Comparison of the structural protein coding sequences of the VR-2332 and Lelystad virus strains of the PRRS virus

Further assessment of fomites and personnel as vehicles for the mechanical transport and transmission of porcine reproductive and respiratory syndrome virus

This study re-evaluated the role of fomites and personnel in the mechanical transport and transmission of porcine reproductive and respiratory syndrome virus (PRRSV) between pig populations. Swabs were collected from hands, boots, coveralls, and other fomites following contact with infected pigs and compared with identical samples collected in the absence of PRRSV exposure. Naïve pigs were provided contact with contaminated fomites/personnel and blood tested periodically post-exposure [positive exposure population (PEP)] and compared with populations that did not gain exposure via these routes [negative exposure population (NEP)]. The majority of swab samples from hands, coveralls, and boots from personnel and fomite samples (cable snare and bleeding equipment) following contact with the PRRSV-infected Source Population. Transmission of PRRSV to the PEP was observed (7/7) cases but not in the NEP. In conclusion, under the proper conditions, transport and transmission of PRRSV by fomites and personnel may occur between swine populations in the absence of intervention.

Changes in lymphocyte subsets and cytokines during European porcine reproductive and respiratory syndrome: increased expression of IL-12 and IL-10 and proliferation of CD4(-)CD8(high)

The changes in peripheral blood mononuclear cells (PBMCs) have been studied in several reports in an attempt to determine the immune response against porcine reproductive and respiratory syndrome virus (PRRSV) infection. However, how these changes are evoked after PRRSV infection has not yet been clarified. The aim of this study was to analyze the changes seen in lymphocyte subsets and immunomodulatory cytokine expression in pigs after an acute experimental infection with a European PRRSV field isolate. Pigs were inoculated intramuscularly with PRRSV field isolate 2982. Samples from blood, medial retropharyngeal and tracheobronchial lymph nodes, and spleen were collected at different time points for flow cytometry studies and for cytokine expression by ELISA. CD21(+) cell counts increased in PBMCs and tracheobronchial lymph node cells from 17 to 24 dpi, coinciding with an increase in PRRSV-specific antibody titer in blood. CD3(+) T-cell counts increased mainly due to an enhancement of CD4(-)CD8(high) and CD4(+)CD8(+) T cells. CD4(-)CD8(low) T cells were decreased in all organs studied, whereas CD4(+)CD8(-) T cells decreased only in the spleen. The drop in viremia correlated with an enhancement of CD4(-)CD8(high) T cells, and with a higher expression of interleukin-10 (IL-10) and interleukin-12 p40 (IL-12 p40). No efficient interferon-gamma (IFN-gamma) response was detected during the acute phase of the infection, and the expression of interferon-alpha (IFN-alpha) was late and reached its maximum expression once the viremia decreased. These results point to IL-10 and IL-12 as cytokines that might play a significant role in the PRRSV immune response, as may CD4(-)CD8(high) T cells.

Genetic analysis of ORF5 of recent Korean porcine reproductive and respiratory syndrome viruses (PRRSVs) in viremic sera collected from MLV-vaccinating or non-vaccinating farms

The 23 open reading frame (ORF) 5 sequences of Korean type II porcine reproductive and respiratory syndrome virus (PRRSV) were collected from viremic sera from the (modified live vaccine) MLV-vaccinating and non-vaccinating farms from 2007 to 2008. The samples were phylogenetically analyzed with previous ORF5 sequences, including type I Korean PRRSV, and previously reported or collected sequences from 1997 to 2008. A MN184-like subgroup of type II Korean PRRSV was newly identified in the viremic sera collected from 2007 to 2008. And of the type I PRRSVs, one subgroup had 87.2~88.9% similarity with the Lelystad virus, showing a close relationship with the 27~2003 strain of Spain. The maximum parsimony tree of type II PRRSV from 1997 to 2008 showed that they had evolved to four lineages, subgroups 1, 2, 3 and 4. Most of the recently collected type II PRRSVs belonged to subgroup 4 (48%). The region of three B-cell epitopes and two T-cell epitopes of ORF5 amino acids sequences was considerably different from the MLV in subgroups 3 and 4. In conclusion, the existence of type I PRRSV, which was genetically different from Lelystad virus (Prototype of type I PRRSV), and heterologous type II PRRSVs of viremic pigs detected even in the MLV-vaccinating farms indicated the need for new vaccine approaches for the control of PRRSV in Korea.

Evidence of long distance airborne transport of porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae

The ability of porcine reproductive and respiratory syndrome virus (PRRSV) and Mycoplasma hyopneumoniae to be transported over long distances via the airborne route was evaluated. A source population of 300 grow-finish pigs was experimentally inoculated with PRRSV MN-184 and M. hyopneumoniae 232 and over a 50-day period, air samples were collected at designated distances from the source herd using a liquid cyclonic collector. Samples were tested for the presence of PRRSV RNA and M. hyopneumoniae DNA by PCR and if positive, further characterized. Of the 306 samples collected, 4 (1.3%) were positive for PRRSV RNA and 6 (1.9%) were positive for M. hyopneumoniae DNA. The PRRSV-positive samples were recovered 4.7 km to the northwest (NW) of the source population. Four of the M. hyopneumoniae-positive samples were obtained at the NW sampling point; 2 samples at approximately 2.3 km and the other 2 samples approximately 4.7 km from the source population. Of the remaining 2 samples, one sample was obtained at the southeast sampling point and the other at the southwest sampling point, with both locations being approximately 4.7 km from the source. The four PRRSV-positive samples contained infectious virus and were ≥ 98.8% homologous to the MN-184 isolate used to inoculate the source population. All 6 of the M. hyopneumoniae-positive samples were 99.9% homologous to M. hyopneumoniae 232. These results support the hypothesis that long distance airborne transport of these important swine pathogens can occur.

Further assessment of houseflies (Musca domestica) as vectors for the mechanical transport and transmission of porcine reproductive and respiratory syndrome virus under field conditions

The purpose of this study was to evaluate the potential for houseflies (Musca domestica) to mechanically transport and transmit porcine reproductive and respiratory syndrome virus (PRRSV) between pig populations under controlled field conditions. The study employed swine housed in commercial livestock facilities and a release-recapture protocol involving marked (ochre-eyed) houseflies. To assess whether transport of PRRSV by insects occurred, ochre-eyed houseflies were released and collected from a facility housing an experimentally PRRSV-inoculated population of pigs (facility A) and collected from a neighboring facility located 120 m to the northwest that housed a naïve pig population (facility B). All samples were tested for PRRSV RNA by polymerase chain reaction (PCR). To assess transmission between the 2 populations, blood samples were collected from naïve pigs in facility B at designated intervals and tested by PCR. A total of 7 replicates were conducted. During 2 of 7 replicates (1 and 5), PCR-positive ochre-eyed houseflies were recovered in facility B and pigs in this facility became infected with PRRSV. Chi-squared analysis indicated that the presence of PRRSV in an insect sample was significantly (P = 0.0004) associated with infection of facility B pigs. Porcine reproductive and respiratory syndrome virus was not recovered from other reported routes of transmission during the study period, including air, fomites, and personnel. In conclusion, while an insufficient number of replicates were conducted to predict the frequency of the event, houseflies may pose some level of risk for the transport and transmission of PRRSV between pig populations under field conditions.

