Development of two novel monoclonal antibody-based ELISAs for the detection of antibodies and the identification of swine isotypes against swine vesicular disease virus

Transboundary Animal Diseases, an Overview of 17 Diseases with Potential for Global Spread and Serious Consequences

Animals provide food and other critical resources to most of the global population. As such, diseases of animals can cause dire consequences, especially disease with high rates of morbidity or mortality. Transboundary animal diseases (TADs) are highly contagious or transmissible, epidemic diseases, with the potential to spread rapidly across the globe and the potential to cause substantial socioeconomic and public health consequences. Transboundary animal diseases can threaten the global food supply, reduce the availability of non-food animal products, or cause the loss of human productivity or life. Further, TADs result in socioeconomic consequences from costs of control or preventative measures, and from trade restrictions. A greater understanding of the transmission, spread, and pathogenesis of these diseases is required. Further work is also needed to improve the efficacy and cost of both diagnostics and vaccines. This review aims to give a broad overview of 17 TADs, providing researchers and veterinarians with a current, succinct resource of salient details regarding these significant diseases. For each disease, we provide a synopsis of the disease and its status, species and geographic areas affected, a summary of in vitro or in vivo research models, and when available, information regarding prevention or treatment.

Development and validation of a competitive ELISA based on virus-like particles of serotype Senecavirus A to detect serum antibodies

Virus-like particles (VLPs) are high-priority antigens with highly ordered repetitive structures, which are similar to natural viral particles. We have developed a competitive enzyme-linked immunosorbent assay (cELISA) for detecting antibodies directed against Senecavirus A (SVA). Our assay utilizes SVA VLPs that were expressed and assembled in an E. coli expression system as the coating antigens. VLPs have better safety and immunogenicity than intact viral particles or peptides. The VLPs-based cELISA was used to test 342 serum samples collected from different pig farms, and the results showed that its specificity and sensitivity were 100% and 94%, respectively. The consistency rates of cELISA with the BIOSTONE AsurDx™ Senecavirus A (SVA) Antibody Test Kit and an indirect immunofluorescent assay were 90.0% and 94.2%, respectively. Therefore, this VLPs-based cELISA can be effectively and reliably used for the detection and discrimination of SVA infection in serum samples.

Eradication of Swine Vesicular Disease in Italy

Swine vesicular disease (SVD) is a contagious viral disease of pigs clinically indistinguishable from other vesicular diseases, such as foot and mouth disease, vesicular stomatitis, vesicular exanthema of swine, and idiopathic vesicular disease. In Italy, where SVD was first reported in 1966, an eradication program started in 1995. The program, updated in 2008, was based on regionalization, complete control on pig movements, improvement of pig farms biosecurity, appropriate cleansing and disinfection procedures of vehicles approved for pig transportation, and a testing program using both serological and virological assays. In cases of confirmed SVD virus infection a stamping-out policy was applied. In the period 2009 to 2019, between 300,000 and 400,000 pigs were serologically tested each year. The last SVD outbreak was notified in 2015, and the last seropositive pig was detected in 2017. SVD surveillance is still ongoing and no proof of virus activity has been detected so far. All available data support the complete SVD virus eradication from the Italian pig industry.

Development and evaluation of swine vesicular disease isotype-specific antibody ELISAs based on recombinant virus-like particles

