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

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.

Development and validation of three Capripoxvirus real-time PCRs for parallel testing

Capripoxviruses have the potential to cause outbreaks with a severe socio-economic impact. The latter, combined with an altered virus dissemination pattern, warrants its status as an important emerging disease. Disease control or eradication programmes can only be applied successfully if the necessary diagnostic tools are available allowing clear and unequivocal identification of the pathogen. Real-time PCR combines high sensitivity/specificity with a reduced analysis time and is thus a proven useful tool for identification of many pathogens, including Capripoxviruses. In order for a real-time PCR to be used in a diagnostic capacity, the different analytical and diagnostic parameters need to be evaluated to assure data quality. The implementation of parallel testing using multiple real-time PCRs with similar characteristics can improve further Capripoxvirus diagnosis. It was therefore the purpose of this study to develop a triplet real-time PCR panel with similar high sensitivity/specificity and provide sufficient validation data regarding the performance characteristics that the panel can be used in parallel, depending on the purpose and local situation.

Development of multiplex real-time PCR for simultaneous detection of three Potyviruses in tobacco plants

AIMS

To develop a multiplex real-time PCR assay using TaqMan probes for the simultaneous detection and quantification of Tobacco etch virus (TEV), Potato virus Y (PVY) and Tobacco vein banding mosaic virus (TVBMV).

METHODS AND RESULTS

Specific primer and probe combinations for TEV and TVBMV were developed from the coat protein region of the viral genome. To detect PVY, a primer and probe combination PVY-Univ F, PVY-Univ R and PVY-Univ P for amplifying the coat protein region of the virus genome was employed. The detection limit of multiplex real-time PCR for these viruses was 10 copies μl(-1) of the standard plasmid. The multiplex reaction was successful in the detection of these three pathogens, with no non-specific amplification and cross-reaction.

CONCLUSIONS

This multiplex real-time PCR provides a rapid, effective, specific and sensitive method for the simultaneous detection and quantification of the three pathogens on infected tobacco plants.

SIGNIFICANCE AND IMPACT OF THE STUDY

This multiplex real-time PCR will be useful not only for diagnostic, ecological, epidemiological and pathogenesis studies, but also for investigating host/virus or virus/virus interactions, in particular during mix infection.

Establishment and characterization of an infectious cDNA clone of a classical swine fever virus LOM strain

Classical swine fever virus (CSFV) causes a highly contagious disease among swine that has an important economic impact worldwide. CSFV strain LOM is an attenuated virus of low virulent strain of Miyagi isolated from Japan in 1956. Eight DNA fragments representing the genome of the CSFV strain LOM were obtained by RT-PCR. These were used to determine the complete nucleotide sequence and construct a full-length cDNA clone which was called Flc-LOM. Sequence analysis of the recombinant clone (Flc-LOM) revealed the presence of eight mutations, resulting in two amino acid substitutions, when compared to the parental sequence. RNA transcripts of both LOM and Flc-LOM were directly infectious in PK-15 cells. The rescued Flc-LOM virus grew more slowly than the parental virus, LOM, in the cells. Intramuscular immunization with Flc-LOM was safe and highly immunogenic in pigs; no clinical signs or virus transmission to sentinel animals were observed after 35 days. CSFV-specific neutralizing antibodies were detected 14 days post-infection. After challenge with the virulent CSFV strain SW03, pigs immunized with Flc-LOM were shown to be fully protected. Thus, our newly established infectious clone of CSFV, Flc-LOM, could serve as a vaccine candidate.

