Porcine reproductive and respiratory syndrome (PRRS) with special reference to clinical aspects and diagnosis. A review

Genetic background influences pig responses to porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly infectious and economically significant virus that causes respiratory and reproductive diseases in pigs. It results in reduced productivity and increased mortality in pigs, causing substantial economic losses in the industry. Understanding the factors affecting pig responses to PRRSV is crucial to develop effective control strategies. Genetic background has emerged as a significant determinant of susceptibility and resistance to PRRSV in pigs. This review provides an overview of the basic infection process of PRRSV in pigs, associated symptoms, underlying immune mechanisms, and roles of noncoding RNA and alternative splicing in PRRSV infection. Moreover, it emphasized breed-specific variations in these aspects that may have implications for individual treatment options.

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.

Classification of porcine reproductive and respiratory syndrome clinical impact in Ontario sow herds using machine learning approaches

Since the early 1990s, porcine reproductive and respiratory syndrome (PRRS) virus outbreaks have been reported across various parts of North America, Europe, and Asia. The incursion of PRRS virus (PRRSV) in swine herds could result in various clinical manifestations, resulting in a substantial impact on the incidence of respiratory morbidity, reproductive loss, and mortality. Veterinary experts, among others, regularly analyze the PRRSV open reading frame-5 (ORF-5) for prognostic purposes to assess the risk of severe clinical outcomes. In this study, we explored if predictive modeling techniques could be used to identify the severity of typical clinical signs observed during PRRS outbreaks in sow herds. Our study aimed to evaluate four baseline machine learning (ML) algorithms: logistic regression (LR) with ridge and lasso regularization techniques, random forest (RF), k-nearest neighbor (KNN), and support vector machine (SVM), for the clinical impact classification of ORF-5 sequences and demographic data into high impact and low impact categories. First, baseline classifiers were evaluated using different input representations of ORF-5 nucleotides, amino acid sequences, and demographic data using a 10-fold cross-validation technique. Then, we designed a consensus voting ensemble approach to aggregate the different types of input representations for genetic and demographic data for classifying clinical impact. In this study, we observed that: (a) for abortion and pre-weaning mortality (PWM), different classifiers gained improvement over baseline accuracy, which showed the plausible presence of both genotypic-phenotypic and demographic-phenotypic relationships, (b) for sow mortality (SM), no baseline classifier successfully established such linkages using either genetic or demographic input data, (c) baseline classifiers showed good performance with a moderate variance of the performance metrics, due to high-class overlap and the small dataset size used for training, and (d) the use of consensus voting ensemble techniques helped to make the predictions more robust and stabilized the performance evaluation metrics, but overall accuracy did not substantially improve the diagnostic metrics over baseline classifiers.

Exostosin glycosyltransferase 1 reduces porcine reproductive and respiratory syndrome virus infection through proteasomal degradation of nsp3 and nsp5

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a serious threat to the swine industry worldwide. Exostosin glycosyltransferase 1 (EXT1), an enzyme involved in the biosynthesis of heparin sulfate, has also been reported to be a host factor essential for a wide variety of pathogens. However, the role of EXT1 in PRRSV infection remains uncharted. Here we identified that PRRSV infection caused an increase of EXT1 expression. EXT1 knockdown promoted virus infection, while its overexpression inhibited virus infection, suggesting an inhibitory function of EXT1 to PRRSV infection. We found that EXT1 had no effects on the attachment, internalization, or release of PRRSV, but did restrict viral RNA replication. EXT1 was determined to interact with viral non-structural protein 3 (nsp3) and nsp5 via its N-terminal cytoplasmic tail and to enhance K48-linked polyubiquitination of these two nsps to promote their degradation. Furthermore, the C-terminal glycosyltransferase activity domain of EXT1 was necessary for nsp3 and nsp5 degradation. We also found that EXT2, a EXT1 homologue, interacted with EXT1 and inhibited PRRSV infection. Similarly, EXT1 effectively restricted porcine epidemic diarrhea virus (PEDV) and porcine enteric alphacoronavirus (PEAV) infection in Vero cells. Taken together, this study reveals that EXT1 may serve as a broad-spectrum host restriction factor and suggests a molecular basis for the potential development of therapeutics against PRRSV infection.

