanced long-term PRRSV-specific cellular responses in vaccinated pigs. Furthermore, heterologous DNA-MLV prime-boost immunization significantly improved the quality of PRRSV-specific cellular and humoral immunity. The information could benefit the future development of PRRSV management and control strategies.

Investigation on host susceptibility of Tibetan pig to infection of porcine reproductive and respiratory syndrome virus through viral challenge study

Previous reports showed that infection of porcine reproductive and respiratory syndrome virus (PRRSV) stimulated a variable host response and pig susceptibility to PRRSV was largely dependent on its genetic composition. In the present study, host susceptibility of Tibetan pig to PRRSV was compared with other two pig breeds, ZangMei black and Large White, by challenge of them with highly pathogenic PRRSV (HP-PRRSV). In the first challenge test, each eight piglets of the three breeds were inoculated with HP-PRRSV and clinical symptoms, viremia and animal mortality were examined up to 28 days post inoculation (DPI). In the secondary pathological study, each twelve piglets of the three breeds were challenged and three pigs of each breed were sacrificed on 4, 7, and 14 DPI for examination of gross damage and lung microscopic lesions. The results showed that no typical clinical signs such as cough, diarrhea and high fever were observed in challenged Tibetan pigs, which however all occurred in Large White accompanied with ∼40% mortality (3/8). In addition, a significant low and short viremia was detected specifically in Tibetan pigs. Based on histopathological analysis of lung sections, a mild to moderate interstitial pneumonia in Tibetan pigs and a much severe pneumonia in Large White were identified on 7-14 DPI. In summary, the study demonstrated that three genetically different pig breeds exhibited a differential host susceptibility to HP-PRRSV and Tibetan pig was much less susceptible to the virus in the three tested pig breeds.

Plant-based porcine reproductive and respiratory syndrome virus VLPs induce an immune response in mice

Porcine reproductive and respiratory syndrome virus (PRRSV) significantly affects the swine industry worldwide. An efficient, protective vaccine is still lacking. Here, we report for the first time the generation and purification of PRRSV virus like particles (VLPs) by expressing GP5, M and N genes in Nicotiana silvestris plants. The particles were clearly visible by transmission electron microscopy (TEM) with a size of 60-70 nm. Hydrodynamic diameter of the particles was obtained and it was confirmed that the VLPs had the appropriate size for PRRS virions and that the VLPs were highly pure. By measuring the Z potential we described the electrophoretic mobility behavior of VLPs and the best conditions for stability of the VLPs were determined. The particles were immunogenic in mice. A western blot of purified particles allowed detection of three coexpressed genes. These VLPs may serve as a platform to develop efficient PRRSV vaccines.

The 15N and 46R Residues of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nucleocapsid Protein Enhance Regulatory T Lymphocytes Proliferation

Porcine reproductive and respiratory syndrome virus (PRRSV) negatively modulates host immune responses, resulting in persistent infection and immunosuppression. PRRSV infection increases the number of PRRSV-specific regulatory T lymphocytes (Tregs) in infected pigs. However, the target antigens for Tregs proliferation in PRRSV infection have not been fully understood. In this study, we demonstrated that the highly pathogenic PRRSV (HP-PRRSV) induced more CD4⁺CD25⁺Foxp3⁺ Tregs than classical PRRSV (C-PRRSV) strain. Of the recombinant GP5, M and N proteins of HP-PRRSV expressed in baculovirus expression systems, only N protein induced Tregs proliferation. The Tregs assays showed that three amino-acid regions, 15–21, 42–48 and 88–94, in N protein played an important role in induction of Tregs proliferation with synthetic peptides covering the whole length of N protein. By using reverse genetic methods, it was firstly found that the 15N and 46R residues in PRRSV N protein were critical for induction of Tregs proliferation. The phenotype of induced Tregs closely resembled that of transforming-growth-factor-β-secreting T helper 3 Tregs in swine. These data should be useful for understanding the mechanism of immunity to PRRSV and development of infection control strategies in the future.

A novel DNA vaccine for reduction of PRRSV-induced negative immunomodulatory effects: A proof of concept

BACKGROUND

Viral-induced interleukin (IL)-10 and regulatory T lymphocytes (Tregs) are believed to play a major role in shaping the immunological and clinical outcomes following Porcine Reproductive and Respiratory Syndrome virus (PRRSV) infection. Recently, it has been shown that PRRSV nucleocapsid (N) protein can induce IL-10 production which is essential for induction of PRRSV-specific Tregs. We hypothesized that immunity to N protein should reduce PRRSV-induced negative immunomodulatory effects which will be essential for establishing proper anti-PRRSV immunity in infected pigs.