Swine vesicular disease (SVD) is a contagious viral disease of pigs. The clinical signs of SVD are indistinguishable from other vesicular diseases, such as senecavirus A infection (SVA) and foot-and-mouth disease (FMD). Rapid and accurate diagnostic tests of SVD are considered essential in countries free of vesicular diseases. Competitive ELISA (cELISA) is the serological test used routinely. However, although cELISA is the standard test for SVD antibody testing, this test produces a small number of false-positive results, which caused problems in international trade. The current project developed a SVD isotype antibody ELISA using recombinant SVD virus-like particles (VLP) and an SVD-specific monoclonal antibody (mAb) to reduce the percentage of false positives. The diagnostic specificities of SVD-VLP isotype ELISAs were 98.7% and 99.6% for IgM and IgG. The SVD isotype ELISAs were SVD-specific, without cross-reactivity to other vesicular diseases. A panel of 16 SVD-positive reference sera was evaluated using the SVD-VLP isotype ELISAs. All sera were correctly identified as positive by the two combined SVD-VLP isotype ELISAs. Comparison of the test results showed a high level of correlation between the SVDV antigen isotype ELISAs and SVD-VLP isotype ELISAs. 303 sera from animals lacking clinical signs and history of SVDV exposure were identified positive using SVD cELISA. These samples were examined using SVD-VLP isotype ELISAs. Of the 303 serum samples, five were positive for IgM, and five of 303 were positive for IgG. Comparable to virus neutralization test results, SVD isotype ELISAs significantly reduced the false-positive samples. Based on above test results, the combined use of cELISA and isotype ELISAs can reduce the number of false-positive samples and the use of time-consuming virus neutralization tests, with benefit for international trade in swine and related products.

The risk of introduction of swine vesicular disease virus into Kenya via natural sausage casings imported from Italy

Pig production in Kenya is hampered by seasonal markets. As an alternative outlet for the finished pigs, several value-added meat-processing firms have been established. Sausage, which is produced using casings derived from intestines of pigs, is one form of processed meats. Kenya imports several kgs of natural casings every year; and a recent concern is Swine vesicular disease virus (SVDV), which has never been reported in Kenya, might be introduced via natural casings imported from Italy. To determine conditions (with associated probabilities) that could lead to the introduction of SVDV, a quantitative risk assessment model was developed. Using Monte Carlo simulations at 10,000 iterations, the probability of introducing SVDV was estimated to be 1.9x10-8. Based on the suggested volume of import and mitigations used in the analysis, contaminated casings derived from an estimated 0.003 (Range = 8.1x10-8 - 0.08) infected pigs will be included in the consignment each year. The critical pathway analysis revealed that rigorous surveillance programs in Italy have a potential to dramatically reduce the risk of introducing SVDV into Kenya by this route.

Generation, characterization, and application in serodiagnosis of recombinant swine vesicular disease virus-like particles

Swine vesicular disease (SVD) is a highly contagious viral disease that causes vesicular disease in pigs. The importance of the disease is due to its indistinguishable clinical signs from those of foot-and-mouth disease, which prevents international trade of swine and related products. SVD-specific antibody detection via an enzyme-linked immunosorbent assay (ELISA) is the most versatile and commonly used method for SVD surveillance and export certification. Inactivated SVD virus is the commonly used antigen in SVD-related ELISA. A recombinant SVD virus-like particle (VLP) was generated by using a Bac-to-Bac baculovirus expression system. Results of SVD-VLP analyses from electron microscopy, western blotting, immunofluorescent assay, and mass spectrometry showed that the recombinant SVD-VLP morphologically resemble authentic SVD viruses. The SVD-VLP was evaluated as a replacement for inactivated whole SVD virus in competitive and isotype-specific ELISAs for the detection of antibodies against SVD virus. The recombinant SVD-VLP assay produced results similar to those from inactivated whole virus antigen ELISA. The VLP-based ELISA results were comparable to those from the virus neutralization test for antibody detection in pigs experimentally inoculated with SVD virus. Use of the recombinant SVD-VLP is a safe and valuable alternative to using SVD virus antigen in diagnostic assays.

Use of Oral Fluids for Detection of Virus and Antibodies in Pigs Infected with Swine Vesicular Disease Virus

The use of swine oral fluid (OF) for the detection of nucleic acids and antibodies is gaining significant popularity. Assays have been developed for this purpose for endemic and foreign animal diseases of swine. Here, we report the use of OF for the detection of virus and antibodies in pigs experimentally infected with swine vesicular disease virus (SVDV), a virus that causes a disease clinically indistinguishable from the economically devastating foot-and-mouth disease. Viral genome was detected in OF by real-time reverse transcription polymerase chain reaction (RRT-PCR) from 1 day post-infection (DPI) to 21 DPI. Virus isolation from OF was also successful at 1-5 DPI. An adapted competitive ELISA based on the monoclonal antibodies 5B7 detected antibodies to SVDV in OF starting at DPI 6. Additionally, using isotype-specific indirect ELISAs, SVDV-specific IgM and IgA were evaluated in OF. IgM response started at DPI 6, peaking at DPI 7 or 14 and declining sharply at DPI 21, while IgA response started at DPI 7, peaked at DPI 14 and remained high until the end of the experiment. These results confirm the potential use of OF for SVD surveillance using both established and partially validated assays in this study.