Genetic variation analysis of porcine reproductive and respiratory syndrome virus isolated in China from 2002 to 2007 based on ORF5

The complete open reading frame 5 (ORF5) sequences of 34 field porcine reproductive and respiratory syndrome virus (PRRSV) isolates from China in 2002-2007 were detected and compared with the different variable Chinese isolates S1, CH-1a, HB-1, HB-2 and JXA1. The results showed that all isolates were of type 2 PRRSV and could be assigned to two clusters. The isolates in cluster sg1 was high similar with the highly pathogenic PRRSV strain JXA1, while sg2 clustered with type 2 PRRSV isolate VR2332. It was interesting that the isolate SH02 which was isolated from Shanghai in 2002 has 98.8% identity with JXA1 emerged in 2006. And the ZJJ07 isolate was found to be a natural recombinant between a Chinese highly pathogenic SY0608 isolate and a VR-2332 derivative NH04 isolate. Analysis of the potential glycosylation sites indicated that they were frequently mutated and formed five putative N-linked glycosylation (NGS) sites patterns based on N30, 33-35, 44 and 51 in those isolates. It indicated that the highly variable PRRSV strain with different NGS patterns spread widely in China. The great genetic diversity could be taken into consideration for the control and prevention of this disease.

Monoclonal antibodies and conserved antigenic epitopes in the C terminus of GP5 protein of the North American type porcine reproductive and respiratory syndrome virus

Glycoprotein 5 (GP5) is the major glycoprotein of porcine reproductive and respiratory syndrome virus (PRRSV). In this study, the gene encoding rtGP5, lacking signal peptide sequence, was expressed as GST-fusion protein in E. coli. Fifteen monoclonal antibodies (MAbs) against rtGP5 were developed and used to probe a series of GP5 peptides by ELISA, in which two MAbs specifically recognized the epitope GP5EP3 (146-156aa), four recognized GP5EP5 (164-180aa) and nine recognized GP5EP7 (192-200aa). After precise analysis by sequential deletion of the terminal amino acid residues, the three minimal epitopes (R(152)LYRWR(156), E(169)GHLIDLKRV(178) and Q(196)WGRL(200)) were determined, which were highly conserved among the North American type isolates, with the exception of one amino acid mutation (L(200) to P(200)). Mutational analysis showed that the mutant (Q(196)WGRP(200)) could be recognized by four of nine anti-GP5EP7 MAbs, indicating Q(196)WGRP(200) was also one minimal epitope. Western blot analysis showed that GP5EP5 and GP5EP7 (L(200) or P(200)) could be recognized by PRRSV-positive sera of CH-1a and/or BJ-4, suggesting GP5EP5 and GP5EP7 (L(200) or P(200)) were antigenic epitopes in the PRRSV-infected pigs. MAbs against GP5EP3, GP5EP5, and GP5EP7 could react with MARC-145 cells infected with the North American type isolates from China in IFA. However, very interestingly, when the highly pathogenic PRRSV, represented by HUN4, was passaged in MARC-145 cells, MAbs against GP5EP7 did not react with HUN4-F20-HUN4-F112 (20-112th passage virus), where Q(196)WGRL(200) had mutated to R(196)WGRL(200). Due to no mutations observed in GP5EP3 and GP5EP5, MAbs against GP5EP3 and GP5EP5 could recognize HUN4-F20-HUN4-F112. All the results herein might deepen the understanding of the antigen structure of in the C terminus of GP5 and facilitate the development of diagnostic antigens of the North American type PRRSV in China.

Molecular characterization of ORFs 2 to 7 of Korean porcine reproductive and respiratory syndrome virus (CA) and its protein expression by recombinant baculoviruses

To determine the characteristics of the Korean porcine reproductive and respiratory syndrome virus (PRRSV), CA, which was isolated from the serum of an infected pig in 2006, we investigated the nucleotide sequence and expression of the structural ORFs (ORFs 2 to 7) using the bApGOZA system. We found that the structural ORFs 2 to 7 of CA consisted of 3188 nucleotides that were the same as those formed from VR-2332. Comparison of the CA with the other strains revealed nucleotide sequence identity ranging from 89.8 to 99.5%. To better understand the genetic relationships between other strains, phylogenetic analyses were performed. The CA strain was closely related to the other North American genotype strains but formed a distinct branch with high bootstrap support. Additionally, expression levels of the PRRSV proteins in insect cells were strong or partially weak. The results of this study have implications for both the taxonomy of PRRSV and vaccine development.

Use of a production region model to assess the airborne spread of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is an emerging and re-emerging disease of pigs and a growing threat to the global swine industry. For sustainable disease control, it is critical to prevent the spread of the etiologic agent, PRRS virus, between pig populations. Therefore, a clear understanding of the role of aerosol transmission in the spread of PRRS virus is needed as well as information on how to reduce this risk. To enhance the knowledge of PRRS aerobiology we used a production region model to quantify infectious virus in bioaerosols, document airborne spread of the virus out to 120m, identify climactic conditions associated with the presence of virus in bioaerosols, and demonstrate the ability to protect at-risk populations using a system of air filtration. These findings confirm the importance of the airborne spread of PRRS virus, provide new information regarding its aerobiology and describe for the first time an effective means of disease control that can protect healthy, vulnerable populations of pigs.

Influence of time on the genetic heterogeneity of Spanish porcine reproductive and respiratory syndrome virus isolates

The aim of the present study was to establish the degree of diversity of porcine reproductive and respiratory virus (PRRSV) isolates that circulate in the same geographical area in different years. Nucleotide sequences of open reading frame (ORF) 5 were determined for 28 Spanish field PRRSV isolates from different years and three European-type modified live virus vaccines. Sequences were aligned using Clustal W software and a phylogenetic tree constructed using the neighbour joining method. The results of pairwise homology comparisons of nucleotide and deduced amino acid sequences of these PRRSV isolates indicate a tendency for heterogeneity to increase with time. The study of the phylogenetic tree revealed that Spanish PRRSV isolates constitute two well-defined clades and a group of unrelated sequences. The observed heterogeneity does not appear to be due to temporal evolution exclusively. Early and recent isolates group themselves into different clusters independently of the time of isolation, indicating the co-circulation of different variants and the maintenance of variants of the original isolates in the field.

Enhanced immune responses of mice inoculated recombinant adenoviruses expressing GP5 by fusion with GP3 and/or GP4 of PRRS virus

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important causes of economic losses of the swine industry. PRRS virus (PRRSV) infection poses a challenge to current vaccination strategies. In this study, three replication-defective adenovirus recombinants expressing fusion protein GP3-GP5, GP4-GP5, or GP3-GP4-GP5 were developed as potential vaccine against PRRSV in a mouse model. Six groups of BALB/c mice (24mice per group) were inoculated subcutaneously twice at 2-week intervals with above mentioned recombinants and other adenoviruses expressing single GP3, GP4, or GP5 protein. The results showed that the mice inoculated with recombinant adenoviruses developed PRRSV-specific antibodies, cellular immune response by 2 weeks post-boost-immunization. However, mice immunized with recombinant adenoviruses rAd-GP3-GP5, rAd-GP4-GP5, and rAd-GP3-GP4-GP5 developed significantly higher titers of neutralizing antibodies to PRRSV and produced stronger lymphocyte proliferation responses compared to mice immunized with rAd-GP3, rAd-GP4 or rAd-GP5 alone. It was also found that mice immunized with rAd-GP3-GP5 and rAd-GP3-GP4-GP5 were primed for significant higher levels of anti-PRRSV CTL responses than mice immunized with rAd-GP3 and rAd-GP5. These findings suggested that the recombinant adenoviruses expressing fusion proteins GP3-GP5 or GP3-GP4-GP5 might be an attractive candidate vaccine for preventing PRRSV infection.

