Evaluation of the epidemiological importance of classical swine fever infected, E2 sub-unit marker vaccinated animals with RT-nPCR positive blood samples

Novel chimeric E2CD154 subunit vaccine is safe and confers long lasting protection against classical swine fever virus

E2CD154 is a vaccine candidate against classical swine fever (CSF) based on a chimeric protein composed of the E2 glycoprotein fused to porcine CD154 antigen, and formulated in the oil adjuvant Montanide™ ISA 50 V2. This vaccine confers early protection in pigs and prevents vertical transmission in pregnant sows. The objectives of this study were to assess the safety of this immunogen in piglets, to compare several doses of antigen in the formulation, and to study the duration of the immunity provided by this vaccine for up to 9 months. Three trials were conducted by immunizing pigs with a two-dose regime of the vaccine. Challenge experiments were carried out with the highly pathogenic Margarita strain. No local or systemic adverse effects were documented, and neither macroscopic nor microscopic pathological findings were observed in the vaccinated animals. The three antigen doses explored were safe and induced CSF protective neutralizing antibodies. The dose of 50 μg was selected for further development because it provided the best clinical and virological protection. Finally, this protective immunity was sustained for at least 9 months. This study demonstrates that E2CD154 vaccine is safe; defines a vaccine dose of 50 μg antigen, and evidences the capacity of this vaccine to confer long term protection from CSFV infection for up to 9 months post- vaccination. These findings complement previous data on the evaluation of this vaccine candidate, and suggest that E2CD154 is a promising alternative to modified live vaccines in CSF endemic areas.

Corrigendum to "Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows"

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and supports the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and support the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Efficacy of CSF vaccine CP7_E2alf in piglets with maternally derived antibodies

There is a need for live DIVA (differentiating infected from vaccinated animals) vaccines against classical swine fever (CSF). The aim of this study was to investigate whether vaccination with the chimeric pestivirus vaccine CP7_E2alf is efficacious to protect young piglets born from vaccinated sows, thus with maternally derived antibodies (MDAs). Groups of 10 piglets each, with or without MDAs, were vaccinated either intramuscularly (IM), at an age of 3 or 6 weeks, or orally (OR), at an age of 6 weeks. Five piglets of each group were challenged with CSFV strain Koslov and protection against clinical disease, virus shedding and transmission were studied. Vaccination with CP7_E2alf, both in the presence of MDA's and in piglets without MDA's, protected against severe clinical signs, but virus shedding from most inoculated piglets and transmission to contact pigs was observed. However, virus transmission in the vaccinated piglets was significantly reduced as compared to non-vaccinated piglets, although the reproduction ratio's R calculated from the results in the vaccinated pigs from our study were not yet significantly below 1. The efficacy of vaccination with CP7_E2alf in the presence of MDAs (R IMvac=0.8, R ORvac=0.4) seemed to be slightly less as compared to vaccination in the absence of MDAs (R IMvac=0.2, R ORvac=0). On a population level, the results suggest that the CP7_E2alf vaccine is an effective tool in the control and eradication of CSF and, moreover, can be applied for both IM and oral use for young age groups, with MDAs having a limited effect on the efficacy.

Classical swine fever (hog cholera): review of aspects relevant to control

Classical swine fever (CSF) has the ability to spread over large distances when human intervention such as illegal swill feeding facilitates its movement. This was apparent during 2005 when CSF appeared in South Africa (SA) after an absence of 87 years. In this review, various newly published developments in terms of the diagnosis of the disease and vaccination are described and applied to situations similar to SA. The role of wildlife such as feral pigs and European wild boar in the dissemination and maintenance of CSF virus are discussed, and the dearth of knowledge on the potential of other wild pig species prevalent on southern Africa noted. The modes of spread and control measures to prevent introduction as well as during outbreaks are discussed.