Towards licensing of CP7_E2alf as marker vaccine against classical swine fever-Duration of immunity

Classical swine fever (CSF) marker vaccine candidate CP7_E2alf was tested in a "duration of immunity" trial according to the World Organisation for Animal Heath (OIE) guidelines. To this means, 15 weaner pigs were either orally or intramuscularly vaccinated with a single dose of CP7_E2alf vaccine produced under Good Laboratory Practice (GLP) conditions. Ten additional pigs were included as controls. Six months later, all animals were oronasally challenged with highly virulent CSF virus (CSFV) strain "Koslov". Upon vaccination, all but one orally and all intramuscularly vaccinated pigs developed rising and later on stable CSFV glycoprotein E2-specific antibodies. In contrast, no CSFV E(rns)-specific "marker" antibodies were detectable prior to challenge infection. None of the co-housed control animals seroconverted. Upon challenge infection, all seropositive animals were protected from lethal challenge, whereas all control animals and the non-responder developed severe signs of CSF. One control animal recovered, the others had to be euthanised due to animal welfare reasons between days 4 and 7 post challenge infection. All protected animals showed quickly rising neutralizing antibodies reaching high titres by the end of the trial. At the end of the trial, the marker ELISA was positive for most challenged animals that survived the CSFV infection (27 out of 30). Using reverse transcription polymerase chain reaction, low level genome detection was seen in all vaccinated animals between days 4 and 10 post challenge infection, but no virus could be isolated from any samples of these animals. The OIE guidelines require seroconversion in at least 8 out of 10 vaccinated animals. This requirement was fulfilled. Moreover, only control animals should die. With this requirement, only the intramuscular vaccination fully complied as one orally vaccinated pig did not respond. Concluding, CP7_E2alf induced stable antibodies that led to protection from lethal challenge with highly virulent CSFV strain "Koslov" six months after vaccination, with the exception of one non-responder after oral vaccination.

Dynamic distribution and tissue tropism of classical swine fever virus in experimentally infected pigs

BACKGROUND

Classical swine fever (CSF), caused by the Classical swine fever virus (CSFV), is an Office International des Epizooties (OIE) notifiable disease. However, we are far from fully understand the distribution, tissue tropism, pathogenesis, replication and excretion of CSFV in pigs. In this report, we investigated the dynamic distribution and tissue tropism of the virus in internal organs of the experimentally infected pigs using real-time RT-PCR and immunohistochemistry (IHC).

RESULTS

A relative quantification real-time PCR was established and used to detect the virus load in internal organs of the experimentally infected pigs. The study revealed that the virus was detected in all 21 of the internal organs and blood collected from pigs at day 1 to day 8 post infections, and had an increasing virus load from day 1 to day 8 post infections. However, there was irregular distribution virus load in most internal organs over the first 2 days post infection. Blood, lymphoid tissue, pancreas and ileum usually contain the highest viral loads, while heart, duodenum and brain show relatively low viral loads.

CONCLUSIONS

All the data suggest that CSFV had an increasing virus load from day 1 to day 8 post infections in experimentally infected pigs detected by real-time RT-PCR, which was in consistent with the result of the IHC staining. The data also show that CSFV was likely to reproduce in blood, lymphoid tissue, pancreas and the ileum, while unlikely to replicate in the heart, duodenum and brain. The results provide a foundation for further clarification of the pathogenic mechanism of CSFV in internal organs, and indicate that blood, lymphoid tissue, pancreas and ileum may be preferred sites of acute infection.

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.

Development of a reverse transcription multiplex real-time PCR for the detection and genotyping of classical swine fever virus

A reverse transcription multiplex real-time PCR (RT-MRT-PCR) was developed for rapid detection and genotyping of classical swine fever virus (CSFV). The universal primers and specific TaqMan probes for each of the three genotypes, genotypes 1, 2, and 3, were designed within the 3′-UTR of the CSFV. Non-CSFV swine virus and clinical samples from specific pathogen-free (SPF) pigs were both demonstrated to be CSFV-negative by RT-MRT-PCR. The diagnostic sensitivity of RT-MRT-PCR was determined to be 1 viral copy/μl for each genotype of standard plasmid. For the analytical sensitivity experiment, 100 samples of 14 CSFV genotype 1 strains and 86 samples from CSFV outbreak farms were all detected as CSFV-positive by RT-MRT-PCR, and the genotype results were consistent with the results of sequencing from a previous study. The intra-assay and inter-assay variations of RT-MRT-PCR were below 3% in all experiments. The sensitivity of RT-MRT-PCR was the same as the reverse transcription nested PCR (RT-nPCR) and higher than reverse transcription PCR (RT-PCR) and viral isolation from clinical samples. This assay was used further to evaluate the duration of viremia of wild-type CSFV in vaccinated exposed pigs. The results indicated that pigs vaccinated with the E2 subunit vaccine had longer viremia than pigs given the C-strain vaccine, which is compatible with the findings of previous studies. Thus, the new RT-MRT-PCR is a rapid, reproducible, sensitive, and specific genotyping tool for CSFV detection.