Validation according to OIE criteria of a monoclonal, recombinant p26–based, serologic competitive enzyme-linked immunosorbent assay as screening method in surveillance programs for the detection of Equine infectious anemia virus antibodies

The Italian National Reference Center for equine infectious anemia (CRAIE; Rome, Italy) developed and validated a monoclonal, recombinant p26–based competitive enzyme-linked immunosorbent assay (cELISA) for the detection of EIA virus antibodies employing the 2010 criteria of the World Organization for Animal Health (OIE). The following parameters were evaluated: cutoff values, repeatability, reproducibility, concordance, analytical sensitivity (Se), absolute analytical specificity (Sp), and diagnostic Se and Sp. Positive and negative predictive values were also defined in relation to the estimated prevalence. When the cELISA was used as a screening test for 96,468 samples in the Italian EIA surveillance program, 17% more EIA cases were detected than by the agar gel immunodiffusion test, and the apparent diagnostic Sp estimated from these samples was 99.8%, which was more than the diagnostic Sp (80.2%) estimated from validation. The high Se and Sp of the cELISA confirm its fit for purpose as a screening test.

Comparison of three in-house ELISAs for the detection of hepatitis E virus infection in pigs under field conditions

Hepatitis E virus (HEV) is a RNA non-enveloped virus that comprises four genotypes. The genome of HEV is organized into three Open Reading Frames (ORFs), and the ORF2 is responsible for encoding capsid proteins. HEV can infect a wide range of hosts, and pigs are considered the main reservoir. HEV infection is considered a zoonosis and it is responsible for acute hepatitis in humans, especially in developing countries. The development of a blocking ELISA would be of high value for screening purpose, because there is no need of species specific reagents. The present study was conducted to assess three in-house ELISAs for the detection of HEV infection in 779 sera collected from breeding and fattening farms under field conditions. Two assays were indirect ELISAs, while the third was a blocking ELISA. Two different recombinant antigens were generated from specific sequences of the HEV-ORF2, and a Latent Class approach in a Bayesian framework was used to evaluate the diagnostic accuracy of each ELISA. Because the three ELISAs cannot be thought of as independent, all possible dependence structures were modelled starting from the general case of conditional independence to the most complex situation of three mutually dependent assays. Results showed that none of the three ELISAs was significantly superior to the others in terms of sensitivity (posterior median value ranging from 89% to 94%, all 95% posterior credible intervals (95%PCI) overlapped). In terms of specificity, one of the indirect ELISAs was superior to blocking ELISA (posterior median indirect ELISA: 99%, 95%PCI: 98-100%; blocking ELISA: 90%; 95%PCI: 86-94%). However, this difference could be due to the potential wider spectrum of antibodies that blocking ELISA can actually detect.

Spatial transmission of Swine Vesicular Disease virus in the 2006-2007 epidemic in Lombardy

In 2006 and 2007 pig farming in the region of Lombardy, in the north of Italy, was struck by an epidemic of Swine Vesicular Disease virus (SVDV). In fact this epidemic could be viewed as consisting of two sub-epidemics, as the reported outbreaks occurred in two separate time periods. These periods differed in terms of the provinces or municipalities that were affected and also in terms of the timing of implementation of movement restrictions. Here we use a simple mathematical model to analyse the epidemic data, quantifying between-farm transmission probability as a function of between-farm distance. The results show that the distance dependence of between-farm transmission differs between the two periods. In the first period transmission over relatively long distances occurred with higher probability than in the second period, reflecting the effect of movement restrictions in the second period. In the second period however, more intensive transmission occurred over relatively short distances. Our model analysis explains this in terms of the relatively high density of pig farms in the area most affected in this period, which exceeds a critical farm density for between-farm transmission. This latter result supports the rationale for the additional control measure taken in 2007 of pre-emptively culling farms in that area.