Recovery of viable porcine reproductive and respiratory syndrome virus from an infectious clone containing a partial deletion within the Nsp2-encoding region

Non-structural protein 2 (Nsp2) of porcine reproductive and respiratory syndrome virus (PRRSV) is the most variable region and postulated to play an important role in cell and tissue tropism of PRRSV. To investigate the role of Nsp2 in the viability and growth of PRRSV in cells in vitro, two cDNA clones were constructed containing a deletion of 63 consecutive nucleotides (pWSK-DCBAd63) or 117 nucleotides (pWSK-DCBAd117) within the Nsp2-encoding region of PRRSV (BJ-4). The clone pWSK-DCBAd63 was infectious and produced viable recombinant virus, whereas clone pWSK-DCBAd117 could not be rescued. The rescued virus was able to induce CPE typical of PRRSV on MARC-145 cells and was stably propagated during sequential in vitro cell passages, like the virus recovered from the full-length cDNA clone of PRRSV BJ-4. In comparison to the parental virus (BJ-4) and the virus recovered from the full-length cDNA clone of the BJ-4 strain, the rescued virus from pWSK-DCBAd63 exhibited enhanced growth kinetics, reaching the peak progeny virus titer by 48 h postinfection. These observations suggest that the Nsp2-encoding region is necessary for productive virus infection, and partial deletion does not influence the viability and propagation of PRRSV in cell culture, which may provide a way to insert a foreign gene into the viral genome as a marker for differentiation.

Emergence of a highly pathogenic porcine reproductive and respiratory syndrome virus in the Mid-Eastern region of China

Herds of pigs in the Mid-Eastern region of China have experienced recent outbreaks of a severe form of porcine reproductive and respiratory syndrome (PRRS) characterised by high fever and morbidity and mortality in animals of different ages. Eighty-one herds were diagnosed with PRRS virus (PRRSV) infection from June to December 2006 on the basis of clinical signs, pathological findings and reverse transcriptase polymerase chain reaction (RT-PCR). Twenty strains of PRRSV were isolated from 20 herds in six provinces. Following experimental inoculation, one isolate, designated SY0608, caused 100% morbidity and 25-50% mortality in 30-, 65- and 105-day-old pigs and the birth of stillborn and weak piglets from affected sows. The ORF5 gene had 99.5-99.8% nucleotide and 99-100% derived amino acid sequence identities among SY0608 and five other field isolates, but only 89.4% and 88.6% identities, respectively, with VR-2332, the prototypic North American isolate. The 2850bp Nsp2 gene of SY0608 had 79.4% nucleotide and 74.9% derived amino acid sequence identities with VR-2332; deletions of 1 and 29 amino acids corresponded to positions 480 and 531-559 of strain VR-2332, respectively. These findings demonstrated that a new highly pathogenic Northern American type PRRSV has spread widely in the Mid-Eastern region of China.

Current status of veterinary vaccines

The major goals of veterinary vaccines are to improve the health and welfare of companion animals, increase production of livestock in a cost-effective manner, and prevent animal-to-human transmission from both domestic animals and wildlife. These diverse aims have led to different approaches to the development of veterinary vaccines from crude but effective whole-pathogen preparations to molecularly defined subunit vaccines, genetically engineered organisms or chimeras, vectored antigen formulations, and naked DNA injections. The final successful outcome of vaccine research and development is the generation of a product that will be available in the marketplace or that will be used in the field to achieve desired outcomes. As detailed in this review, successful veterinary vaccines have been produced against viral, bacterial, protozoal, and multicellular pathogens, which in many ways have led the field in the application and adaptation of novel technologies. These veterinary vaccines have had, and continue to have, a major impact not only on animal health and production but also on human health through increasing safe food supplies and preventing animal-to-human transmission of infectious diseases. The continued interaction between animals and human researchers and health professionals will be of major importance for adapting new technologies, providing animal models of disease, and confronting new and emerging infectious diseases.

Emergence of fatal PRRSV variants: unparalleled outbreaks of atypical PRRS in China and molecular dissection of the unique hallmark

Porcine reproductive and respiratory syndrome (PRRS) is a severe viral disease in pigs, causing great economic losses worldwide each year. The causative agent of the disease, PRRS virus (PRRSV), is a member of the family Arteriviridae. Here we report our investigation of the unparalleled large-scale outbreaks of an originally unknown, but so-called "high fever" disease in China in 2006 with the essence of PRRS, which spread to more than 10 provinces (autonomous cities or regions) and affected over 2,000,000 pigs with about 400,000 fatal cases. Different from the typical PRRS, numerous adult sows were also infected by the "high fever" disease. This atypical PRRS pandemic was initially identified as a hog cholera-like disease manifesting neurological symptoms (e.g., shivering), high fever (40-42 degrees C), erythematous blanching rash, etc. Autopsies combined with immunological analyses clearly showed that multiple organs were infected by highly pathogenic PRRSVs with severe pathological changes observed. Whole-genome analysis of the isolated viruses revealed that these PRRSV isolates are grouped into Type II and are highly homologous to HB-1, a Chinese strain of PRRSV (96.5% nucleotide identity). More importantly, we observed a unique molecular hallmark in these viral isolates, namely a discontinuous deletion of 30 amino acids in nonstructural protein 2 (NSP2). Taken together, this is the first comprehensive report documenting the 2006 epidemic of atypical PRRS outbreak in China and identifying the 30 amino-acid deletion in NSP2, a novel determining factor for virulence which may be implicated in the high pathogenicity of PRRSV, and will stimulate further study by using the infectious cDNA clone technique.

Cross-reactive antibody responses to nsp1 and nsp2 of Porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) non-structural proteins (nsps) play a key role in processing and maturation of the repertoire of structural and nsps of the virion, but little is known about the anti-nsp immune response. Here, it was hypothesized that pronounced antibody responses are generated to PRRSV nsp1 and nsp2, as they are present in infected cells and cytolytic infection releases viral proteins into interstitial spaces. Accordingly, nsp1 and nsp2 were cloned and expressed, and antibody responses in the sera of infected and vaccinated pigs were determined. Pigs mounted significant cross-reactive antibody responses that appeared equivalent to or greater than the response to nucleocapsid (N). Antibody reactivity to nsp1 and N was highly dependent on refolding of denatured proteins, suggesting that the porcine antibody response is directed primarily to conformational epitopes. The proteins reacted with sera from pigs infected with other PRRSV strains, indicating that multiple epitopes are conserved. Antibody responses to nsp1 and nsp2 were much higher than those to nsp4, which is encoded on the same RNA molecule and is equivalent in predicted antigenicity. These findings suggest either that nsp1 and nsp2 are highly immunogenic or that they are expressed at higher levels than nsp4 in PRRSV-infected cells, or both. Strong antibody responses to nsp1 and nsp2 may benefit the host by limiting potentially pathological consequences of viral protease activities encoded in these proteins that are released from dying cells. The identification of strain-specific antibody responses to a highly variable region of nsp2 may also provide the basis for immunoassays that differentiate serological responses of vaccines from field isolates.

Monoclonal antibody and porcine antisera recognized B-cell epitopes of Nsp2 protein of a Chinese strain of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important pathogens for swine industry. The non-structural protein 2 (Nsp2) is considered to be one of the immunogenic proteins of PRRSV. In this study, the B-cell epitopes of the Nsp2 protein of a North American type Chinese strain PRRSV BJ-4 were identified on a prokaryotic expressed Nsp2 fragment (73-567aa). A total of six monoclonal antibodies (mAbs) recognizing different epitopes on the expressed protein were prepared. All six mAbs exhibited immunoreactivity with the denatured Nsp2 protein in Western blotting and produced strong perinuclear staining in PRRSV infected MARC-145 cells in an immunofluorescence assay. Pepscan analysis revealed six distinct linear epitopes for the six mAbs, respectively, and of which four were identified to be novel linear Nsp2 B-cell epitopes: T(73)LPERVRPPDDWAT(86), D(385)ELKDQMEED(394), P(452)VPAPRRKVGSDCGS(466), and P(467)VSLGGDVPNS(477). All of the six mAb specific peptides could be recognized by porcine PRRSV antiserum, indicating that the epitopes involving these synthetic peptides were immunogenic and immunodominant during PRRSV infection in pigs. Our results provided valuable information for developing novel PRRSV vaccines using the Nsp2 epitopes as potential serological markers.