Immunological response to porcine reproductive and respiratory syndrome virus in young pigs obtained from a PRRSV-positive exposure status herd in a PRRSV endemic area

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), remains a major economic threat to swine production throughout the world. The aim of this study was to investigate the humoral and cell-mediated immune responses to PRRSV in 10 PRRSV vaccinated and 10 non-vaccinated young pigs obtained from a PRRSV-seropositive herd under field conditions. On day 35 days of post-vaccination (dpv), two PRRSV seropositive mixed-litter pigs were added to each group to co-mingle the animals. Serum and whole blood samples were collected from all pigs on the first day of vaccination, as well as on the 21, 35, 49, and 63 dpv. The PRRSV-specific humoral and cell-mediated immune response was determined by ELISA and flow cytometry analysis. The PRRSV ELISA sample to positive (S/P) ratio was found to be positive at the threshold level until the age of 84 days in both non-vaccinated and vaccinated groups, whereas the IFN-γ positive staining cytotoxic (CD8⁺) cells were rapidly expressed in the early periods of vaccination and co-mingling, but were not found to be specific to PRRSV. This result might have been due to an unspecific response to stress antigens. Further studies should be conducted to obtain more immune response data over long-term observation periods and to study the effect of PRRSV endemic strain vaccinations in endemically-infected herds.

Detection of porcine reproductive and respiratory syndrome virus (PRRSV) 1-7-4-type strains in Peru

Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses to the swine industry worldwide. While PRRSV has been endemic in North America since 1989, it was not until 1999 that the virus was first described in South America. Notably, recently an increased number of PRRSV outbreaks have been reported in South American countries. However, epidemiological information related to these outbreaks is limited and the genetic characteristics of the PRRSV strains circulating in the region are poorly understood. In this study, we describe the genetic analyses of PRRSV strains associated with severe PRRS outbreaks in Peru. Samples originating from 14 farms located in two Departments in Peru (Lima and Arequipa), were subjected to RT-PCR amplification of the PRRSV ORF5 gene and sequencing followed by restriction fragment length polymorphism (RFLP) analysis. Results demonstrated the circulation of PRRSV-2 in Peru. Notably ORF5 RFLP typing revealed that 15 (75%) of the PRRSV strains detected in this study belong to the RFLP 1-7-4 type. Phylogenetic analysis showed that the Peruvian strains are closely related to the highly virulent PRRSV 1-7-4 strains that emerged in the US in 2013-2014. Results here indicate the presence of highly virulent PRRSV 1-7-4 strains in Peru and provide important information on the geographical distribution of PRRSV, confirming the recent geographical expansion of this important swine pathogen towards South America.

Establishment and Characterization of a High and Stable Porcine CD163-Expressing MARC-145 Cell Line

Isolation and identification of diverse porcine reproductive and respiratory syndrome viruses (PRRSVs) play a fundamental role in PRRSV research and disease management. However, PRRSV has a restricted cell tropism for infection. MARC-145 cells are routinely used for North American genotype PRRSV isolation and vaccine production. But MARC-145 cells have some limitations such as low virus yield. CD163 is a cellular receptor that mediates productive infection of PRRSV in various nonpermissive cell lines. In this study, we established a high and stable porcine CD163- (pCD163-) expressing MARC-145 cell line toward increasing its susceptibility to PRRSV infection. Indirect immunofluorescence assay (IFA) and Western blotting assays showed that pCD163 was expressed higher in pCD163-MARC cell line than MARC-145 cells. Furthermore, the ability of pCD163-MARC cell line to propagate PRRSV was significantly increased as compared with MARC-145 cells. Finally, we found that pCD163-MARC cell line had a higher isolation rate of clinical PRRSV samples and propagated live attenuated PRRS vaccine strains more efficiently than MARC-145 cells. This pCD163-MARC cell line will be a valuable tool for propagation and research of PRRSV.