Generation and diagnostic application of monoclonal antibodies against Seneca Valley virus

Seneca Valley virus (SVV), a member of the Picornaviridae family, was implicated in a suspicious vesicular disease discovered in pigs from Canada in 2007. Because any outbreak of vesicular disease in pigs is assumed to be foot-and-mouth disease (FMD) until confirmed otherwise, a test for diagnosing the presence of SVV would be a very useful tool. To develop the diagnostic tests for SVV infection, 5 monoclonal antibodies (mAbs) were produced from mice immunized with binary ethylenimine (BEI)-inactivated SVV. Using a dot blot assay, the reactivity of the mAbs was confirmed to be specific for SVV, not reacting with any of the other vesicular disease viruses tested. The mAbs demonstrated reactivity with SVV antigen in infected cells by an immunohistochemistry assay. An SVV-specific competitive enzyme-linked immunosorbent assay (cELISA) was developed using BEI-inactivated SVV antigen and a mAb for serodiagnosis. The cELISA results were compared to the indirect isotype (immunoglobulin [Ig]M and IgG) ELISA and the virus neutralization test. All SVV experimentally inoculated pigs exhibited a positive SVV-specific antibody response at 6 days postinoculation, and the sera remained positive until the end of the experiment on day 57 (>40% inhibition) using the cELISA. The cELISA reflected the profile of the indirect ELISA for both IgM and IgG. This panel of SVV-specific mAbs is valuable for the identification of SVV antigen and the serological detection of SVV-specific antibodies.

Differentiation of foot-and-mouth disease-infected pigs from vaccinated pigs using antibody-detecting sandwich ELISA

The presence of serum antibodies for nonstructural proteins of the foot-and-mouth disease virus (FMDV) can differentiate FMDV-infected animals from vaccinated animals. In this study, a sandwich ELISA was developed for rapid detection of the foot-and-mouth disease (FMD) antibodies; it was based on an Escherichia coli-expressed, highly conserved region of the 3ABC nonstructural protein of the FMDV O/TW/99 strain and a monoclonal antibody derived from the expressed protein. The diagnostic sensitivity of the assay was 98.4%, and the diagnostic specificity was 100% for naïve and vaccinated pigs; the detection ability of the assay was comparable those of the PrioCHECK and UBI kits. There was 97.5, 93.4 and 66.6% agreement between the results obtained from our ELISA and those obtained from the PrioCHECK, UBI and CHEKIT kits, respectively. The kappa statistics were 0.95, 0.87 and 0.37, respectively. Moreover, antibodies for nonstructural proteins of the serotypes A, C, Asia 1, SAT 1, SAT 2 and SAT 3 were also detected in bovine sera. Furthermore, the absence of cross-reactions generated by different antibody titers against the swine vesicular disease virus and vesicular stomatitis virus (VSV) was also highlighted in this assay's specificity.

Molecular characterization and phylogenetic analysis of VP1 of porcine enteric picornaviruses isolates in Italy

Porcine enterovirus (PEV), Porcine Teschovirus and Porcine sapelovirus, belonging to the family Picornaviridae, are ubiquitous and mainly cause asymptomatic infections in pigs. In this study, a total of 40 Italian porcine picornavirus isolates were characterized by sequencing the capsid VP1-encoding gene. This procedure turned out to be a useful diagnostic tool for the molecular identification of porcine enterovirus, teschovirus and sapelovirus strains and for the study of molecular epidemiology and evolution of these viruses confirming the possibility of correlating virus genotype to serotype.