Immune responses of pigs inoculated with a recombinant fowlpox virus coexpressing GP5/GP3 of porcine reproductive and respiratory syndrome virus and swine IL-18

Two recombinant fowlpox viruses (rFPV-ORF5-ORF3 and rFPV-IL-18-ORF5-ORF3) containing the ORF5/ORF3 cDNAs of PRRSV (strain Chang Chun) and IL-18 of swine were constructed and evaluated for theirs abilities to induce humoral and cellular responses in piglets. In addition, their abilities to protect piglets against homologous virus challenge were examined. All piglets were given booster vaccinations at 21 days after the initial inoculation, and all piglets were challenged at 60 after the initial inoculation. Control groups were inoculated with wild-type fowlpox virus (wtFPV). All animals vaccinated with rFPV-ORF5-ORF3 and rFPV-IL-18-ORF5-ORF3 developed specific anti-PRRSV ELISA antibody and neutralizing antibody, as well as T-lymphocyte proliferation response. To evaluate the cellular immune function, IFN-gamma production in pigs serum and T-lymphocytes (CD4 and CD8 T cells) in peripheral blood were examined. Following challenge with a pathogenic strain of PRRSV (strain Chang Chun), piglets inoculated with recombinant fowlpox virus (rFPV) showed lower (P<0.05) temperature, viremia and virus load in bronchial lymph nodes than control animals, suggesting the establishment of partial protection against PRRSV infection. The results demonstrated the potential use of a fowlpox virus-based recombinant vaccine in the control and prevention of PRRSV infections.

Influence of isolate pathogenicity on the aerosol transmission of Porcine reproductive and respiratory syndrome virus

The objectives of this study were to evaluate the role of isolate pathogenicity in the aerosol transmission of Porcine reproductive and respiratory syndrome virus (PRRSV) and to determine whether PRRSV could be detected in air samples. To assess transmission, we exposed naive recipient pigs to aerosols from pigs inoculated with PRRSV MN-30100, an isolate of low pathogenicity, or MN-184, a highly pathogenic isolate. Blood samples and nasal-swab samples were collected from the inoculated pigs during the exposure period and tested for the presence of PRRSV RNA by quantitative (real-time) reverse-transcriptase polymerase chain reaction (RT-PCR); the amount of RNA was expressed as the median tissue culture dose per milliliter (TCID50/mL). The recipient pigs were clinically evaluated for 14 d after exposure and tested on days 7 and 14 by qualitative RT-PCR and enzyme-linked immunosorbent assay (ELISA). To prove the presence of PRRSV in aerosols, air samples were collected from each recipient-pig chamber by means of an air sampler. The PRRSV RNA concentrations were significantly higher (P = 0.01) in the blood samples from the pigs infected with PRRSV MN-184 than in the blood samples from those infected with PRRSV MN-30100; however, the concentrations in the nasal-swab samples were not significantly different (P = 0.26). Recipient pigs exposed to aerosols from pigs infected with PRRSV MN-184 became infected, whereas those exposed to aerosols from pigs infected with PRRSV MN-30100 did not; the difference in transmission rate was significant at P = 0.04. We detected PRRSV MN-184 RNA but not PRRSV MN-30100 RNA in air samples by PCR. Under the conditions of this study, PRRSV isolate pathogenicity may influence aerosol transmission of the virus.

Long-term administration of a commercial porcine reproductive and respiratory syndrome virus (PRRSV)-inactivated vaccine in PRRSV-endemically infected sows

The purpose of this study was to investigate the safety and efficacy of a commercial European porcine reproductive and respiratory syndrome virus (PRRSV)-inactivated vaccine after 18-month use in gilts/sows at a farm with high seroprevalence. In a farrow-to-finish farm with 1100 sows, all sows and gilts were systematically vaccinated with the PRRS-inactivated PROGRESSIS vaccine for a period of 18 months. Farm's reproductive and litter characteristics were longitudinally recorded for this period and historically compared with those of the year prior to vaccination. Serology, employing immunoperoxidase monolayer assay, had confirmed a high prevalence of PRRS-specific antibodies in most age groups within the farm prior to vaccination. Seroprevalence during the experiment ranged between 0% and 100% in weaners and growers, but remained at stable high levels (> 93%) in finishing pigs and gilts throughout all 2-year period of serology measurements. No local or systemic vaccine side effects were noted throughout the trial period. Vaccinations had resulted over time in a significant improvement of sow reproductive performance (e.g. reduction of premature farrowings, abortions and increase of farrowing rate) and litter characteristics (e.g. increase of the number of live born and weaned pigs and decrease of stillborn, mummified, weak and splay-legged piglets). It has also been observed that the higher the degree of immunization of a sow, the better the improvement of her reproductive parameters. Sows after vaccination have shown improved characteristics compared to homoparous sows prior to the application of vaccinations in the farm.

Genetic variation of Chinese PRRSV strains based on ORF5 sequence

Thirteen isolates of porcine reproductive and respiratory syndrome virus (PRRSV) from different provinces of China were studied and compared with several PRRSV isolates from other countries. Phylogenetic analysis shows that all Chinese isolates of PRRSV in this study belong to the American genotype, except for one strain, B13, which clustered as a European genotype. Sequence analysis revealed that PRRSV Chinese isolates of the American genotype were highly similar in the ORF5 sequence and could be classified into two subclades. One contains PRRSV isolates that are more closely related to the American vaccine strain MLV Resp and its parent strain VR-2332, and the other contains ones only distantly related to them. Within the Chinese isolates slight genetic variation occurred, and some strains may originate directly from the vaccine virus.

Recombinant adenovirus expressing GP5 and M fusion proteins of porcine reproductive and respiratory syndrome virus induce both humoral and cell-mediated immune responses in mice

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important contagious agents of swine in the world. PRRSV infection poses a challenge to current vaccination strategies. In this study, three replication-defective adenovirus recombinants were developed as potential vaccine against PRRSV in a mouse model. Three groups of BALB/c mice (24 mice per group) were inoculated subcutaneously twice at 2-week intervals with the recombinants expressing PRRSV GP5 (rAd-GP5), M (rAd-M), and M-GP5 fusion protein (rAd-M-GP5). Two additional groups were injected with wild-type adenovirus (wtAd) or PBS as control. The results showed that the mice inoculated with recombinant adenoviruses developed PRRSV-specific antibodies, cellular immune response by 2 weeks post second inoculation. However, only mice immunized with recombinant adenovirus rAd-M-GP5 developed significantly higher titers of neutralizing antibodies to PRRSV and produced stronger lymphocyte proliferation responses compared to mice immunized with rAd-M or rAd-GP5 alone. It was also found that mice immunized with rAd-M-GP5 were primed for significant higher levels of anti-PRRSV CTL responses than mice immunized with rAd-M. Mice receiving rAd-GP5 also mounted PRRSV-specific response, but levels were lower. It suggested that the recombinant adenovirus expressing M-GP5 fusion protein might be an attractive candidate vaccine to be tested for preventing PRRSV infection.

Identification of a Conserved Epitope Cluster in the N Protein of Porcine Reproductive and Respiratory Syndrome Virus

In this study, 4 overlapping fragments and 12 overlapping peptides of the nucleocapsid (N) protein from porcine reproductive and respiratory syndrome virus (PRRSV) were expressed as glutathione S-transferase (GST) fusion proteins and used to probe a panel of 16 anti-N protein monoclonal antibodies (mAbs) by ELISA. The minimal epitope sequence of the following seven mAbs was determined by sequential deletion of terminal amino acid residues from each peptide: N2H7 corresponded to H54FPLA58; N2F7 corresponded to K52PHFPLA58; and N1A2, N1E3, N1G4, and N2E5 were reactive against E51KPHFP56. Furthermore, a polypeptide containing this epitope cluster was recognized by PRRSV-immune pig serum by Western blot, suggesting that residues 51-58 represent an immunodominant region of the N protein. Sequence alignment revealed that these epitopes are well conserved among North American and European genotypes of PRRSV. These findings enhance our knowledge of the antigenic structure of N protein and may facilitate the development of better diagnostic methods for PRRSV.