OBJECTIVES

To investigate the immunomodulatory effects of DNA vaccine encoding a linearized, truncated form of PRRSV-N protein (pORF7t) which was designed to preferentially induce cell-mediated immunity against PRRSV N protein.

METHOD

Immunomodulatory effects of the novel DNA vaccine were investigated in an experimental vaccinated-challenged model. In addition, long-term immunomodulatory effects of the DNA vaccine were investigated in vaccinated pigs kept at the PRRSV-positive environment until the end of the fattening period. Pigs were vaccinated either prior to or following natural PRRSV infection.

RESULT

The results indicated that pORF7t could modulate the anti-PRRSV immune responses and promote the control of viral replication in the vaccinated-challenged pigs. Immunized pigs exhibited increased numbers of PRRSV-specific activated CD4(+)CD25(+) lymphocytes, reduced numbers of PRRSV-specific Tregs, and rapid viral clearance following infection. In a long-term study, regardless of the time of vaccination, DNA vaccine could modulate the host immune responses, resulted in enhanced PRRSV-specific IFN-γ producing cells, and reduced numbers of PRRSV-specific Tregs, without evidence of enhanced antibody responses. No vaccine adverse reaction was observed throughout the study.

CONCLUSION

This study revealed the novel concept that PRRSV-specific immunity can be modulated by induction of cell-mediated immunity against the nucleocapsid protein. This concept could potentially benefit the development of PRRSV management and control strategies.

Cecropin P1 inhibits porcine reproductive and respiratory syndrome virus by blocking attachment

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is a continuous threat to the pig industry, causing high economic losses worldwide. Current vaccines have specific limitations in terms of their safety and efficacy, so the development of novel antiviral drugs is urgently required. The aim of this study was to evaluate the inhibitory effects and underlying molecular mechanisms of the antimicrobial peptide cecropin P1 (CP1) against PRRSV infection in vitro.

Results

CP1 not only displayed extracellular virucidal activity against PRRSV, but also exerted a potent inhibitory effect when added either before, simultaneously with, or after viral inoculation. The inhibitory effect of CP1 occurred during viral attachment, but not at viral entry into Marc-145 cells. CP1 also inhibited viral particle release and attenuated virus-induced apoptosis during the late phase of infection. CP1 exerted similar inhibitory effects against PRRSV infection in porcine alveolar macrophages, the cells targeted by the virus in vivo during its infection of pigs. The expression of interleukin 6 was elevated by CP1 in porcine alveolar macrophages, which might contribute to its inhibition of PRRSV infection.

Conclusions

Collectively, our findings provide a new direction for the development of potential therapeutic drugs against PRRSV infection.

Immune Control of PRRS: Lessons to be Learned and Possible Ways Forward

Porcine reproductive and respiratory syndrome (PRRS) is an elusive model of host/virus relationship in which disease is determined by virus pathogenicity, pig breed susceptibility and phenotype, microbial infectious pressure, and environmental conditions. The disease can be controlled by farm management programs, which can be supported by vaccination or conditioning of animals to circulating PRRS virus (PRRSV) strains. Yet, PRRS still represents a cause of heavy losses for the pig industry worldwide. Immunological control strategies are often compounded by poor and late development of adaptive immunity in both vaccinated and infected animals. Also, there is evidence that results of field trials can be worse than those of experimental studies in isolation facilities. Neutralizing antibody (NA) was shown to prevent PRRSV infection. Instead, the role of NA and adaptive immunity on the whole in virus clearance after established PRRSV infections is still contentious. Pigs eventually eliminate PRRSV infection, which may be correlated with an “educated,” innate immune response, which may also develop following vaccination. In addition to vaccination, an immunomodulation strategy for PRRS can be reasonably advocated in pig “problem” farms, where a substantial control of disease prevalence and disease-related losses is badly needed. This is not at odds with vaccination, which should be preferably restricted to PRRSV-free animals bound for PRRSV-infected farm units. Oral, low-dose, interferon-α treatments proved effective on farm for the control of respiratory and reproductive disease outbreaks, whereas the results were less clear in isolation facilities. Having in mind the crucial interaction between PRRSV and bacterial lipopolysaccharides for occurrence of respiratory disease, the strong control actions of low-dose type I interferons on the inflammatory response observed in vitro and in vivo probably underlie the rapid clinical responses observed in field trials.