Noninfectious virus-like particle antigen for detection of swine vesicular disease virus antibodies in pigs by enzyme-linked immunosorbent assay

An inactivated SVDV antigen is used in current enzyme-linked immunosorbent assays (ELISAs) for the detection of antibodies to swine vesicular disease virus (SVDV). To develop a noninfectious recombinant alternative, we produced SVDV-like particles (VLPs) morphologically and antigenically resembling authentic SVDV particles by using a dual baculovirus recombinant, which expresses simultaneously the P1 and 3CD protein genes of SVDV under different promoters. Antigenic differences between recombinant VLPs and SVDV particles were not statistically significant in results obtained with a 5B7-ELISA kit, indicating that the VLPs could be used in the place of SVDV antigen in ELISA kits. We developed a blocking ELISA using the VLPs and SVDV-specific neutralizing monoclonal antibody 3H10 (VLP-ELISA) for detection of SVDV serum antibodies in pigs. The VLP-ELISA showed a high specificity of 99.9% when tested with pig sera that are negative for SVDV neutralization (n=1,041). When tested using sera (n=186) collected periodically from pigs (n=19) with experimental infection with each of three different strains of SVDV, the VLP-ELISA detected SVDV serum antibodies as early as 3 days postinfection and continued to detect the antibodies from all infected pigs until termination of the experiments (up to 121 days postinfection). This test performance was similar to that of the gold standard virus neutralization test and indicates that the VLP-ELISA is a highly specific and sensitive method for the detection of SVDV serum antibodies in pigs. This is the first report of the production and diagnostic application of recombinant VLPs of SVDV. Further potential uses of the VLPs are discussed.

Use of automated real-time reverse transcription-polymerase chain reaction (RT-PCR) to monitor experimental swine vesicular disease virus infection in pigs

Automated real-time RT-PCR was evaluated as a diagnostic tool for swine vesicular disease virus (SVDV) infection on a range of samples (vesicular epithelium, serum, nasal swabs, faeces) from four inoculated and three in-contact pigs over a period of 28 days. Traditional diagnostic procedures (virus isolation, and ELISAs for antigen and antibody) were used in parallel. Each inoculated pig developed a significant viraemia and clinical disease, and excreted virus, which was transmitted to the in-contact animals. The latter, however, developed only a short-lived, low-level viraemia and no clinical disease. The RT-PCR and virus isolation were generally comparable in detecting SVDV in the serum and nasal swabs from inoculated and in-contact pigs up to day 6 after infection; it was possible, however, to isolate virus for a longer period from the faeces of a few pigs. This suggested that further optimization of the template extraction method was required to counteract the effects of RT-PCR inhibitors in faeces. It was concluded that the automated real-time RT-PCR is a useful diagnostic method for SVD in clinically or subclinically affected pigs and contributed to the study of the pathogenesis of SVD in the pigs.

Importance of arginine 20 of the swine vesicular disease virus 2A protease for activity and virulence

A major virulence determinant of swine vesicular disease virus (SVDV), an Enterovirus that causes an acute vesicular disease, has been mapped to residue 20 of the 2A protease. The SVDV 2A protease cleaves the 1D-2A junction in the viral polyprotein, induces cleavage of translation initiation factor eIF4GI, and stimulates the activity of enterovirus internal ribosome entry sites (IRESs). The 2A protease from an attenuated strain of SVDV (Ile at residue 20) is significantly defective at inducing cleavage of eIF4GI and the activation of IRES-dependent translation compared to the 2A protease from a pathogenic strain (J1/73, Arg at residue 20), but the two proteases have similar 1D-2A cleavage activities (Y. Sakoda, N. Ross-Smith, T. Inoue, and G. J. Belsham, J. Virol. 75:10643-10650, 2001). Residue 20 has now been modified to every possible amino acid, and the activities of each mutant 2A protease has been analyzed. Selected mutants were reconstructed into full-length SVDV cDNA, and viruses were rescued. The rate of virus growth in cultured swine kidney cells reflected the efficiency of 2A protease activity. In experimentally infected pigs, all four of the mutant viruses tested displayed much-reduced virulence compared to the J1/73 virus but a significant, albeit reduced, level of viral replication and excretion was detected. Direct sequencing of cDNA derived from samples taken early and late in infection indicated that a gradual selection-reversion to a more efficient protease occurred. The data indicated that extensive sequence change and selection may introduce a severe bottleneck in virus replication, leading to a decreased viral load and reduced or no clinical disease.