Antigenic structure analysis of glycosylated protein 3 of porcine reproductive and respiratory syndrome virus

The function of the glycosylated protein 3 (GP3), a porcine reproductive and respiratory syndrome virus (PRRSV) associated protein is poorly known. In the present study, the gene encoding GP3 (ORF3), lacking the highly hydrophobic domain in the N- and C-termini was expressed as GST-fusion proteins in E. coli. Monoclonal antibodies (MAbs) against GP3 were developed and used to probe a series of GP3 peptides using ELISA. After precise analysis by sequential deletion of the terminal amino acid residues from each peptide, the minimal epitopes recognized by the MAbs were localized to W(74)CRIGHDRCGED(85) and Y(67)EPGRSLW(74). The epitope sequences were well conserved among most of the North American-type isolates, with the exception of two amino acid mutations in both epitopes in a few of these isolates. Mutational analysis revealed that these mutants were not recognized by any of the five MAbs, indicating that genetic variation could lead to altered antigenicity. Eight out of nine peptide fragments, 58-72aa, 73-87aa, 88-101aa, 102-115aa, 50-65aa, 66-81aa, 80-95aa and 94-109aa were recognized by PRRSV-positive pig serum as determined by Western blot analysis. The results herein may elucidate partially the antigenic structure of GP3 and variations of PRRSV.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV): A Ring Test Performed in Germany to Assess RT-PCR Detection Methods

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major pathogen of swine. Due to genetic variation between the European and the US genotype as well as within both genotypes detection of PRRSV is a diagnostic challenge. This paper reports on a ring test to compare different established reverse transcriptase polymerase chain reaction methods applied routinely in 16 different laboratories in Germany. Three different sets of samples were sent to the laboratories which were to be analysed as follows: (i) basis package: detection of PRRS (yes/no); (ii) differentiation package I: differentiation of EU and US genotypes; and (iii) differentiation package II: differentiation of EU field isolates and EU vaccine strain. A total of 80% of the samples of the basic package were analysed correctly, the analysis of the differentiation package I revealed 61.82% correctly tested samples and the two laboratories that analysed the differentiation package II showed only one correct result. The ring test showed that the majority of incorrect diagnoses were false-negative results.

γδ Lymphocyte Response to Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be one of the most important diseases facing swine industry today. Following PRRSV infection pigs develop both humoral and cell-mediated responses following PRRSV exposure; however, the relative importance in protection and clearance of the virus is not yet completely understood. Swine contain a large percentage of gammadelta T-lymphocytes in peripheral circulation capable of responding to various pathogens in both an innate and specific immune response. The objectives of this study were to determine whether gammadelta lymphocytes functionally respond to PRRSV upon initial exposure and re-exposure. Four month old PRRSV free gilts were intranasally inoculated with a field isolate MN-30100 then assessed at various time points post infection. On day 120, pigs were re-exposed with MN-30100 PRRSV strain and subsequently were bled on days 0, 7, and 14 post re-exposure. Lymphocyte subpopulations, antigen specific proliferation, and IFN-gamma production were evaluated throughout the study. Circulating gammadelta lymphocytes in PRRSV exposed animals expanded between days 14 to 70 (d14-d70, p = 0.016); following antigen stimulation, gammadelta lymphocyte proliferated by day 14 (d0-d14, p = 0.001) continuing through day 60. gammadelta lymphocytes produced IFN-gamma by day 14 pi continuing through day 50 (d0-d50, p = 0.004). Following re-exposure both gammadelta+ and CD4+ lymphocytes increased in IFN-gamma production. These results are not fully conclusive on the role of gammadelta lymphocytes against PRRSV; the data indicate that gammadelta lymphocytes specifically respond to PRRSV.

Immune responses of pigs after experimental infection with a European strain of Porcine reproductive and respiratory syndrome virus

The purpose of this experiment was to study the immune response of pigs during an experimental infection with a European strain of Porcine reproductive and respiratory syndrome virus (PRRSV). Five pigs were challenged intranasally with PRRSV strain VP21 and another five were kept as controls. Clinical course and humoral and cell-mediated responses were monitored for 70 days post-infection (p.i.). Infected pigs developed mild signs at 24 h p.i. Viraemia was detectable by nested RT-PCR until day 14 p.i. Earliest seroconversions (ELISA) were seen by day 7 p.i. (three of five animals) and, by day 14, all inoculated pigs had seroconverted (ELISA and immunoperoxidase monolayer assay). Virus-neutralizing antibodies were undetectable until day 56 p.i. and, by day 70 p.i., two inoculated pigs still were negative. Flow-cytometry assays using peripheral blood mononuclear cells (PBMC) showed an upshift in CD8+ cells (day 7 p.i.) and a downshift of CD21+ cells (days 7 and 28 p.i.). Regarding cell-mediated responses, development of PRRSV-specific gamma interferon-secreting cells (IFN-γ-SC) and interleukin 4-secreting cells (IL4-SC) in PBMC was examined by ELISPOT assay. IFN-γ-SC were not detected significantly until day 14 p.i., whereas, for IL4-SC, no differences between groups were seen. Concurrently with the onset of viraemia and the development of clinical signs, serum haptoglobin levels and interleukin 10 (IL10) in PRRSV-stimulated PBMC-culture supernatants increased significantly. These differences disappeared later on. For IL2, IL4, IL8 or transforming growth factor beta, no differences were seen among groups. These results are compatible with a model in which the immune response does not fully control the outcome of the infection.

New insights into the genetic diversity of European porcine reproductive and respiratory syndrome virus (PRRSV)

The complete ORF5 sequences of 66 porcine reproductive and respiratory syndrome (PRRS) field virus strains (1991-2001) and three European modified live vaccine strains were determined, as well as ORFs 6 and 7 of 19 selected strains. The variability of the deduced ORF5 amino acid sequences was analysed using statistical process control (SPC), allowing for the objective assessment of variable and conserved regions. Four variable and four conserved regions as well as five hypervariable amino acid positions were defined. The effects of genetic variability on possible structural and functional properties were discussed with emphasis on immunogenic features. Phylogenetic analysis and pairwise comparison of the nucleotide sequences revealed that the genetic distances between the strains has greatly increased over time. The data do not support an evolutionary influence of the geographical location or the time of sample collection, nor of PRRSV vaccination on strain development. In contrast to other authors who tended to concentrate on the samples from either a common geographic origin or a short sampling period, we could not confirm geographically separate PRRSV clusters nor did we find evidence of positive selective pressure as measured by the ratio of synonymous to non-synonymous substitutions in ORF5, 6 or 7. Immunological implications and vaccination strategies are discussed.

Spatial dispersal of porcine reproductive and respiratory syndrome virus-contaminated flies after contact with experimentally infected pigs

OBJECTIVE

To determine whether flies can acquire porcine reproductive and respiratory syndrome virus (PRRSV) and disperse the virus throughout a designated area.

ANIMALS

60 four-month-old pigs.

PROCEDURE

On day 0, 28 of 60 pigs were inoculated with PRRSV MN 30-100 (index variant). On the same day, 100,000 pupae of ochre-eyed houseflies and 100,000 pupae of red-eyed (wild-type) houseflies were placed in the swine facility for a release-recapture study. Flies were recaptured at 2 locations within the swine facility, 6 locations immediately outside the facility, and 30 locations 0.4, 0.8, 1.3, 1.7, 1.9, and 2.3 km from the facility. Traps were emptied on days 2, 7, 8, 10, and 14. Samples derived from flies were tested by use of a polymerase chain reaction assay, virus DNA was sequenced, and viruses were tested for infectivity by means of a swine bioassay.