Proof of concept for the reduction of classical swine fever infection in pigs by a novel viral polymerase inhibitor

5-[(4-Bromophenyl)methyl]-2-phenyl-5H-imidazo[4,5-c]pyridine (BPIP) is a representative of a class of imidazopyridines with potentin vitroantiviral activity against pestiviruses including classical swine fever virus (CSFV). This study analysed whether the lead compound, BPIP, was able to reduce virus replication in infected piglets. The compound, administered in feed, was readily bioavailable and was well tolerated. Eight specific-pathogen-free pigs received a daily dose of 75 mg kg−1(mixed in feed) for a period of 15 consecutive days, starting 1 day before infection with the CSFV field isolate Wingene. BPIP-treated pigs developed a short, transient viraemia (one animal remained negative) and leukopenia (three animals did not develop leukopenia). Virus titres at peak viraemia (7 days post-infection) were markedly lower (∼1000-fold) than in untreated animals (P=0.00005) and the viral genome load in blood was also significantly lower (P≤0.001) in drug-treated animals than in untreated animals over the entire experiment. At the end of the experiment (day 33), no infectious virus was detectable in the tonsils of BPIP-treated animals, although low levels of viral RNA were detected. The inability to isolate infectious virus from the tonsils indicates that the risk of a persistent CSFV infection is negligible. Further optimization of the antiviral potency and bioavailability of this lead compound may result in molecules completely suppressing virus replication. A potent antiviral could potentially be used as a primary control measure against virus spread in case of an outbreak, in addition to present countermeasures. This study provides the first proof of concept for the prophylaxis/treatment of CSFV infection in pigs.

Modelling the effectiveness and risks of vaccination strategies to control classical swine fever epidemics

In a recent update of the Dutch contingency plan for controlling outbreaks of classical swine fever (CSF), emergency vaccination is preferred to large-scale pre-emptive culling. This policy change raised two questions: can emergency vaccination be as effective as pre-emptive culling, and what are the implications for showing freedom of infection? Here, we integrate quantitative information available on CSF virus transmission and vaccination effects into a stochastic mathematical model that describes the transmission dynamics at the level of animals, farms and livestock areas. This multilevel approach connects individual-level interventions to large-scale effects. Using this model, we compare the performance of five different control strategies applied to hypothetical CSF epidemics in The Netherlands and, for each of these strategies, we study the properties of three different screening scenarios to show freedom of infection. We find that vaccination in a ring of 2 km radius around a detected infection source is as effective as ring culling in a 1 km radius. Feasible screening scenarios, adapted to the use of emergency vaccination, can reduce the enhanced risks of (initially) undetected farm outbreaks by targeting vaccinated farms. Altogether, our results suggest that emergency vaccination against CSF can be equally effective and safe as pre-emptive culling.

Evaluation of real-time RT-PCR assay for the routine intra vitam diagnosis of classical swine fever

The aim of the study was to evaluate the diagnostic quality of the real-time RT-PCR assay described by Hoffmann et al. [Hoffmann, B., Beer, M., Schelp, C., Schirrmeier, H., Depner, K., 2005. Validation of a real-time RT-PCR assay for sensitive and specific detection of classical swine fever. J. Virol. Methods 130, 36-44] for the routine intra vitam diagnosis of classical swine fever (CSF). We compared the assay with conventional diagnostic methods by using defined diagnostic material from an animal experiment with pigs showing different clinical forms of CSF. Compared to virus isolation and antigen ELISA an enhanced sensitivity of the real-time RT-PCR could be shown. We were able to detect all infected pigs regardless of the clinical course of CSF. CSF infection was detected already during the incubation period, during the entire clinical phase as well as at the beginning of convalescence when the first antibodies were detected and no virus could be isolated any more. In most cases, positive PCR results were obtained 2 days earlier than with virus isolation and 2-4 days earlier than with the antigen ELISA.