Mapping of linear epitopes on the capsid proteins of swine vesicular disease virus using monoclonal antibodies

The antigenic linear map of swine vesicular disease virus (SVDV) has been studied using a repertoire of monoclonal antibodies (mAbs) raised against a recombinant SVDV polyprotein, P1. Peptide-scanning analyses, cross-reactivity studies with homologous and heterologous viruses and predicted location on a computer-generated three-dimensional model of the capsid proteins have allowed the identification of five main linear sites. Two sites, the N terminus of VP3 and amino acids 51-60 on VP1, correspond to internal areas, conserved not only between SVDV isolates but also in the related enterovirus coxsackievirus B5. In contrast, three other regions, amino acids 142-161 of VP2, 61-70 of VP3 and the C terminus of VP1, are exposed on the external face of the capsid and subjected to antigenic variation, even among different SVDV isolates. Further minor sites that were antigenically conserved were identified on VP4. In contrast with conformational sites described previously, none of the linear epitopes identified in this work is involved in neutralization of virus infectivity and post-infection swine sera did not inhibit the binding of mAbs with the relevant epitopes. Both of these observations suggest that linear epitopes are poorly immunogenic in pigs. The characterization of linear sites has contributed to a better understanding of the antigenic structure of SVDV and mAbs used to this purpose may provide a useful tool for the improvement of diagnostic methods, such as antigen detection systems, and analyses of the antigenic profile of SVDV isolates.

Characterization of neutralization sites on the circulating variant of swine vesicular disease virus (SVDV): a new site is shared by SVDV and the related coxsackie B5 virus

Using a panel of new monoclonal antibodies (mAbs), five neutralizing, conformation-dependent sites have been identified on the antigenic variant of swine vesicular disease virus (SVDV) circulating currently. In studies on the antigenic conservation of these sites, the four antigenic/genetic groups of SVDV described showed distinguishable patterns, confirming this classification. By sequencing mAb-resistant mutants, the five sites have been mapped precisely and localized on a three-dimensional model of the SVDV capsid. All were found to be orientated, to a different extent, towards the external surface of the capsid. Three of the five sites, located in VP1, VP2 and VP3, correspond to epitopes identified previously in historic isolates as sites 1, 2a and 3b, respectively. Another site, site IV, which maps to position 258 of VP1, corresponds to an epitope reported recently and is described in this study to be specific for isolates of the most recent antigenic group of SVDV. A fifth site is described for the first time and corresponds to the unique neutralizing site that is common to both SVDV and coxsackie B5 virus; it maps to positions 95 and 98 of VP1, but may also include positions nearby that belong to site 1 on the BC-loop of VP1, suggesting the classification of site Ia. These results may have useful diagnostic and epidemiological applications, since mAbs to the new conserved site Ia provide universal reagents for SVDV detection systems, while the specificity of mAbs to site IV make them unique markers for the most recent strains of SVDV.

Swine vesicular disease virus. Pathology of the disease and molecular characteristics of the virion

Swine vesicular disease is a highly contagious disease of pigs that is caused by an enterovirus of the family Picornaviridae. The virus is a relatively recent derivative of the human coxsackievirus B5, with which it has high molecular and antigenic homology. The disease is not severe, and affected animals usually show moderate general weakening and slight weight loss that is recovered in few days, as well as vesicular lesions in the mucosa of the mouth and nose and in the interdigital spaces of the feet. However, the similarity of these lesions to those caused by foot-and-mouth disease virus has led to the inclusion of this virus in list A of the Office International des Epizooties. The disease has been eradicated in the European Union except in Italy, where it is considered endemic in the south. Nevertheless, as occasional outbreaks still appear and must be eliminated rapidly, European countries are on the alert and farms are monitored routinely for the presence of the virus. This circumstance has led to a considerable effort to study the pathology of the disease and the molecular biology and antigenicity of the virus, andto the development of optimized methods for the diagnosis of the infection.

