Application of a computer program for genetic typing of classical swine fever virus isolates from Germany

E-2 Glycoprotein Structural Variations Analysed within the CSFV 2.2. Genogroup in a “Closed Grid” Sampling Study from Meghalaya, India

CSF is enzootic in most of pig-producing states, particularly in the NorthEastern (NE) region of India. In this study, a total of 249 sera and 190 tissue samples were collected from different parts of Meghalaya. Samples were processed by ELISA and RT-PCR for serological and molecular diagnosis. Representative positive samples from the Khasi Hills region were selected for sequencing and “close grid” phylogenetic relationship using partial genomic regions of 5′UTR and E2. High seroprevalence (74.7%) of CSFV was recorded. Detection of the CSFV genome in serologically positive serum samples and tissue samples was 61.29% and 18.94%, respectively. BLAST and phylogenetic analyses indicate the clustering of all the field samples in subgroup 2.2, with high identity with EF014334 from China. Molecular structural modelling of the E2 partial sequence using representative sequences MG563797 from Meghalaya and EF014334 from China indicate potential changes in the protein motif and its conformation, which may explain the emergence of subgroup 2.2 CSFV replacing the predominant subgroup 1.1 viruses in NorthEast India. The epidemiological information presented in this study may be helpful for determination of disease incidence in this region, whereas the virus profile may be useful for framing disease control programs.

Molecular detection and characterization of African swine fever virus from field outbreaks in domestic pigs, Mizoram, India

African swine fever (ASF) is a devastating haemorrhagic disease of domestic pigs, which can cause mortality up to 100%. Sudden mortality in pigs following an acute course of systemic disease was investigated in Mizoram state of India and confirmed the outbreak as ASF. Affected pigs suffered from severe depression, high fever, bloody diarrhoea, cutaneous haemorrhages and showed haemorrhagic lesions in visceral organs. The outbreak was confirmed by detection of p72, p54 and the central variable region of B602L genes by PCR in representative tissue samples collected from dead pigs. The nucleotide and phylogenetic analyses of p72, p54 and B602L characterized the ASFV as genotype II. Interestingly, the analysis of B602L gene has revealed that the ASFV from Mizoram state of India is more closely linked to the Eurasian ASFV strains isolated prior to 2014 and discriminated the Indian strains in two separate groups indicating that the source of origin for the Mizoram outbreak could be different from that of the other states of India.

Molecular detection and characterization of highly pathogenic porcine reproductive and respiratory syndrome virus from a natural outbreak in wild pigs, Mizoram, India

This study reports for the first time a natural outbreak of highly pathogenic porcine reproductive and respiratory syndrome (HP-PRRS) caused by HP-PRRS virus (HP-PRRSV) in wild pigs characterized by sudden onset of depression, anorexia, respiratory distress, and high fever. The disease has caused severe haemorrhagic pneumonia, haemorrhagic lymphadenitis, enlarged spleen with areas of infarction, and petechial haemorrhages on the myocardium and on the surface of kidneys. HP-PRRSV was detected in representative tissue samples by reverse transcription-PCR, and the field strain was isolated in the MA104 cell line. The phylogenetic analyses based on the whole genome sequences and nucleotide sequences of open reading frame 5 (ORF5) gene showed close grouping with the subtype IV of lineage 8/8.7 of PRRSV II, which represents the HP-PRRSV strains that predominate in the pig population of China since 2010. The amino acid sequence analysis of the ORF5 gene revealed the replacement of leucine (L) at position 39 to isoleucine (I) in the primary neutralizing epitope. Among the four potential N glycosylation sites, the N34 was mutated and found to be restricted to only three N glycosylation sites. The present findings have indicated that HP-PRRSV can cause fatal outbreaks and may emerge as a major threat to the wild pig population.

Molecular detection and seroprevalence of classical swine fever virus from 2016 to 2018 in pigs of Mizoram, India

Classical swine fever (CSF) is a fatal endemic disease of pig population of North eastern India in particular and India in general. Present study revealed molecular detection of CSFV and seroprevalence of the disease in pig population of Mizoram, India during 2016–2018. Serum samples from apparently healthy, unvaccinated pigs were collected in collaboration with the State Animal Husbandry and Veterinary Department, Mizoram and a total of 594 serum samples from 7 districts were subjected to detection of CSFV specific antibodies by indirect ELISA. A total of 206 (34.68%) serum samples were positive for CSFV antibodies by ELISA. District wise, Saiha district showed highest seroprevalence of the disease followed by Kolasib and Serchhip. Apart from this, during the same time period, CSFV suspected samples received in the Department of Veterinary Microbiology consisting of 269 serum samples, 10 whole blood and 83 tissue samples obtained from 8 districts of Mizoram were subjected to detection of NS5b and E2 mRNA transcripts by nRT-PCR of which a total of 42 (11.60%) samples including serum (5.58%), tissues (27.71%) and whole blood (40%) were positive for the NS5b and E2 mRNA transcripts, specific for CSFV. District wise analysis revealed that Aizawl has the highest percentage of positive samples of CSFV followed by Saiha and Lawngtlai district.