Epidemiological investigations of the introduction of porcine reproductive and respiratory syndrome virus in Chile, 2013-2015

Porcine reproductive and respiratory syndrome (PRRS) is endemic in most pork producing countries. In Chile, eradication of PRRS virus (PRRSV) was successfully achieved in 2009 as a result of the combined efforts of producers and the animal health authorities. In October 2013, after several years without detecting PRRSV under surveillance activities, suspected cases were confirmed on a commercial swine farm. Here, we describe the PRRS epidemic in Chile between October 2013 and April 2015, and we studied the origins and spread of PRRSV throughout the country using official surveillance data and Bayesian phylogenetic analysis. Our results indicate that the outbreaks were caused by a PRRSV closely related to viruses present in swine farms in North America, and different from the strain that circulated in the country before 2009. Using divergence time estimation analysis, we found that the 2013–2015 PRRSV may have been circulating in Chile for at least one month before the first detection. A single strain of PRRSV spread into a limited number of commercial and backyard swine farms. New infections in commercial systems have not been reported since October 2014, and eradication is underway by clearing the disease from the few commercial and backyard farms that remain positive. This is one of the few documented experiences of PRRSV introduction into a disease-free country.

Review on the transmission porcine reproductive and respiratory syndrome virus between pigs and farms and impact on vaccination

Porcine reproductive and respiratory syndrome (PRRS) is considered to be one of the most costly diseases affecting intensive pig production worldwide. Control of PRRS is a complex issue and involves a combination of measures including monitoring, diagnosis, biosecurity, herd management, and immunization. In spite of the numerous studies dealing with PRRS virus epidemiology, transmission of the infection is still not fully understood. The present article reviews the current knowledge on PRRSV transmission between and within farm, and the impact of vaccination on virus transmission.

Porcine semen as a vector for transmission of viral pathogens

Different viruses have been detected in porcine semen. Some of them are on the list of the World Organization for Animal Health (OIE), and consequently, these pathogens are of socioeconomic and/or public health importance and are of major importance in the international trade of animals and animal products. Artificial insemination (AI) is one of the most commonly used assisted reproductive technologies in pig production worldwide. This extensive use has enabled pig producers to benefit from superior genetics at a lower cost compared to natural breeding. However, the broad distribution of processed semen doses for field AI has increased the risk of widespread transmission of swine viral pathogens. Contamination of semen can be due to infections of the boar or can occur during semen collection, processing, and storage. It can result in reduced semen quality, embryonic mortality, endometritis, and systemic infection and/or disease in the recipient female. The presence of viral pathogens in semen can be assessed by demonstration of viable virus, nucleic acid of virus, or indirectly by measuring serum antibodies in the boar. The best way to prevent disease transmission via the semen is to assure that the boars in AI centers are free from the disease, to enforce very strict biosecurity protocols, and to perform routine health monitoring of boars. Prevention of viral semen contamination should be the primary focus because it is easier to prevent contamination than to eliminate viruses once present in semen. Nevertheless, research and development of novel semen processing treatments such as single-layer centrifugation is ongoing and may allow in the future to decontaminate semen.

Development of a network based model to simulate the between-farm transmission of the porcine reproductive and respiratory syndrome virus