Integrative model of the immune response to a pulmonary macrophage infection: what determines the infection duration?

The immune mechanisms which determine the infection duration induced by pathogens targeting pulmonary macrophages are poorly known. To explore the impact of such pathogens, it is indispensable to integrate the various immune mechanisms and to take into account the variability in pathogen virulence and host susceptibility. In this context, mathematical models complement experimentation and are powerful tools to represent and explore the complex mechanisms involved in the infection and immune dynamics. We developed an original mathematical model in which we detailed the interactions between the macrophages and the pathogen, the orientation of the adaptive response and the cytokine regulations. We applied our model to the Porcine Respiratory and Reproductive Syndrome virus (PRRSv), a major concern for the swine industry. We extracted value ranges for the model parameters from modelling and experimental studies on respiratory pathogens. We identified the most influential parameters through a sensitivity analysis. We defined a parameter set, the reference scenario, resulting in a realistic and representative immune response to PRRSv infection. We then defined scenarios corresponding to graduated levels of strain virulence and host susceptibility around the reference scenario. We observed that high levels of antiviral cytokines and a dominant cellular response were associated with either short, the usual assumption, or long infection durations, depending on the immune mechanisms involved. To identify these mechanisms, we need to combine the levels of antiviral cytokines, including , and . The latter is a good indicator of the infected macrophage level, both combined provide the adaptive response orientation. Available PRRSv vaccines lack efficiency. By integrating the main interactions between the complex immune mechanisms, this modelling framework could be used to help designing more efficient vaccination strategies.

Spatial epidemiology of porcine reproductive and respiratory syndrome in Thailand

BACKGROUND

Porcine reproductive and respiratory syndrome (PRRS) has become a worldwide endemic disease of pigs. In 2006, an atypical and more virulent PRRS (HP-PRRS) emerged in China and spread to many countries, including Thailand. This study aimed to provide a first description of the spatio-temporal pattern of PRRS in Thailand and to quantify the statistical relationship between the presence of PRRS at the sub-district level and a set of risk factors. This should provide a basis for improving disease surveillance and control of PRRS in Thailand.

RESULTS

Spatial scan statistics were used to detect clusters of outbreaks and allowed the identification of six spatial clusters covering 15 provinces of Thailand. Two modeling approaches were used to relate the presence or absence of PRRS outbreaks at the sub-district level to demographic characteristics of pig farming and other epidemiological spatial variables: autologistic multiple regressions and boosted regression trees (BRT). The variables showing a statistically significant association with PRRS presence in the autologistic multiple regression model were the sub-district human population and number of farms with breeding sows. The predictive power of the model, as measured by the area under the curve (AUC) of the receiver operating characteristics (ROC) plots was moderate. BRT models had higher goodness of fit the metrics and identified the sub-district human population and density of farms with breeding sows as important predictor variables.

CONCLUSIONS

The results indicated that farms with breeding sows may be an important group for targeted surveillance and control. However, these findings obtained at the sub-district level should be complemented by farm-level epidemiological investigations in order to obtain a more comprehensive view of the factors affecting PRRS presence. In this study, the outbreaks of PRRS could not be differentiated from the potential novel HP-PPRS form, which was recently discovered in the country.

Actinobacillus pleuropneumoniae possesses an antiviral activity against porcine reproductive and respiratory syndrome virus

Pigs are often colonized by more than one bacterial and/or viral species during respiratory tract infections. This phenomenon is known as the porcine respiratory disease complex (PRDC). Actinobacillus pleuropneumoniae (App) and porcine reproductive and respiratory syndrome virus (PRRSV) are pathogens that are frequently involved in PRDC. The main objective of this project was to study the in vitro interactions between these two pathogens and the host cells in the context of mixed infections. To fulfill this objective, PRRSV permissive cell lines such as MARC-145, SJPL, and porcine alveolar macrophages (PAM) were used. A pre-infection with PRRSV was performed at 0.5 multiplicity of infection (MOI) followed by an infection with App at 10 MOI. Bacterial adherence and cell death were compared. Results showed that PRRSV pre-infection did not affect bacterial adherence to the cells. PRRSV and App co-infection produced an additive cytotoxicity effect. Interestingly, a pre-infection of SJPL and PAM cells with App blocked completely PRRSV infection. Incubation of SJPL and PAM cells with an App cell-free culture supernatant is also sufficient to significantly block PRRSV infection. This antiviral activity is not due to LPS but rather by small molecular weight, heat-resistant App metabolites (<1 kDa). The antiviral activity was also observed in SJPL cells infected with swine influenza virus but to a much lower extent compared to PRRSV. More importantly, the PRRSV antiviral activity of App was also seen with PAM, the cells targeted by the virus in vivo during infection in pigs. The antiviral activity might be due, at least in part, to the production of interferon γ. The use of in vitro experimental models to study viral and bacterial co-infections will lead to a better understanding of the interactions between pathogens and their host cells, and could allow the development of novel prophylactic and therapeutic tools.