Swine vesicular disease: an overview

Swine vesicular disease (SVD) is a notifiable viral disease of pigs included on the Office International des Epizooties List A. The first outbreak of the disease was recognized in Italy in 1966. Subsequently, the disease has been reported in many European and Asian countries. The causative agent of the disease is SVD virus which is currently classified as a porcine variant of human coxsackievirus B5 and a member of the genus enterovirus in the family picornaviridae. From a clinical point of view, SVD is relatively unimportant, rarely causing deaths and usually only a minor setback to finishing schedules. However, the clinical signs which it produces are indistinguishable from those caused by foot-and-mouth disease, and its presence prevents international trade in pigs and pig products. This article reviews recent findings on all aspects of the virus and the disease which it causes.

Swine vesicular disease, studies on pathogenesis, diagnosis, and epizootiology: a review

Swine vesicular disease (SVD) is a contagious viral disease of swine. It causes vesicular lesions indistinguishable from those observed of foot-and-mouth disease. Infection with SVD virus (SVDV) can lead to viraemia within 1 day and can produce clinical signs 2 days after a pig has come into contact with infected pigs or a virus-contaminated environment. Virus can be detected 3.5 hours after infection using immunohistochemistry. In these in vitro studies, this technique was superior to in-situ hybridization. In SVDV-infected tissues, however, more infected cells were positive using in-situ hybridization, and these were already seen 4.5 hours after infection. For serological diagnosis of SVD several new enzyme-linked immunosorbent assays (ELISA's) have been developed. The newest ELISAs, based on monoclonal antibodies, are superior to the previous tests. The new tests produce fewer less false-negative results and enable large-scale serological screening. In screening programmes a small percentage of false positive reactors have been detected. The cause of these false-positive reactions has not been identified, though infections with human Coxsackie B5 virus can be excluded.

Chimeric swine vesicular disease viruses produced by fusion PCR: a new method for epitope mapping

A new method of epitope mapping based on chimeric swine vesicular disease (SVD) viruses produced by fusion PCR (polymerase chain reaction). Seven out of 16 neutralising and non-neutralising newly produced monoclonal antibodies (MAbs) discriminated between SVD isolate ITL/1/66 and NET/1/92. Using fusion PCR eight chimeric viruses were produced containing different supplementary pieces of the P1 region of both parent strains. Using these chimeric viruses we were able to map the epitope regions recognised by these seven neutralising and non-neutralising Mabs. This new method, using chimeric viruses produced by fusion PCR, is particularly valuable for the epitope mapping of non-neutralising MAbs.

Immune recognition of swine vesicular disease virus structural proteins: novel antigenic regions that are not exposed in the capsid

Swine vesicular disease virus (SVDV) is an enterovirus of the Picornaviridae family that belongs to the coxsackievirus B group. A number of antigenic sites have been identified in SVDV by analysis of neutralizing monoclonal antibody-resistant mutants and shown to be exposed on the surface of the capsid. In this paper we have identified seven new immunodominant antigenic regions in SVDV capsid proteins by a peptide scanning method, using a panel of sera from infected pigs. When these antigenic regions were located in the capsid by using a computer-generated three-dimensional model of the virion, one was readily exposed on the surface of the virus and the remaining sites were located facing the inner side of the capsid shell, at subunit contacts, or in the interior of the subunit structure.