Contact structure within a population can significantly affect the outcomes of infectious disease spread models. The objective of this study was to develop a network based simulation model for the between-farm spread of porcine reproductive and respiratory syndrome virus to assess the impact of contact structure on between-farm transmission of PRRS virus. For these farm level models, a hypothetical population of 500 swine farms following a multistage production system was used. The contact rates between farms were based on a study analyzing movement of pigs in Canada, while disease spread parameters were extracted from published literature. Eighteen distinct scenarios were designed and simulated by varying the mode of transmission (direct versus direct and indirect contact), type of index herd (farrowing, nursery and finishing), and the presumed network structures among swine farms (random, scale-free and small-world). PRRS virus was seeded in a randomly selected farm and 500 iterations of each scenario were simulated for 52 weeks. The median epidemic size by the end of the simulated period and percentage die-out for each scenario, were the key outcomes captured. Scenarios with scale-free network models resulted in the largest epidemic sizes, while scenarios with random and small-world network models resulted in smaller and similar epidemic sizes. Similarly, stochastic die-out percentage was least for scenarios with scale-free networks followed by random and small-world networks. Findings of the study indicated that incorporating network structures among the swine farms had a considerable impact on the spread of PRRS virus, highlighting the importance of understanding and incorporating realistic contact structures when developing infectious disease spread models for similar populations.

Simulation of between-farm transmission of porcine reproductive and respiratory syndrome virus in Ontario, Canada using the North American Animal Disease Spread Model

Porcine reproductive and respiratory syndrome (PRRS), a viral disease of swine, has major economic impacts on the swine industry. The North American Animal Disease Spread Model (NAADSM) is a spatial, stochastic, farm level state-transition modeling framework originally developed to simulate highly contagious and foreign livestock diseases. The objectives of this study were to develop a model to simulate between-farm spread of a homologous strain of PRRS virus in Ontario swine farms via direct (animal movement) and indirect (sharing of trucks between farms) contacts using the NAADSM and to compare the patterns and extent of outbreak under different simulated conditions. A total of 2552 swine farms in Ontario province were allocated to each census division of Ontario and geo-locations of the farms were randomly generated within the agriculture land of each Census Division. Contact rates among different production types were obtained using pig movement information from four regions in Canada. A total of 24 scenarios were developed involving various direct (movement of infected animals) and indirect (pig transportation trucks) contact parameters in combination with alternating the production type of the farm in which the infection was seeded. Outbreaks were simulated for one year with 1000 replications. The median number of farms infected, proportion of farms with multiple outbreaks and time to reach the peak epidemic were used to compare the size, progression and extent of outbreaks. Scenarios involving spread only by direct contact between farms resulted in outbreaks where the median percentage of infected farms ranged from 31.5 to 37% of all farms. In scenarios with both direct and indirect contact, the median percentage of infected farms increased to a range from 41.6 to 48.6%. Furthermore, scenarios with both direct and indirect contact resulted in a 44% increase in median epidemic size when compared to the direct contact scenarios. Incorporation of both animal movements and the sharing of trucks within the model indicated that the effect of direct and indirect contact may be nonlinear on outbreak progression. The increase of 44% in epidemic size when indirect contact, via sharing of trucks, was incorporated into the model highlights the importance of proper biosecurity measures in preventing transmission of the PRRS virus. Simulation of between-farm spread of the PRRS virus in swine farms has highlighted the relative importance of direct and indirect contact and provides important insights regarding the possible patterns and extent of spread of the PRRS virus in a completely susceptible population with herd demographics similar to those found in Ontario, Canada.

Membrane proteins of arterivirus particles: structure, topology, processing and function

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Arteriviruses are important pathogens in veterinary medicine.

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We review the structure and processing of their membrane proteins.

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Some features are unique from a cell biological point of view.

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New data on this topic are also presented.

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We speculate on the role of the membrane proteins during virus entry and budding.

Arteriviruses, such as equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV), are important pathogens in veterinary medicine. Despite their limited genome size, arterivirus particles contain a multitude of membrane proteins, the Gp5/M and the Gp2/3/4 complex, the small and hydrophobic E protein and the ORF5a protein. Their function during virus entry and budding is understood only incompletely.