Reproductive performance of sows with and without PRRS modified live virus vaccination in PRRS-virus-seropositive herds

Porcine reproductive and respiratory syndrome (PRRS) virus infection causes reproductive failures including return to oestrus, abortion, mummified foetuses, stillborn, and weak-born piglets. The objective of the present study was to investigate reproductive performance of sows in PRRS-virus-seropositive herds with and without PRRS modified live virus (PRRS-MLV) vaccination. The study was conducted in 20 PRRS-virus-seropositive commercial swine herds in Thailand. The data included 211,009 mating and 180,935 farrowing records. The analysed variables included farrowing rate (FR), return rate (RR), abortion rate (AR), total number of piglets born per litter (TB), number of piglets born alive per litter (BA), percentage of stillborn (SB), percentage of mummified foetuses (MM), and number of piglets weaned per litter (WP). The results revealed that FR in non-vaccinated sows was lower than that in vaccinated sows (85.0 vs 89.7%, respectively, P < 0.001), and RR in non-vaccinated sows was higher than that in vaccinated sows (6.9 vs 3.7%, respectively, P < 0.001). AR did not differ significantly between non-vaccinated and vaccinated sows (1.6 and 2.0%, respectively, P = 0.964). TB (11.2 and 11.5, respectively, P < 0.001), BA (10.0 and 10.6, respectively, P < 0.001), and WP (9.2 and 9.6, respectively, P < 0.001) in non-vaccinated sows were lower than those in vaccinated sows. SB (6.9 and 5.1%, respectively, P < 0.001) and MM (3.2 and 2.2%, respectively, P < 0.001) in PRRS-MLV-vaccinated sows were higher than those in non-vaccinated sows. The improvement in sow reproductive performance in PRRS-MLV-vaccinated herds was most pronounced in gilts and primiparous sows.

Development of a chimeric strain of porcine reproductive and respiratory syndrome virus with an infectious clone and a Korean dominant field strain

The K418 chimeric virus of porcine reproductive and respiratory syndrome virus (PRRSV) was engineered by replacing the genomic region containing structure protein genes of an infectious clone of PRRSV, FL12, with the same region obtained from a Korean dominant field strain, LMY. The K418 reached 10⁶ TCID₅₀/ml of viral titer with similar growth kinetics to those of parental strains and had a cross-reactive neutralizing antibody response to field serum from the entire country. The chimeric clone pK418 can be used as a practical tool for further studying the molecular characteristics of PRRSV proteins through genetic manipulation. Furthermore, successful construction of the K418 will allow for the development of customized vaccine candidates against PRRSV, which has evolved rapidly in Korea.

In Vitro Virucidal and Virustatic Properties of the Crude Extract of Cynodon dactylon against Porcine Reproductive and Respiratory Syndrome Virus

The in vitro virustatic and virucidal tests of the crude extract of Cynodon dactylon against infection with porcine reproductive and respiratory syndrome virus (PRRSV), a cause of major devastating pig disease, were described. Crude extract of C. dactylon was prepared for cytotoxicity on tissue-culture cells that were used to measure virustatic and virucidal activities against PRRSV. Crude extract of C. dactylon at 0.78 mg/mL showed no cytotoxicity on the cell line, and at that concentration significantly inhibited replication of PRRSV as early as 24 hours post infection (hpi). C. dactylon also inactivated PRRSV as determined by immunoperoxidase monolayer assay (IPMA) compared to the control experiments. In summary, the present study may be among the earliest studies to describe virustatic and virucidal activities of C. dactylon crude extract against PRRSV in vitro. Extracts of C. dactylon may be useful for PRRSV control and prevention on pig farms.