Investigation of congenital tremor associated with Classical swine fever virus genotype 2.2 in an organized pig farm in north-eastern India

The present study describes an outbreak of Classical swine fever (CSF) in an organized pig farm followed by an episode of CSF virus (CSFV) associated congenital tremors in piglets. The outbreak was recorded in a newly procured herd of Hampshire pigs housed adjacent to the existing pigs of the farm. The recorded CSF outbreak caused a mortality of 100% in the newly procured and 54.28% in the existing herd. As the disease subsides, the clinically recovered boars were served naturally with Tamworth gilts. Though, the sows farrowed on usual gestation period, litters born to each sow showed congenital tremors and eventually died within 24 h of birth. Necropsy analysis of affected piglets was indicative of CSFV infection and was further confirmed using RT-PCR signifying a transplacental infection. The CSFV strains from the initial outbreak and post outbreak episode of congenital tremors were successfully isolated in PK-15 cells and detected in indirect FAT and RT-PCR. Phylogenetic analysis based on E2 gene and 5′NTR of CSFV grouped the isolates within the genotype 2.2 and revealed close resemblance with previously reported Indian isolates of CSFV genotype 2.2 origin. To the best of our knowledge, this is the first report of CSFV induced congenital form reported from India under natural conditions.

Adaptability of a field isolate of Classical swine fever virus to PK-15 cells

Classical swine fever (CSF) is an economically important pig disease affecting rural pig farming and vaccines are not freely available for control. In the present study an Indian isolate of CSF virus was passaged 75 times in PK-15 cells. At different passages, presence of virus was confirmed by Fluorescent Antibody Virus Neutralization (FAVN) test and Reverse Transcription-Polymerase Chain Reaction (RTPCR) for NS5B, E2 and 5-UTR genes. TCID50 titers were found to range between 4.00 and 8.97 at 10th and 75th passages respectively. Back passage and pathogenicity studies in susceptible pigs, the natural host, indicated that the virus was found to be attenuated following PK-15 passages and did not produce any clinical signs after 45th and 75th passages.

Prevalence and Genetic Diversity of Atypical Porcine Pestivirus (APPV) Detected in South Korean Wild Boars

Atypical porcine pestivirus (APPV), currently classified as pestivirus K, causes congenital tremor (CT) type A-II in piglets. Eighteen APPV strains were identified from 2297 South Korean wild boars captured in 2019. Phylogenetic analysis of the structural protein E2 and nonstructural proteins NS3 and Npro classified the APPV viruses, including reference strains, into Clades I, II and III. Clade I was divided into four subclades; however, the strains belonging to the four subclades differed slightly, depending on the tree analysis, the NS3, E2, and Npro genes. The maximum-likelihood method was assigned to South Korean wild boar APPV strains to various subclades within the three trees: subclades I.1 and I.2 in the E2 tree, subclade I.1 in the Npro tree, and subclades I.1 and I.4 in the NS3 ML tree. In conclusion, APPV among South Korean wild boars belonging to Clade I may be circulating at a higher level than among the South Korean domestic pig populations.

Identification of E2 with improved secretion and immunogenicity against CSFV in piglets

BACKGROUND

Outbreaks of Classical swine fever virus (CSFV) cause significant economic losses in the swine industry. Vaccination is the major method to prevent and control the disease. As live attenuated vaccines fail to elicit differentiable immunity between infected and vaccinated animals, subunit vaccine was considered as an alternative candidate to prevent and eradicate CSFV. Subunit vaccines present advantages in DIVA immunogenicity and safety. The technology was limited due to the low yield and the high cost with multiple and large doses. The native E2 signal peptide has not been well defined before. Here, the aim of this study is to develop a cost-effective and efficacious E2 vaccine candidate against CSFV with signal peptide and E2 sequence selection.