We summarize current knowledge of their primary structure, membrane topology, (co-translational) processing and intracellular targeting to membranes of the exocytic pathway, which are the budding site. We profoundly describe experimental data that led to widely believed conceptions about the function of these proteins and also report new results about processing steps for each glycoprotein. Further, we depict the location and characteristics of epitopes in the membrane proteins since the late appearance of neutralizing antibodies may lead to persistence, a characteristic hallmark of arterivirus infection. Some molecular features of the arteriviral proteins are rare or even unique from a cell biological point of view, particularly the prevention of signal peptide cleavage by co-translational glycosylation, discovered in EAV-Gp3, and the efficient use of overlapping sequons for glycosylation. This article reviews the molecular mechanisms of these cellular processes. Based on this, we present hypotheses on the structure and variability of arteriviral membrane proteins and their role during virus entry and budding.

Seroprevalence and associated risk factors of important pig viral diseases in Bhutan

A cross-sectional serological study was conducted in Bhutan between October 2011 and February 2012 to determine the prevalence of antibodies to classical swine fever virus (CSFV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2), swine influenza virus (SIV) subtype H1N1 and Aujeszky's disease virus (ADV). Furthermore, risk factors for the seropositive status were investigated. Antibodies to SIV, subtype H1N1 (likely pandemic H1N1 2009) were detected in 49% of the pigs in the government farms, and 8% of the village backyard pigs. For PCV2, these percentages were 73% and 37% respectively. For CSFV, the percentages were closer together, with 62% and 52% respectively. It should be taken into consideration that vaccination of piglets is routine in the government herds, and that piglets distributed to backyard farms are also vaccinated. No direct evidence of CSFV infections was found, either by clinical signs or virus isolation. Antibodies to PRRSV and Aujeszky's disease, on the other hand, were not found at all. Risk factors found are mainly related to practices of swill feeding and other biosecurity measures. For CSFV, these were swill feeding (OR=2.25, 95% CI: 1.01-4.99) and contact with neighbour's pigs (OR=0.31, 95% CI: 0.13-0.75). For PCV2 this was lending of boars for local breeding purposes (OR=3.30, 95% CI: 1.43-7.59). The results of this study showed that PCV2 and SIV infections are important in pigs in Bhutan and thus appropriate control strategies need to be designed and applied which could involve strict regulation on the import of live pigs and vaccination against these diseases.

Genomic evolution of porcine reproductive and respiratory syndrome virus (PRRSV) isolates revealed by deep sequencing

Most studies on PRRSV evolution have been limited to a particular region of the viral genome. A thorough genome-wide understanding of the impact of different mechanisms on shaping PRRSV genetic diversity is still lacking. To this end, deep sequencing was used to obtain genomic sequences of a diverse set of 16 isolates from a region of Hong Kong with a complex PRRSV epidemiological record. Genome assemblies and phylogenetic typing indicated the co-circulation of strains of both genotypes (type 1and type 2) with varying Nsp2 deletion patterns and distinct evolutionary lineages (“High Fever”-like and local endemic type). Recombination analyses revealed genomic breakpoints in structural and non-structural regions of genomes of both genotypes with evidence of many recombination events originating from common ancestors. Additionally, the high fold of coverage per nucleotide allowed the characterization of minor variants arising from the quasispecies of each strain. Overall, 0.56–2.83% of sites were found to be polymorphic with respect to cognate consensus genomes. The distribution of minor variants across each genome was not uniform indicating the influence of selective forces. Proportion of variants capable of causing an amino acid change in their respective codons ranged between 25–67% with many predicted to be non-deleterious. Low frequency deletion variants were also detected providing one possible mechanism for their sudden emergence as cited in previous reports.

Risk factors for porcine reproductive and respiratory syndrome virus infection and resulting challenges for effective disease surveillance

BACKGROUND

This study aimed to identify risk factors for active porcine reproductive and respiratory syndrome virus (PRRSV) infection at farm level and to assess the probability of an infected farm being detected through passive disease surveillance in England. Data were obtained from a cross-sectional study on 147 farrow-to-finish farms conducted from April 2008-April 2009. The risk factors for active PRRSV infection were identified using multivariable logistic regression analysis. The surveillance system was evaluated using a stochastic scenario tree model.