Detection of classical swine fever vaccine virus in blood and tissue samples of pigs vaccinated either with a conventional C-strain vaccine or a modified live marker vaccine

Attenuated live classical swine fever (CSF) viruses are the most efficacious vaccines against the disease. However, little is known about the distribution and detection of CSF vaccine viruses in the host. We therefore compared the new recombinant attenuated marker vaccine virus CP7_E2alf with the conventional C-strain vaccine concerning virus isolation, antigen-, and genome-detection in different samples within the first 42 days post-vaccination (p.v.). Leukocytes and several organs such as tonsils, lymph nodes, spleen, thymus, parotis and kidney were also tested using highly sensitive real-time reverse transcription-polymerase chain reaction (RT-PCR) techniques. It was demonstrated that vaccine virus could be detected by live animal sampling only in a few leukocytes samples at very low titres and genome copy numbers within the first 14 days after immunisation. Vaccine virus could also be isolated from individual tonsil samples within the first 6 days after vaccine application. In contrast, vaccine virus genomes were consistently detected in the tonsils up to day 42 by real-time RT-PCR. Distribution, amount of virus and viral genome levels were similar for both tested vaccines. In conclusion, blood samples could be the sample material of choice for detecting CSF wild type virus infection even in vaccinated animals after more than 14 days p.v., while tonsil sampling provided appropriate material for long-term detection of both tested CSF vaccine viruses using real-time RT-PCR methods.

Marker vaccine strategies and candidate CSFV marker vaccines

Classical swine fever (CSF) is an economically important highly contagious disease of swine worldwide. Classical swine fever virus (CSFV) is its etiological agent, and the only natural hosts are domestic pigs and wild boars. Although field CSFV strains vary in the virulence, they all result in serious losses in pig industry. Highly virulent field strains generally cause acute disease and high mortality; moderately virulent field strains raise subacute or chronic infections; postnatal infection by low virulent field strains produces subclinical infection and mortality in the new-born piglets. CSFV can cross the placental barrier, and this transplacental transmission usually results in mortality of fetuses and birth of congenitally infected pigs with a late-onset disease and death. Two main strategies to control CSF epidemic are systematic prophylactic vaccination with live attenuated vaccines (such as C-strain) and non-vaccination stamping-out policy. But neither of them is satisfying enough. Marker vaccine and companion serological diagnostic test is thought to be a promising strategy for future control and eradication of CSF. During the past 15 years, various candidate marker vaccines were constructed and evaluated in the animal experiments, including recombinant chimeric vaccines, recombinant deletion vaccines, DNA vaccines, subunit vaccines and peptide vaccines. Among them, two subunit vaccines entered the large scale marker vaccine trial of EU in 1999. Although they failed to fulfil all the demands of the Scientific Veterinary Committee, they successfully induced solid immunity against CSFV in the vaccinated pigs. It can be expected that new potent marker vaccines might be commercially available and used in systematic prophylactic vaccination campaign or emergency vaccination in the next 15 years. Here, we summarized current strategies and candidate CSFV marker vaccines. These strategies and methods are also helpful for the development of new-generation vaccines against other diseases.

Factors critical for successful vaccination against classical swine fever in endemic areas

Classical swine fever (CSF) or hog cholera, caused by the classical swine fever virus (CSFV), is one of the most important viral diseases that cause serious economic loss to the swine industry worldwide. During the past 5 years, several techniques for measuring porcine cell-mediated immunity (CMI) were applied, in conjunction with other conventional techniques, to study factors that influence the induction of CSFV-specific immunity. Information, obtained from a series of experiments, demonstrated cell-mediated immune responses in providing protective immunity against CSF infection. Although it has been confirmed that commercially available modified live CSF vaccines are able to induce complete protection in vaccinated pigs, several factors including maternal immunity, the age of primary vaccination, vaccination protocol and complications caused by other pathogens, can greatly affect the effectiveness of CSF vaccines in the field.