RESULTS

A novel CSFV E2 sequence (E2ZJ) was identified from an epidemic strain of Zhejiang for outstanding secretion in baculovirus and enhanced immunogenicity. E2 secretion induced with the selected signal peptide, SPZJ (SP23), increase at least 50% as compared to any other signal peptides tested. Besides, unique antigenic features were identified in E2ZJ. As indicated with immunized sera in IFA against CSFV infection, E2ZJ elicited CSFV antibodies at the earlier stage than other E2 types tested in mice. Moreover, higher level of neutralizing and CSFV antibodies against CSFV with E2ZJ was detected than other E2s with the same dosage at 28 dpi. Further, E2ZJ successfully elicited neutralizing immunity in piglets. A single dose of 5 μg of E2ZJ was sufficient to induce protective antibodies against CSFV in piglets and provided 100% protection against lethal virus challenge.

CONCLUSIONS

Our studies provide evidence that E2ZJ guided by a novel E2 signal peptide (SPZJ) was efficiently secreted and presented significantly improved immunogenicity than conventional E2 vaccines. Moreover, a single dose of 5 μg E2ZJ is efficacious against CSFV in piglets.

Prevalence and genome characteristics of atypical porcine pestivirus in southwest China

Atypical porcine pestivirus (APPV) causes congenital tremor (CT) in piglets and has a wide geographical distribution. In this study, we evaluated APPV prevalence using 165 piglet sera from southwest China. Viral RNA was detectable by qRT-PCR in 43.6 % (17/39, 95 % CI 27.8-60.4 %) of piglets with CT, while viral RNA was not detected in the sera of any healthy piglets. The seven complete APPV genomes were obtained from distinct farms and were 11 269-11 459 nucleotides in length. The genomes of the seven strains shared 82.8-98 % identity with the APPV reference strains. Phylogenetic analysis of the complete genomes as well as E2 and Nrpo sequences revealed that the seven APPVs clustered into two groups: four strains belonged to genogroups A and D and three strains belonged to a novel APPV genotype, tentatively called genogroup E. This study provides important insights into the epidemiological features and genetic diversity of APPV.

Genetic variation of highly pathogenic Indian porcine reproductive and respiratory syndrome viruses after introduction in 2013

To study its possible link to pathogenicity, the genomic variation in full ORF5 and ORF7 genes, and their encoded proteins in 26 field HP-PRRSV isolates from three major HP-PRRS outbreaks occurred in India, since 2013 was analysed. Sequence analysis and phylogenetic tree revealed involvement of genetically different strain in each outbreak of India rather persistence of a single strain. Analysis and comparison of N protein amino acid sequences of HP-PRRSV with VR2332 revealed consistent mutation at position 15D to N or K and 46 K to R in all the HP-PRRSV. GP5 protein showed consistent mutations at 29 positions from that of VR2332. The potential Nglycosylation sites in GP5 was found variable from 4–5 with one additional N-glycan moiety around the neutralizing epitope B. However, the ‘decoy’ epitope A was found highly conserved in all the HP-PRRSV.

Molecular characterization of classical swine fever virus isolates from India during 2012-14

Classical swine fever is a highly contagious and economically important viral disease of pigs. Outbreaks of classical swine fever virus (CSFV) were recorded in different places in the Kamrup district of Assam in India between the years 2012 and 2014. The nucleotide sequences of the 10 CSFV isolates were analyzed based on the partial nucleotide sequences of the E2, 5'NTR and NS5B genes. Phylogenetic analysis indicated the dominance of subgroup 2.2 along with 2.1 strains in the northeast part of India. Variation in the nucleotide sequences of E2, 5'NTR and 3'NS5B genes of CSFV allows tracking changes in the virus population over time. The study will provide epidemiological information useful for assessing CSFV circulating genogroups in India.

Phylogenetic analysis of Classical swine fever virus from archival formalin fixed clinical tissues reveals vietnamese origin of the isolates

Detection of Classical swine fever virus (CSFV) nucleic acid in archival formalin fixed tissue samples and their use for phylogenetic analysis was investigated. Ten samples were examined for the presence of CSFV nucleic acid by reverse transcriptase polymerase chain reaction (RT-PCR) amplification of 5'UTR and E2 gene. RT-PCR was found positive for 5'UTR fragment in eight samples while only one tissue samples showed amplification for E2 gene target fragment. For molecular epidemiology of the disease, 5'UTR PCR product of sample from Darbhanga (Bihar), was cloned and sequenced. The sequence was compared with the sequences available in database. The phylogenetic analysis reveals that the isolate belongs to subgroup 2.2 sharing 98.7% nucleotide identities with Vietnamese isolate (CaTh05-1, AB252170), indicating towards the possible origin of genogroup 2.2 CSFV isolates involved in the outbreak from Vietnam. From the study, it can be concluded that the tissue samples collected and stored in buffer formalin for years can be used to detect CSFV nucleic acid. Results are also suggestive of that the 5'UTR region of genome is more suitable target for RT-PCR based detection of CSFV in archival formalin fixed specimens. The study also indicates the potential of archival formalin fixed tissues for molecular epidemiology and genotyping of the CSF virus.