RESULTS

Evidence of PRRSV circulation was confirmed on 35.1% (95%CI: 26.8-43.4) of farms in the cross sectional study, with a higher proportion of infected farms in areas with high pig density (more than 15000 pigs within 10 km radius from the farm). Farms were more likely to have active PRRSV infection if they used the live virus vaccine-Porcilis PRRS (OR=7.5, 95%CI: 2.5-22.8), were located in high pig density areas (OR=2.9, 95%CI: 1.0-8.3) or had dead pigs collected (OR=5.6, 95%CI: 1.7-18.3). Farms that weaned pigs at 28 days of age or later had lower odds of being PRRSV positive compared to those weaning at 21-27 days (OR=0.2, 95%CI: 0.1-0.7). The probability of detecting an infected farm through passive surveillance for disease was low (mode=0.074, 5th and 95th percentiles: 0.067; 0.083 respectively). In particular farms which used live virus vaccine had lower probabilities for detection compared to those which did not.

CONCLUSIONS

Risk factors identified highlight the importance of biosecurity measures for the incursion of PRRSV infection. The results further indicate that a combined approach of surveillance for infection and disease diagnosis is needed to assist effective control and/or elimination of PRRSV from the pig population.

A live-attenuated chimeric porcine circovirus type 2 (PCV2) vaccine is transmitted to contact pigs but is not upregulated by concurrent infection with porcine parvovirus (PPV) and porcine reproductive and respiratory syndrome virus (PRRSV) and is efficacious in a PCV2b-PRRSV-PPV challenge model

The live chimeric porcine circovirus type 2 (PCV2) vaccine with the capsid gene of the emerging subtype 2b cloned in the genomic backbone of the nonpathogenic PCV1 is attenuated in vivo and induces protective immunity against PCV2. To further determine the safety and efficacy of this experimental vaccine, we tested for evidence of pig-to-pig transmission by commingling nonvaccinated and vaccinated pigs, determined potential upregulation by simultaneous vaccination and infection with porcine parvovirus (PPV) and porcine reproductive and respiratory syndrome virus (PRRSV), and determined vaccine efficacy by challenging pigs 4 weeks after vaccination with PCV2b, PRRSV, and PPV. Forty-six 21-day-old, PCV2-naïve pigs were randomly assigned to one of six groups. Twenty-nine of 46 pigs were challenged with PCV2b, PRRSV, and PPV at day 28, 8/46 remained nonvaccinated and nonchallenged and served as negative controls, and 9/46 remained nonchallenged and served as vaccination controls. All animals were necropsied at day 49. PCV1-PCV2 viremia was detected in nonvaccinated contact pigs commingled with vaccinated pigs, indicating pig-to-pig transmission; however, PCV1-PCV2 DNA levels remained low in all vaccinated and contact pigs regardless of concurrent infection. Finally, vaccination 28 days before challenge resulted in significantly (P < 0.05) decreased amounts of PCV2 in tissues and sera and significantly (P < 0.05) reduced macroscopic and microscopic lesions. The results of this study indicate that the experimental live-attenuated chimeric PCV2 vaccine, although transmissible to contact pigs, remains attenuated in pigs concurrently infected with PRRSV and PPV and induces protective immunity against PCV2b when it is administered 28 days before PCV2 exposure.