Validation of a monoclonal antibody-based ELISA to detect antibodies directed against swine vesicular disease virus

A simple, rapid and sensitive competitive monoclonal antibody-based ELISA for the detection of antibodies directed against swine vesicular disease virus (SVDV) was developed. The ELISA was validated using field sera originating from SVDV-infected and non-infected Dutch pig herds, reference sera obtained from the Community Reference Laboratory for Swine Vesicular Disease at the Institute for Animal Health, Pirbright Laboratory, UK, and sera from animals infected experimentally. When testing 4277 sera originating from non-infected Dutch pig herds and collected as part of the national screening program, this ELISA had only 0.6% false positive results, whereas approximately 2% of false positive results were obtained with a conventional blocking ELISA used until recently. A sensitivity relative to the virus neutralisation test of > 97% was achieved when testing sera collected from Dutch pig farms where an outbreak of SVDV had occurred. All international reference sera scored consistently correct. Sera collected sequentially from pigs experimentally infected with SVDV isolates representing all currently recognized antigenic groups, were scored positive slightly earlier by the ELISA compared to the virus neutralisation test. This monoclonal antibody-based competitive ELISA for SVDV antibodies designated the Ceditest ELISA for SVDV-Ab, is as sensitive but more specific than the ELISA used until recently. Because sera are tested at a single dilution (1:5), incubations are carried out at room temperature and test results are available within 3 h, this ELISA is simple, easy to automate and therefore very suitable for screening large numbers of serum samples.

Molecular cloning, expression and immunological analysis of the capsid precursor polypeptide (P1) from swine vesicular disease virus

Swine vesicular disease virus (SVDV) is the aetiological agent of a highly contagious viral disease of pigs, whose symptoms are indistinguishable from those caused by foot-and-mouth disease virus (FMDV). The gene coding for the capsid protein precursor of SVDV (P1) from a recent spanish isolate (SPA/1/'93) was cloned and expressed in bacteria, and the antigenicity and immunogenicity of the recombinant product were evaluated. The recombinant P1 was recognised by antibodies against SVDV induced in pigs infected experimentally with different SVDV strains. Immunisation of swine with recombinant P1-induced SVDV-specific cellular and humoral immune responses. The implications of these results in SVD diagnostic as well as in vaccine development are discussed.

Reduction of singleton reactors against swine vesicular disease virus by a combination of virus neutralisation test, monoclonal antibody-based competitive ELISA and isotype specific ELISA

Pigs which are serologically positive for swine vesicular disease virus (SVDV) but which show no clinical signs and for which there is neither a relevant history of the disease on the holding nor contact with a known outbreak are considered as singleton reactors. False positive serological results for an epizootic disease, like SVD, in a non-vaccinated population or in imported animals are of great concern to international trade. For the virus neutralisation test, the gold standard for SVD, singleton reactors are found at a level of 1-3/1000. Singleton reactors also occur when other serological testing methods are used. The number of animals finally considered as singleton reactors can be reduced considerably by performing three different serological tests (virus neutralisation test, monoclonal antibody-based competitive ELISA and isotype specific ELISA) on the same serum. A serological profile of the animal can be derived by analysing the results in greater detail. This procedure can reduce considerably the number of pig holdings on which a prohibition of movement and trade needs to be imposed without requiring analysis of supplementary samples.

Rapid and sensitive polymerase chain reaction based detection and typing of foot-and-mouth disease virus in clinical samples and cell culture isolates, combined with a simultaneous differentiation with other genomically and/or symptomatically related viruses

Reverse transcription followed by the polymerase chain reaction method (PCR) allowed the detection of foot-and-mouth disease virus (FMDV), regardless of the serotype. A primer set corresponding to highly conserved regions of the 2B sequence was selected. By combining in a single reaction tube specific primer pairs for FMDV, swine vesicular disease virus, (SVDV), encephalomyocarditis virus (EMCV) and bovine viral diarrhea virus (BVDV), all four viruses could be identified and differentiated in one amplification reaction, based on the different lengths of the respective amplified segments. In a similar way, FMDV types O, A, C, SAT 2 and Asia 1 could be identified and differentiated, using primers selected from the ID (VP1) genome region. All results were confirmed by direct sequencing of the PCR product. The very fast RNA extraction, reverse transcription and PCR permitted us to read the agarose gels within three hours after samples (cell culture isolates as well as clinical material) arrived, which is of great importance in case of an FMDV suspicion. Furthermore, a very high sensitivity was achieved (less than one PFU). Therefore, our powerful detection assay by means of PCR for FMDV as well as for SVDV, EMCV and BVDV, has advantages compared to the presently used procedures.