Changing pattern of classical swine fever virus genogroup from classical 1.1 to emerging 2.2 in India

Classical swine fever (CSF) is one of the most important viral diseases of pigs with high economic impact. The causative agent, Classical swine fever virus (CSFV) is a member of genus Pestivirus in family Flaviviredae and is structurally and antigenically related to other members of the genus. The identification of virus strains and genotypes can conveniently be used to trace the origin and patterns of virus spread, which contribut substantially in control strategies. In the present study, we have partially sequenced and analysed the 5' untranslated region (UTR) and E2 regions of CSFV clinical samples (n = 24) from various parts of the country. Among the samples, the sequence alignment of 5'UTR and E2 regions revealed 96.7-100 and 94.7-100% identities at the nucleotide level, respectively. The samples under study showed the close resemblance to the other CSFV isolates reported in India. In phylogenetic analysis, all the field samples were clustered in subgroup 2.2. Thus the study presents a further phylogenetic evidence for the emergence of subgroup 2.2 CSFV replacing the predominant subgroup 1.1 viruses in India. As the information regarding the molecular epidemiology the CSFV in india is very little, generation of such epidemiological data is warranted to help in comprehensing the nationwide disease control program to sustain the growth of pig industry in India.

Quasispecies composition and diversity do not reveal any predictors for chronic classical swine fever virus infection

Classical swine fever (CSF) can run acute, chronic, and prenatal courses in both domestic pigs and wild boar. Although chronic infections are rare events, their epidemiological impact is very high due to the long-term shedding of virus. So far, little is known about the factors that influence disease course and outcome from either the host or virus's perspective. To elucidate the viral determinants, we analyzed the role of the viral populations for the development of chronic CSF virus (CSFV) infections. Three different animal trials that had led to both chronic and acute infections were chosen for a detailed analysis by deep sequencing. The three inocula represented sub-genogroups 2.1 and 2.3, and two viruses were wild-type CSFV, one derived from an infectious cDNA clone. These viruses and samples derived from acutely and chronically infected animals were subjected to next-generation sequencing. Subsequently, the derived full-length genomes were compared at both the consensus and the quasispecies level. At consensus level, no differences were observed between the parental viruses and the viruses obtained from chronically infected animals. Despite a considerable level of variability at the quasispecies level, no indications were found for any predictive pattern with regard to the chronicity of the CSFV infections. While there might be no direct marker for chronicity, moderate virulence of some CSFV strains in itself seems to be a crucial prerequisite for the establishment of long-term infections which does not need further genetic adaption. Thus, general host and virus factors need further investigation.

Genetic variability and distribution of Classical swine fever virus

Classical swine fever is a highly contagious disease that affects domestic and wild pigs worldwide. The causative agent of the disease is Classical swine fever virus (CSFV), which belongs to the genus Pestivirus within the family Flaviviridae. On the genome level, CSFV can be divided into three genotypes with three to four sub-genotypes. Those genotypes can be assigned to distinct geographical regions. Knowledge about CSFV diversity and distribution is important for the understanding of disease dynamics and evolution, and can thus help to design optimized control strategies. For this reason, the geographical pattern of CSFV diversity and distribution are outlined in the presented review. Moreover, current knowledge with regard to genetic virulence markers or determinants and the role of the quasispecies composition is discussed.

Genetic Diversity and Positive Selection Analysis of Classical Swine Fever Virus Envelope Protein Gene E2 in East China under C-Strain Vaccination

Classical swine fever virus (CSFV) causes an economically important and highly contagious disease of pigs worldwide. C-strain vaccination is one of the most effective ways to contain this disease. Since 2014, sporadic CSF outbreaks have been occurring in some C-strain vaccinated provinces of China. To decipher the disease etiology, 25 CSFV E2 genes from 169 clinical samples were cloned and sequenced. Phylogenetic analyses revealed that all 25 isolates belonged to subgenotype 2.1. Twenty-three of the 25 isolates were clustered in a newly defined subgenotype, 2.1d, and shared some consistent molecular characteristics. To determine whether the complete E2 gene was under positive selection pressure, we used a site-by-site analysis to identify specific codons that underwent evolutionary selection, and seven positively selected codons were found. Three positively selected sites (amino acids 17, 34, and 72) were identified in antigenicity-relevant domains B/C of the amino-terminal half of the E2 protein. In addition, another positively selected site (amino acid 200) exhibited a polarity change from hydrophilic to hydrophobic, which may change the antigenicity and virulence of CSFV. The results indicate that the circulating CSFV strains in Shandong province were mostly clustered in subgenotype 2.1d. Moreover, the identification of these positively selected sites could help to reveal molecular determinants of virulence or pathogenesis, and to clarify the driving force of CSFV evolution in East China.