Genome-wide transcriptional response of primary alveolar macrophages following infection with porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome is a major cause of economic loss for the swine industry worldwide. Porcine reproductive and respiratory syndrome virus (PRRSV) triggers weak and atypical innate immune responses, but key genes and mechanisms by which the virus interferes with the host innate immunity have not yet been elucidated. In this study, genes that control the response of the main target of PRRSV, porcine alveolar macrophages (PAMs), were profiled in vitro with a time-course experiment spanning the first round of virus replication. PAMs were obtained from six piglets and challenged with the Lelystad PRRSV strain, and gene expression was investigated using Affymetrix microarrays and real-time PCR. Of the 1409 differentially expressed transcripts identified by analysis of variance, two, five, 25, 16 and 100 differed from controls by a minimum of 1.5-fold at 1, 3, 6, 9 and 12 h post-infection (p.i.), respectively. A PRRSV infection effect was detectable between 3 and 6 h p.i., and was characterized by a consistent downregulation of gene expression, followed by the start of the host innate immune response at 9 h p.i. The expression of beta interferon 1 (IFN-β), but not of IFN-α, was strongly upregulated, whilst few genes commonly expressed in response to viral infections and/or induced by interferons were found to be differentially expressed. A predominance of anti-apoptotic transcripts (e.g. interleukin-10), a shift towards a T-helper cell type 2 response and a weak upregulation of tumour necrosis factor-α expression were observed within 12 h p.i., reinforcing the hypotheses that PRRSV has developed sophisticated mechanisms to escape the host defence.

Genetic parameters for performance traits in commercial sows estimated before and after an outbreak of porcine reproductive and respiratory syndrome

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is globally the most economically important disease in commercial pigs, and novel control strategies are sought. This paper explores the potential to use host genetics to decrease the impact of PRRS on reproductive sows. Commercial pig data (7,542 phenotypic records) from a farm undergoing an outbreak of PRRSV were analyzed to assess the impact of PRRS on reproductive traits and the inheritance of such traits. First, differing methodologies were used to partition the data into time periods when the farm was disease free and when the farm was experiencing PRRSV outbreaks. The methods were a date/threshold method based on veterinary diagnosis and a threshold/threshold method based on trends in underlying performance data, creating the DTD and TTD data sets, respectively. The threshold/threshold method was more stringent in defining periods when PRRS was likely to be having an impact on reproductive performance, resulting in a data set (TTD) that was slightly smaller (1,977 litters from 1,526 sows) than that from the date/threshold method (3,164 litters and 1,662 sows), and it showed more pronounced impacts of PRRS on performance. Impacts on performance included significant increases in mean values of mummified and stillborn piglets (0.04 to 1.13 and 0.63 to 1.02, respectively) with a significant decrease in total born alive (10.3 to 9.08). Estimated heritabilities during the healthy phase were generally less (mummified piglets = 0.03 +/- 0.01, matings per conception = 0.04 +/- 0.01) than during the PRRSV outbreak (TTD data set; mummified piglets = 0.10 +/- 0.03, matings per conception = 0.46 +/- 0.04). These results imply genetic variation for host resistance to, or tolerance of, PRRSV, particularly with the TTD data set. Genetic correlations between reproductive traits measured in the healthy phase and TTD data set varied from effectively zero for traits describing numbers of mummified or dead piglets to strongly positive for litter size traits. This indicates genetic variation in piglet losses during PRRSV outbreaks is independent of genetic variation in the same traits in healthy herds. In summary, our findings show that there is within-breed genetic variation for commercially relevant traits that could be exploited in future breeding programs against PRRSV infection. Selection for increased PRRS resistance would be desirable to the industry because effective control measures remain elusive.

CD163 expression confers susceptibility to porcine reproductive and respiratory syndrome viruses

Direct functional screening of a cDNA expression library derived from primary porcine alveolar macrophages (PAM) revealed that CD163 is capable of conferring a porcine reproductive and respiratory syndrome virus (PRRSV)-permissive phenotype when introduced into nonpermissive cells. Transient-transfection experiments showed that full-length CD163 cDNAs from PAM, human U937 cells (histiocytic lymphoma), African green monkey kidney cells (MARC-145 and Vero), primary mouse peritoneal macrophages, and canine DH82 (histocytosis) cells encode functional virus receptors. In contrast, CD163 splice variants without the C-terminal transmembrane anchor domain do not provide PRRSV receptor function. We established several stable cell lines expressing CD163 cDNAs from pig, human, and monkey, using porcine kidney (PK 032495), feline kidney (NLFK), or baby hamster kidney (BHK-21) as the parental cell lines. These stable cell lines were susceptible to PRRSV infection and yielded high titers of progeny virus. Cell lines were phenotypically stable over 80 cell passages, and PRRSV could be serially passed at least 60 times, yielding in excess of 10(5) 50% tissue culture infective doses/ml.