RESULTS

PRRSV RNA homologous to the index PRRSV was detected in trapped flies collected inside and immediately outside the facility and from 9 of 48 samples collected at 0.4 km, 8 of 24 samples collected at 0.8 km, 5 of 24 samples collected at 1.3 km, and 3 of 84 samples collected at > 1.7 km from the facility. Two samples collected at 0.8 km contained genetically diverse variants of PRRSV. Swine bioassays revealed the virus in flies was infectious.

CONCLUSIONS AND CLINICAL RELEVANCE

Flies appeared to become contaminated with PRRSV from infected pigs and transported the virus > or = 1.7 km. Fly-born transmission may explain how PRRSV is seasonally transported between farms.

Porcine reproductive and respiratory syndrome virus field isolates differ in in vitro interferon phenotypes

Type I interferons (IFN-α and -β) play an important role in the innate host defense against viral infection by inducing antiviral responses. In addition to direct antiviral activities, type I IFN serves as an important link between the innate and adaptive immune response through multiple mechanisms. Therefore, the outcome of a viral infection can be affected by IFN induction and the IFN sensitivity of a virus. North American porcine reproductive and respiratory syndrome virus (PRRSV) field isolates were studied with regard to IFN-α sensitivity and induction in order to understand the role of type I IFN in PRRSV pathogenesis. PRRSV isolates were differentially sensitive to porcine recombinant IFN-α (rIFN-α) and varied in their ability to induce IFN-α in porcine alveolar macrophages (PAM) cultures as measured by a porcine IFN-α specific ELISA on cell culture supernatants. Fifty-two plaques were purified from three PRRSV isolates (numbers 3, 7, and 12) and tested for IFN sensitivity and IFN induction. Plaque-derived populations were composed of heterogeneous populations in terms of IFN-inducing capacity and sensitivity to rIFN-α. When macrophages infected with isolates 3, 7, or 12 were treated with polycytidylic acid (polyI:C), IFN-α production was enhanced. Cells infected with isolate 3 and treated with polyI:C showed the most consistent and strongest enhancement of IFN-α production. It was demonstrated that the relatively low concentrations of IFN-α produced by isolate 3 contributed to the enhanced IFN-α synthesis in response to polyI:C. Isolates 7 and 12 significantly suppressed the enhanced IFN-α production by isolate 3 in polyI:C treated cells. To determine if suppression was at the level of IFN-α transcription, quantitative RT-PCR was performed for IFN-α mRNA and compared to GAPDH and cyclophilin mRNA quantification. However, the relative number of IFN-α transcript copies did not correlate with IFN-α protein levels, suggesting a post-transcriptional mechanism of suppression. In summary, these results demonstrate that PRRSV field isolates differ both in IFN-α sensitivity and induction. Furthermore, a PRRSV field isolate strongly enhance polyI:C-induced IFN-α production in PAM cultures and this priming effect was suppressed by other PRRSV isolates.

Differential host cell gene expression regulated by the porcine reproductive and respiratory syndrome virus GP4 and GP5 glycoproteins

The porcine reproductive and respiratory syndrome virus (PRRSV) GP4 and GP5 proteins are two membrane-associated viral glycoproteins that have been shown to induce neutralizing antibodies. In the present study, the host cell gene expression profiles altered by the GP4 and GP5 proteins were investigated by the use of DNA microarrays. Sublines of Marc-145 and HeLa cells were established by stable transfection with open reading frame (ORF)4 and ORF5 of PRRSV, respectively, and differential gene expressions were studied using microarray chips embedded with 1718 human-expressed sequence tags. The genes for protein degradation, protein synthesis and transport, and various other biochemical pathways were identified. No genes involved in the apoptosis pathway appeared to be regulated in GP5-expressing cells. The microarray data may provide insights into the specific cellular responses to the GP4 and GP5 proteins during PRRSV infection.

DNA vaccination of pigs with open reading frame 1-7 of PRRS virus

We cloned all open reading frames of a Danish isolate of porcine reproductive and respiratory syndrome (PRRS) virus in DNA vaccination vectors. Pigs were vaccinated using a gene gun with each single construct (ORF1, ORF2, ORF3, ORF4, ORF5, ORF6, or ORF7) or combinations thereof. Vaccination with ORF7 consistently induced antibodies after three vaccinations, while antibodies were only sporadically detected in the remaining groups. After six vaccinations, all pigs were inoculated with PRRS virus and the post-inoculation antibody response was studied. Pigs vaccinated with ORF1 or ORF4 were primed for antibody response against NSP2 or GP4, respectively. Neutralising antibodies were detected in all pigs, with ORF5 vaccinated pigs showing the highest titres.

Molecular characterization of PL97-1, the first Korean isolate of the porcine reproductive and respiratory syndrome virus

We determined the complete nucleotide and predicted amino acid sequence of the genomic RNA of PL97-1, the first Korean strain of porcine reproductive and respiratory syndrome virus (PRRSV), which was isolated from the serum of an infected pig in 1997. We found that the 15411-nucleotide genome of PL97-1 consisted of a 189-nucleotide 5' noncoding region (NCR), a 15071-nucleotide protein-coding region, and a 151-nucleotide 3'NCR, followed by a poly (A) tail. The 5'-end of PL97-1 began with 1ATG ACG TAT AGG12. Comparison of the PL97-1 genome with the 11 fully sequenced PRRSV genomes currently available revealed sequence divergence ranging from 0.3% (the VR-2332-derived vaccine MLV RespPRRS/Repro strain) to 38% (the Dutch Lelystad strain). To better understand the genetic relationships between these different strains, phylogenetic analyses were performed on the full-length PRRSV genomes. Significantly, the phylogenetic tree based on the ORF1b or ORF7 genes most closely resembled the tree based on the full-length genomes. Thus, these single genes will be the most useful in revealing the genetic relationships between the different strains relative to their geographical distribution. Extensive phylogenetic analyses using the ORF7 sequences of 111 PRRSV isolates available revealed that PL97-1 is most closely related to the North American genotype VR-2332, a VR-2332-derived vaccine strain, and Chinese BJ-4. It is distantly related to the European genotype Lelystad. This study provides the largest full-length genome phylogenetic analysis of PRRSV that has been published to date, and supports an earlier genetic grouping of the many temporally and geographically diverse PRRSV strains currently isolated.

Virological and immunological responses to porcine reproductive and respiratory syndrome virus in a large population of gilts

Porcine reproductive and respiratory syndrome virus (PRRSV) causes a prolonged active infection followed by a persistent infection in lymphoid tissues lasting for several months. Pigs develop both an antibody and cell-mediated immune response following PRRSV infection, but the specific role of each type in the development of protective immunity and clearance of the virus is not yet known. The aims of this study were to characterize the dynamics of PRRSV persistence from 0 to 135 d post infection (pi), characterize the kinetics of the antibody mediated immune response following PRRSV infection, and characterize the cell mediated immune responses to PRRSV infection. Eighty, 4-month-old PRRSV-free gilts were obtained from a source known to be negative for PRRSV. On day 0, gilts were infected intranasally with 10(2.4) TCID/50 MN 30-100 PRRSV. Following infection, animals were bled between days 0 to 135 pi. Viremia was detected up to day 30. Serum antibody response (by enzyme-linked immunosorbent assay [ELISA] and virus neutralization antibody) was detected from day 14 to 120 pi. Cell-mediated immune response represented by interferon gamma (IFN-gamma) was detected from day 14 to 120 pi. Persistence of PRRSV in tissues was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) between days 30 to 135. These results indicate that serum neutralizing antibodies and IFN-gamma play an important role in the clearance of PRRSV. Nevertheless none of the parameters measured (virus neutralizing antibodies), either alone or in combination, are solely responsible for the clearance of the virus from the host and the development of sterilizing immunity.