Protection of Gruntlings against Classical Swine Fever Virus-Infection after Oral Vaccination of Sows with C-strain Vaccine

The objective of this study was to investigate the maternal protection of gruntlings derived from wild sows vaccinated orally against classical swine fever (CSF) using C-strain vaccine. Three vaccinated sows and one unvaccinated control sow were included. Challenge infection of the progeny was carried out either intranasally or by contact at the beginning of the third month of life (61-65 days post-natum). Whereas, two of three litters had maternal antibodies, the progeny of one vaccinated sow was seronegative at challenge. The progeny of the control sow, which was challenged by contact infection, developed moderate clinical signs except for one animal which became ill and died. Two gruntlings derived from the vaccinated sows also died of CSF, although one of them had a relatively high maternal antibody titre (128 ND(50)). The transient infection and partial virus shedding observed in a small number of gruntlings with maternal antibodies and the fact that one animal with maternal antibodies became ill and died confirm the incomplete maternal protection at this age. The reason for this incomplete protection is discussed. As none of the surviving gruntlings could be shown to carry CSFV or viral RNA at the end of the experiment (36 or 70 d.p.i.), it may be concluded that these animals do not represent a potential CSFV reservoir.

Diagnostic Evaluation of a Real-Time RT-PCR assay for Routine Diagnosis of Classical Swine Fever in Wild Boar

The aim of the study was to evaluate a real-time reverse transcription-polymerase chain reaction (RT-PCR) assay for routine diagnosis of classical swine fever (CSF) in wild boar by using 1000 spleen homogenates that were tested previously negative by virus isolation and 26 homogenates from which CSF virus could be isolated. All 26 positive samples in the virus isolation assay also were found to be positive in real-time RT-PCR. Additionally further 10 samples were detected by real-time RT-PCR out of the 1000 negative samples in the virus isolation. With a commercial CSF antigen-ELISA only 14 out of the 36 real-time RT-PCR positive samples could be detected. The sequence analysis of all ten samples that tested positive by real-time RT-PCR and negative by virus isolation revealed CSF virus-specific sequences. Based on the assumption that all samples with a CSF virus-specific sequence or positive in the virus isolation test originated from truly CSF virus-infected wild boar, the following sensitivity values were calculated as antigen-ELISA, 39%; virus isolation, 72% and real-time RT-PCR, 100%. The use of real-time RT-PCR instead of antigen-ELISA and virus isolation as a routine tool for control and eradication of CSF in wild boar populations is recommended.

Characterisation of the discrepancy between PCR and virus isolation in relation to classical swine fever virus detection

In order to confirm and characterise further the discrepancies observed between diagnostic RT-nPCR and virus isolation results for the detection of classical swine fever virus (CSFV), a test panel of three new RT-PCRs was designed, amplifying parts of the NS2, NS3 and NS5A regions. Screening of negative samples by virus isolation with the new panel not only confirmed the discrepancies previously observed but also indicated that these were not associated with a specific genomic region. However, none of the PCR-positive samples were positive on all the different PCRs and preferential amplification was not obtained even when a more sensitive real-time RT-PCR was used. Furthermore, the primer-dependent amplification, most likely caused by the presence of viral fragments, demonstrates the necessity of confirming a single positive PCR result, certainly in the presence of contradictory virus isolation results. The new PCR panel, in combination with sequencing, can be used as a tool to provide additional information on the nature of the viral RNA present in the sample.

ImmunoTrack®: The novel antibody-based technology for tracing in animal health

This paper describes a novel antibody-based livestock movement control tool and method of meat allocation, both in livestock husbandry as well as during the meat-processing chain. Immuno Track fulfills diverse prerequisites and meets regulatory demands which are substantial for a successful monitoring technology: (i) the induction of long-lasting antibody responses detectable onsite throughout the whole mast period of pigs, (ii) a single immunization injection with protein derivatives is sufficient to evoke a strong epitope-specific antibody response, and (iii) the complete degradation of the protein markers after the antibody response has been triggered in meatproducing animals such as cattle or pigs. There are diverse fields of application for the Immuno-Track marker technology, such as in quality meat programs, as compliance markers for animal vaccines or as a tool for verification of origin. Combination of this monitoring technology with the husbandry and identification databases for cattle and pigs within the European Community will lead to greater transparency in meat production, thereby regaining consumers' trust in concomitant structures of the meat-producing industry.