Rapid pre-clinical detection of classical swine fever by reverse transcription loop-mediated isothermal amplification

The usefulness of reverse transcription loop-mediated isothermal amplification (RT-LAMP) for rapid pre-clinical detection of classical swine fever virus (CSFV) infection was evaluated. The RT-LAMP reaction could be finished in 60 min under isothermal condition at 65 °C by employing a set of four primers targeting the 5′ untranslated region of CSFV. The RT-LAMP assay of CSFV showed higher sensitivities than that of RT-PCR, with a detection limit of 5 copies per reaction. No cross-reactivity was observed from the samples of other related viruses including porcine circovirus type 2, porcine parvovirus, porcine pseudorabies virus, Japanese encephalitis virus, and porcine reproductive and respiratory syndrome virus. The detection rates of CSFV RT-LAMP, RT-PCR and virus isolation for samples including blood, tonsil, nasal and rectal swabs from uninoculated pigs without any clear clinical symptom were 89%, 78% and 71%, respectively. Furthermore, all of the assays showed higher sensitivity for blood and tonsil swabs samples than nasal and rectal swabs. These results indicate that the CSFV RT-LAMP assay is a valuable tool for its rapid, cost-effective detection and has potential usefulness for rapid pre-clinical detection and surveillance of classical swine fever in developing countries.

Validation of two commercial real-time RT-PCR kits for rapid and specific diagnosis of classical swine fever virus

Two real-time RT-PCR kits, developed by LSI (TaqVet CSF) and ADIAGENE (Adiavet CSF), obtained an agreement to be commercialised in France, subject to conditions, defined by the French Classical Swine Fever (CSF) National Reference Laboratory. The producers were asked to introduce an internal control to check the RNA extraction efficacy. The different criteria assessed were sensitivity, "pestivirus specificity", reproducibility and ease of handling, using 189 different samples. These samples were either CSFV inactivated strains or blood/serum/organs collected from CSFV experimentally infected pigs or naturally infected wild boars. The reproducibility of the assays was confirmed by the analysis of a batch-to-batch panel control that was used for inter-laboratory tests involving nine laboratories. The two kits were also tested for the use in mass diagnostics and the results proved the kits to be suited using pools of blood, serum and tonsils. Moreover, a field evaluation, carried out on spleen samples collected from the CSF surveillance of wild boars in an area known to be infected and from domestic pigs at a slaughterhouse, confirmed the high sensitivity and specificity of the two kits. This step-by-step evaluation procedure confirmed that the two commercial CSF real-time RT-PCR kits have a higher predictive value than the current diagnostic standard, Virus Isolation.

A multiplex nested RT-PCR for the detection and differentiation of wild-type viruses from C-strain vaccine of classical swine fever virus

A multiplex nested RT-PCR (RT-nPCR) was developed for the detection and differentiation of classical swine fever virus (CSFV). A fragment of 447 or 343 bp was amplified from the genomic RNA of C-strain or virulent Shimen strain, respectively, and two fragments of 447 and 343 bp were simultaneously amplified from the mixed samples of C-strain and Shimen. When detecting several wild-type isolates representative of different subgroups (1.1, 2.1, 2.2, and 2.3) circulating in Mainland China and samples from pigs experimentally infected with Shimen strain, the RT-nPCR resulted in an amplification pattern similar to Shimen. No amplification was achieved for uninfected cells, or cells infected with bovine viral diarrhea virus (BVDV), and other viruses of porcine origin. The RT-nPCR was able to detect as little as 0.04 pg of CSFV RNA. The restrictive fragment length polymorphism (RFLP) demonstrated unique patterns of wild-type viruses and C-strain. Among the 133 field samples, 42 were tested to contain wild-type viruses and 18 showing presence of C-strain. The RT-nPCR can be used to detect and differentiate pigs infected with wild-type CSFV from those vaccinated with C-strain vaccine, thus minimizing the risk of culling vaccinates during outbreaks.