Immunogenicity and protective efficacy of recombinant pseudorabies virus expressing the two major membrane-associated proteins of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) infection still remains today as the most significant health threat to swine and poses a challenge to current vaccination strategies. To develop a new generation of vaccine against PRRSV, a live attenuated pseudorabies virus (PRV) was used as vaccine vector to express the two major membrane-associated proteins (GP5 or M) of PRRSV in various forms. Four PRV recombinants, rPRV-GP5 (expressing native GP5), rPRV-GP5m (expressing GP5m, a modified GP5), rPRV-GP5-M (co-expressing GP5 and M proteins), rPRV-GP5m-M (co-expressing GP5m and M proteins) were generated. Mouse immunized with all these recombinants developed comparable PRV-specific humoral immune responses and provided complete protection against a lethal PRV challenge. However, the highest level of PRRSV-specific neutralizing antibodies and lymphocyte proliferative responses was observed in mice immunized with rPRV-GP5m-M. The immunogenicity and protective efficiency of rPRV-GP5m-M were further evaluated in the piglets. Compared to commercial PRRSV killed vaccine, detectable PRRSV-specific neutralizing antibody and higher lymphocyte proliferative responses could be developed in piglets immunized with rPRV-GP5m-M before virus challenge. Furthermore, more efficient protection against a PRRSV challenge was obtained in piglets immunized with rPRV-GP5m-M, as showed by the balanced body-temperature fluctuation, shorter-term viremia, lower proportion of virus load in nasal and oropharyngeal scrapings and tissues, and milder lung lesions. These data indicate that the recombinant rPRV-GP5m-M is a promising candidate bivalent vaccine against both PRV and PRRSV infection.

Genetic variation and geographic distribution of porcine reproductive and respiratory syndrome virus in Japan

Porcine reproductive and respiratory syndrome virus (PRRSV) has two genotypes, the North American-type (NA-type) and the European-type (EU-type), and each genotype is also genetically diverged. We sequenced the ORF5 gene of 30 PRRSVs isolated from 23 prefectures of Japan during 1992 and 1993 and during 2000 and 2001. All of the isolates were of the NA-type. Phylogenetic analysis of the overall NA-type viruses isolated from around the world identified five major genetic clusters. The 1992-1993 Japanese samples belonged to only two genetic clusters, while the 2000-2001 samples included more diverged ORF5 genomes. One genetic cluster, which included 63% (20/32) of Japanese isolates, one Taiwanese isolate and one Chinese isolate, was mainly found in the eastern part of Japan. Another genetic cluster, which was found in various areas around the world, was distributed in the western part of Japan.

Quasispecies variation of porcine reproductive and respiratory syndrome virus during natural infection

Porcine reproductive and respiratory syndrome virus (PRRSV) displays notorious genetic, antigenic, and clinical variability. Little is known, however, about the nature and extent of viral variation present within naturally infected animals. By amplifying and cloning the open reading frame 5 gene from tonsils of naturally infected swine, and by sequencing individual clones, we characterized viral diversity in nine animals from two farms. All animals harbored multiple PRRSV variants at both the nucleic and the amino acid levels. Structural variation and rates of synonymous and nonsynonymous nucleotide substitution were no different within known epitopes than elsewhere. Analysis of molecular variance indicated that differences between farms, among animals within farms, and within individual animals accounted for 92.94, 3.84, and 3.22% of the total viral genetic variability observed, respectively. PRRSV exists during natural infection as a quasispecies distribution of related genotypes. Positive natural selection for immune evasiveness does not appear to maintain this diversity.