Genomic characterization of virulent, attenuated, and revertant passages of a North American porcine reproductive and respiratory syndrome virus strain

Pigs were exposed to three passages of the NADC-8 strain of porcine reproductive and respiratory syndrome virus (PRRSV) to investigate the relationship between genotypic and phenotypic properties. Differences were found in the virulence of the three passages called virulent, intermediate, and avirulent. Avirulent virus was derived by attenuation of virulent virus in cell culture and intermediate virus was derived by passage of avirulent virus in a pig. Nucleotide sequence differences between virulent and avirulent virus consisted of 50 nucleotide changes and a three-nucleotide deletion, and between avirulent and intermediate virus consisted of 8 nucleotide changes resulting in six amino acid changes. Three of these amino acid changes were direct reversions to virulent virus. Genetic changes, especially those seemingly associated with attenuation followed by some degree of reversion to virulence, in ORF1a, ORF1b, and ORF 6 regions of the genome may be involved in the control of PRRSV replication and virulence.

Immune response of pigs inoculated with Mycobacterium bovis BCG expressing a truncated form of GP5 and M protein of porcine reproductive and respiratory syndrome virus

Pigs were immunised with recombinant BCG (rBCG) expressing a truncated form of GP5 (lacking the first 30 NH(2)-terminal residues) (rBCGGP5) and M protein (rBCGM) of porcine reproductive and respiratory syndrome virus (PRRSV). At 30 days post-inoculation (dpi), pigs inoculated with rBCGGP5 and rBCGM developed a specific humoral immune response against the viral proteins, as detected by commercial ELISA and Western blot tests, and at 60 dpi, three out of five animals developed neutralizing antibodies with titers ranging from 1:4 to 1:8. At 67 dpi, an IFN-gamma response against BCG antigens, but not against the viral proteins, was detected by ELISPOT in inoculated pigs. Following challenge with a pathogenic strain of PRRSV, pigs inoculated with rBCG showed lower (P<0.05) temperature, viremia and virus load in bronchial lymph nodes than control animals, suggesting the establishment of partial protection against PRRSV infection.

Studies on the carriage and transmission of porcine reproductive and respiratory syndrome virus by individual houseflies (Musca domestica )

The objectives of the study were to determine the site of porcine reproductive and respiratory syndrome virus (PRRSV) in individual houseflies, to assess whether an individual housefly could transmit PRRSV to a susceptible pig, and to compare the ability of PCR, virus isolation and a pig bioassay to detect PRRSV in houseflies. In the first experiment 26 houseflies were fed on a pig infected experimentally with PRRSV; 13 were processed as a whole fly homogenate, while an exterior surface wash and a gut homogenate were collected from the other 13. Infectious PRRSV was recovered from nine of the whole fly homogenates, 12 of the gut homogenates and one of the exterior surface washes. In the second experiment, two of 10 individual houseflies, which had fed on an infected pig, transmitted PRRSV to a susceptible pig in a controlled manual transmission protocol. In the third experiment, single flies or pools of 30 flies were immersed in different concentrations of a PRRSV inoculum, then tested by PCR, virus isolation and bioassay. The virus was detected at a concentration of 10(1) TCID50/ml by PCR, 10(2) TCID50/ml by the bioassay and 10(3) TCID50/ml by virus isolation.

Survival of porcine reproductive and respiratory syndrome virus in houseflies

The objectives of the study were to determine the duration of porcine reproductive and respiratory syndrome virus (PRRSV) survival in houseflies (Musca domestica Linnaeus) following feeding on an infected pig, and to determine whether the virus was present on the exterior surface or within the internal viscera of the fly. A total of 210 laboratory-colonized houseflies were allowed to feed to repletion on a pig, experimentally infected with PRRSV on day 7 postinoculation, and then maintained alive under laboratory conditions (27 degrees C). Two subsets (A and B) of 30 flies were collected at each of the following sampling points; 0, 6, and 12 hours post feeding (pf). Subset A contained an extra group of 30 flies collected at 24 hours pf due to the availability of extra flies. Flies in subset A were processed as whole fly homogenates, while the exterior surface washes and digestive organs were collected from flies in subset B. Whole fly homogenates, collected at 0, 6, and 12 hours pf, were positive by both polymerase chain reaction (PCR) and swine bioassay. Digestive organs, collected at 0 and 12 hours pf, were positive by PCR and swine bioassay. The PRRSV RNA was detected by PCR from the exterior surface wash of subset B flies collected at 0, 6, and 12 hours pf; however, only the subset collected at 0 hour pf was swine bioassay-positive. This study indicates that infectious PRRSV can survive within the intestinal tract of houseflies for up to 12 hours following feeding on an infected pig, but only for a short period on the exterior surface of the flies.

Safety and protective efficacy of porcine reproductive and respiratory syndrome recombinant virus vaccines in young pigs

Three porcine reproductive and respiratory syndrome virus (PRRSV) recombinants, generated by mutagenesis of an infectious cDNA clone of the Lelystad virus (LV) isolate, were tested for their safety and protective efficacy as potential PRRSV vaccines in pigs. Recombinant vABV688 contains two amino acid substitutions in the minor structural protein GP(2) resulting in improved growth on cell line CL2621; in recombinant vABV707 the region encoding the ectodomain of the major unglycosylated membrane protein M has been replaced by that of the murine lactate dehydrogenase-elevating arterivirus; recombinant vABV746 lacks the six C-terminal amino acids of the nucleocapsid protein N. First, we determined the safety of these recombinant viruses by monitoring the stability of the introduced mutations in 8-week-old pigs. We showed that the introduced genomic mutations were maintained throughout the viraemic period. Second, the protective efficacy of immunization with the recombinant viruses against challenge with a homologous and a heterologous PRRSV strain was determined in two pigs and compared with the efficacy of vABV437, a virus derived from the parental LV cDNA. The viraemia in pigs immunized with the recombinant viruses was reduced compared to pigs immunized with vABV437. In addition, the length of viraemia was reduced in the sentinel pigs that were introduced into the groups immunized with vABV746, vABV688, and vABV707, however, all of the sentinel pigs became infected. Pigs immunized with vABV707 and vABV437 were protected against challenge with homologous virus LV-Ter Huurne and transmission of the latter virus. None of the immunized pigs were protected against heterologous challenge with the virulent US isolate SDSU#73, but the vABV707- and vABV746-immunized pigs were protected against transmission of this virus from challenged pigs. In conclusion, the obtained viral recombinants are interesting candidates to be further explored for their use as vaccines against PRRSV.

Development of a heteroduplex mobility assay to identify field isolates of porcine reproductive and respiratory syndrome virus with nucleotide sequences closely related to those of modified live-attenuated vaccines

Porcine reproductive and respiratory syndrome has been devastating the swine industry since the late 1980s. The disease has been controlled, to some extent, through the use of modified live-attenuated (MLV) vaccines once available. However, such a practice periodically resulted in isolation or detection of vaccine-like viruses from pigs as determined by a partial genomic sequencing. In this study, we developed a heteroduplex mobility assay (HMA) for quickly identifying porcine reproductive and respiratory syndrome virus (PRRSV) isolates with significant nucleotide sequence identities (>/=98%) with the modified live-attenuated vaccines. The major envelope gene (ORF5) of 51 PRRSV field isolates recovered before and after the introduction of the vaccines was amplified, denatured, and reannealed with the HMA reference vaccine strains Ingelvac PRRS MLV and Ingelvac PRRS ATP, respectively. Nine of the 51 field isolates and the VR2332 parent virus of Ingelvac PRRS MLV, which were all highly related to Ingelvac PRRS MLV with Collapse

Key Words Collapse

MESH Headings

• Animals
• Heteroduplex Analysis/methods
• Molecular Sequence Data
• Phylogeny
• Porcine Reproductive and Respiratory Syndrome/virology
• Porcine respiratory and reproductive syndrome virus/classification
• Porcine respiratory and reproductive syndrome virus/genetics
• Porcine respiratory and reproductive syndrome virus/isolation & purification
• Sequence Analysis, DNA
• Swine
• Swine Diseases/virology
• Vaccines, Attenuated
• Viral Vaccines