Evaluation of a multiplex real-time RT-PCR for quantitative and differential detection of wild-type viruses and C-strain vaccine of Classical swine fever virus

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), one of OIE listed diseases. Most of the currently available detection methods do not allow discrimination between wild-type CSF viruses and the vaccine strains. This study was designed to develop a multiplex real-time RT-PCR for the quantitative and differential detection of wild-type viruses and C-strain vaccine widely used in China. CSFV specific primers and two differently labeled TaqMan probes for the differentiation of wild-type viruses from C-strain vaccine were designed in the 5'-untranslated region of the viral genome of CSFV. The two TaqMan probes specifically hybridize wild-type viruses of different subgroups and C-strain vaccine, respectively, in the multiplex real-time RT-PCR, with no cross-reaction to a number of non-CSFV porcine viruses. The sensitivity of the assay for detecting wild-type and C-strain-type vaccine viruses was determined to be 41.8 and 81.5copies/microL viral RNA, respectively. Completely correct differentiation of wild-type viruses from C-strain vaccine was achieved when testing reference strains and characterized field isolates of CSFV in China. The multiplex real-time RT-PCR was able to detect the viral RNA in the whole blood samples of experimentally infected pigs as early as 2 days post-infection, 3 to 4 days prior to the onset of clinical signs in co-housed pigs. The agreements between the multiplex real-time RT-PCR and a multiplex RT-nested PCR for detection of wild-type and C-strain-type viruses were 96.9% and 100%, respectively, when detecting 106 different field samples. There is a positive correlation between the titers of C-strain vaccines titrated in rabbits and RNA copies quantitated by the multiplex real-time RT-PCR. The novel assay described here is rapid and sensitive, and is useful for differentiating field strains and C-strain of CSFV in China.

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.

CP7_E2alf: a safe and efficient marker vaccine strain for oral immunisation of wild boar against Classical swine fever virus (CSFV)

Wild boar are an important reservoir of Classical swine fever virus (CSFV) in several European countries, where most of the primary outbreaks in domestic pigs are directly related to the endemic disease situation in the wild boar population. Oral immunisation has been introduced as an additional control measure to accelerate CSF eradication in wild boar in Germany since 1993. Immunisation with an oral bait vaccine based on the conventionally attenuated live vaccine strain "C" proved to be safe and effective, but does not allow differentiation between infected and vaccinated animals. Therefore, we examined the vaccine efficacy of the recently constructed chimeric pestivirus CP7_E2alf, whose coding sequences for the major envelope protein E2 of BVDV strain CP7 are replaced by E2 of the CSFV strain Alfort187 [Reimann I, Depner K, Trapp S, Beer M. An avirulent chimeric pestivirus with altered cell tropism protects pigs against lethal infection with classical swine fever virus. Virology 2004;322(1):143-57]. Following oral immunisation of wild boar, CP7_E2alf proved to be completely avirulent. Furthermore, all vaccinees were fully protected from clinical disease after a highly virulent CSFV challenge infection. The immunised animals seroconverted within 3 weeks after vaccination for CSFV E2-specific and CSFV neutralising antibodies, whereas prior to challenge infection no antibodies against CSFV E(rns) were detected with an appropriate CSFV-specific marker ELISA test. Thus, the BVDV backbone of CP7_E2alf enables serological and genetic differentiation from wild type CSFV infection. In conclusion, CP7_E2alf represents the first efficient and safe marker vaccine candidate for oral immunisation of wild boar against CSFV.