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Grants Collapse

Affiliation(s)

K F Key Center for Molecular Medicine and Infectious Diseases, Department of Biomedical Sciences and Pathobiology, College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060-0342, USA
D K Guenette
K-J Yoon
P G Halbur
T E Toth
X J Meng

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Strain specificity of the immune response of pigs following vaccination with various strains of porcine reproductive and respiratory syndrome virus

The primary objective of the study was to determine strain specificity of the immune response of pigs following vaccination with selected strains of porcine reproductive and respiratory syndrome virus (PRRSV). The experimental design included five groups (I through V, six pigs per group) free of antibody for PRRSV at the beginning of the experiment (day 0). On day 0, groups III, IV, and V were vaccinated with attenuated versions of PRRSV strains 8, 9, and 14, respectively. On day 21, the immunity of group II (non-vaccinated/challenged controls) and groups III, IV, and V was challenged by exposing each pig to a composite of the virulent versions of these same three strains. On day 35, all pigs, including non-vaccinated/non-challenged pigs of group I, were necropsied. Lungs and selected lymph nodes were examined for lesions. Serum samples obtained at weekly intervals throughout the study and lung lavage fluids obtained at necropsy were tested for the presence of PRRSV and its strain identity. Before challenge the strain of PRRSV identified in the sera of vaccinated pigs was always that with which the particular pig had been vaccinated (i.e. homologous strain), whereas, with one exception, only heterologous strains were identified after challenge. This apparent strain exclusion as a result of vaccination was statistically significant (P = 0.004). The tendency for heterologous strains to predominate after challenge suggests that a pig's immune response to PRRSV has some degree of strain specificity. Whether this finding has any clinical relevance in regard to immunoprophylaxis remains to be determined.

Homo-oligomerization of the porcine reproductive and respiratory syndrome virus nucleocapsid protein and the role of disulfide linkages

As a step toward understanding the assembly pathway of the porcine reproductive and respiratory syndrome virus (PRRSV), the oligomeric properties of the nucleocapsid (N) protein were investigated. In this study, we have demonstrated that under nonreducing conditions the N protein forms disulfide-linked homodimers. However, inclusion of an alkylating agent (N-ethylmaleimide [NEM]) prevented disulfide bond formation, suggesting that these intermolecular disulfide linkages were formed as a result of spurious oxidation during cell lysis. In contrast, N protein homodimers isolated from extracellular virions were shown to have formed NEM-resistant intermolecular disulfide linkages, the function of which is probably to impart stability to the virion. Pulse-chase analysis revealed that N protein homodimers become specifically disulfide linked within the virus-infected cell, albeit at the later stages of infection, conceivably when the virus particle buds into the oxidizing environment of the endoplasmic reticulum. Moreover, NEM-resistant disulfide linkages were shown to occur only during productive PRRSV infection, since expression of recombinant N protein did not result in the formation of NEM-resistant disulfide-linked homodimers. Mutational analysis indicated that of the three conserved cysteine residues in the N protein, only the cysteine at position 23 was involved in the formation of disulfide linkages. The N protein dimer was shown to be stable both in the presence and absence of intermolecular disulfide linkages, indicating that noncovalent interactions also play a role in dimerization. Non-disulfide-mediated N protein interactions were subsequently demonstrated both in vitro by the glutathione S-transferase (GST) pull-down assay and in vivo by the mammalian two-hybrid assay. Using a series of N protein deletion mutants fused to GST, amino acids 30 to 37 were shown to be essential for N-N interactions. Furthermore, since RNase A treatment markedly decreased N protein-binding affinity, it appears that at least in vitro, RNA may be involved in bridging N-N interactions. In cross-linking experiments, the N protein was shown to assemble into higher-order structures, including dimers, trimers, tetramers, and pentamers. Together, these findings demonstrate that the N protein possesses self-associative properties, and these likely provide the basis for PRRSV nucleocapsid assembly.

Genetic diversity and phylogenetic analysis of glycoprotein 5 of European-type porcine reproductive and respiratory virus strains in Spain

The gene encoding glycoprotein 5 (ORF5) of 21 porcine reproductive and respiratory syndrome virus (PRRSV) isolates from Spain and two European-type vaccines currently available in that country were analysed using RT-PCR and sequencing. Sequences were then compared with other European-type sequences available through GenBank. Results showed percentages of similarity to Lelystad virus (LV), which, in most cases, were below 90 %. In contrast, two strains were very similar (>99 %) to a PRRSV variant from the Czech Republic. Evolutionary trees showed three types of strains: one grouped old Spanish sequences; a second grouped isolates from this study together with two Czech variant strains; and the third comprised other GenBank sequences. Regarding the predicted protein sequences, some isolates from this study showed a low degree of similarity to LV (below 50 %) and most of the strains examined had additional N-linked glycosylation sites compared to LV. These results provide evidence of the existence of variant PRRSV strains in Spain with characteristics that may be advantageous for immune evasion.

Mechanical transmission of porcine reproductive and respiratory syndrome virus throughout a coordinated sequence of events during warm weather

Mechanical transmission of porcine reproductive and respiratory syndrome virus (PRRSV) throughout a coordinated sequence of events that replicated common farm worker behavior during warm weather (10 degrees C to 16 degrees C) was assessed using a field-based model. The model involved fomites (boots and containers), vehicle sanitation, transport, and personnel movement. In a previous study, the model successfully demonstrated mechanical transmission of PRRSV in 8 out of 10 replicates during cold weather. A field strain of PRRSV was inoculated into carriers consisting of soil samples, which were adhered to the undercarriage of a vehicle. The vehicle was driven approximately 50 km to a commercial truck washing facility where the driver's boots contacted the carriers during washing, introducing the virus to the vehicle interior. The vehicle was then driven 50 km to a simulated farm site, and the driver's boots mechanically spread virus into the farm anteroom. Types of containers frequently employed in swine farms contacted drippings from the footwear on the anteroom floor. The truck wash floor, vehicle cab floor mats, boot soles, anteroom floor, and the ventral surface of containers were sampled to track the virus throughout the model. Ten replicates were conducted, along with sham-inoculated controls, and control replicates. In 2 replicates, infectious PRRSV was detected on the anteroom floor and in 1 replicate, infectious PRRSV was detected on the surface of the container by swine bioassay. All sham-inoculated controls and protocol controls were negative. These results indicate that mechanical transmission of PRRSV throughout a coordinated sequence of events in warm weather can occur, but in contrast to data from studies conducted during cold weather, it appears to be a relatively infrequent event.

Heterodimerization of the two major envelope proteins is essential for arterivirus infectivity

The two major envelope proteins of arteriviruses, the membrane protein (M) and the major glycoprotein (GP(5)), associate into a disulfide-linked heterodimer that is incorporated into the virion and has been assumed to be a prerequisite for virus assembly. Using an equine arteritis virus (EAV) infectious cDNA clone, we have analyzed the requirement for GP(5)-M heterodimerization and have identified the Cys residues involved in the formation of the GP(5)-M disulfide bond. The single Cys residue (Cys-8) in the M ectodomain was crucial for heterodimerization and virus infectivity. Mutagenesis of any of the five Cys residues in the GP(5) ectodomain or removal of the single GP(5) N-glycosylation site also rendered the full-length clone noninfectious. However, an analysis of revertants yielded an exceptional pseudorevertant in which residues 52 to 79 of the GP(5) ectodomain had been deleted and the original Cys-80-->Ser mutation had been maintained. Consequently, this revertant lacked the GP(5) N-glycosyation site (Asn-56) and retained only a single cysteine residue (Cys-34). By using this GP(5) deletion, we confirmed that Cys-34 of GP(5) and Cys-8 of M are essential for GP(5)-M heterodimerization, a key event in the assembly of the EAV envelope.