Evaluation of an Erns-based enzyme-linked immunosorbent assay to distinguish Classical swine fever virus-infected pigs from pigs vaccinated with CP7_E2alf

Infections with Classical swine fever virus (CSFV) are a major economic threat to pig production. To combat CSF outbreaks and to maintain trade, new marker vaccines were developed that allow differentiation of infected from vaccinated animals (DIVA principle). The chimeric pestivirus CP7_E2alf was shown to be safe and efficacious. Its DIVA strategy is based on the detection of CSFV E(rns)-specific antibodies that are only developed on infection. However, for the new marker vaccine to be considered a valuable control tool, a validated discriminatory assay is needed. One promising candidate is the already commercially available enzyme-linked immunosorbent assay, PrioCHECK CSFV E(rns) ELISA (Prionics BV, Lelystad, The Netherlands). Four laboratories of different European Union member states tested 530 serum samples and country-specific field sera from domestic pigs and wild boar. The ELISA displayed a good robustness. However, based on its reproducibility and repeatability, ranges rather than single values for diagnostic sensitivity and specificity were defined. The ELISA displayed a sensitivity of 90-98% with sera from CSFV-infected domestic pigs. A specificity of 89-96% was calculated with sera from domestic pigs vaccinated once with CP7_E2alf. The ELISA detected CSFV infections in vaccinated domestic pigs with a sensitivity of 82-94%. The sensitivity was lower with sera taken ≤21 days post-challenge indicating that the stage of CSFV infection had a considerable influence on testing. Taken together, the PrioCHECK CSFV E(rns) ELISA can be used for detection of CSFV infections in CP7_E2alf-vaccinated and nonvaccinated domestic pig populations, but should only be applied on a herd basis by testing a defined number of animals.

Transfection of RNA from organ samples of infected animals represents a highly sensitive method for virus detection and recovery of classical swine fever virus

Translation and replication of positive stranded RNA viruses are directly initiated in the cellular cytoplasm after uncoating of the viral genome. Accordingly, infectious virus can be generated by transfection of RNA genomes into susceptible cells. In the present study, efficiency of conventional virus isolation after inoculation of cells with infectious sample material was compared to virus recovery after transfection of total RNA derived from organ samples of pigs infected with Classical swine fever virus (CSFV). Compared to the conventional method of virus isolation applied in three different porcine cell lines used in routine diagnosis of CSF, RNA transfection showed a similar efficiency for virus rescue. For two samples, recovery of infectious virus was only possible by RNA transfection, but not by the classical approach of virus isolation. Therefore, RNA transfection represents a valuable alternative to conventional virus isolation in particular when virus isolation is not possible, sample material is not suitable for virus isolation or when infectious material is not available. To estimate the potential risk of RNA prepared from sample material for infection of pigs, five domestic pigs were oronasally inoculated with RNA that was tested positive for virus rescue after RNA transfection. This exposure did not result in viral infection or clinical disease of the animals. In consequence, shipment of CSFV RNA can be regarded as a safe alternative to transportation of infectious virus and thereby facilitates the exchange of virus isolates among authorized laboratories with appropriate containment facilities.

Pathology and molecular diagnosis of classical swine fever in Mizoram

AIM

Clinical histopathological and molecular diagnosis of classical swine fever disease in pigs of Mizoram.

MATERIALS AND METHODS

Totally, 31 clinically suspected pigs from 6 districts of Mizoram were examined, and clinical symptoms were recorded. Detailed post mortem examination of all the 31 dead animals was conducted, and gross changes were recorded. Tissue samples were collected for histopathological examination and molecular diagnosis. The collected tissues (tonsil, lymph nodes, spleen) were also processed for RNA extraction. Reverse transcription polymerase chain reaction (RT-PCR) was performed to detect the specific gene fragments of classical swine fever virus (CSFV).

RESULTS

Clinical examination of all the 31 suspected pigs revealed typical clinical signs of CSF. All the animals also showed typical gross and microscopic lesions of CSF. RT-PCR on tissue samples amplified the 421bp, 449bp and 735bp region of 5´NCR, non-structural protein 5B and E(rns) gene regions of CSFV, respectively. Nested PCR for internal region of E2 gene also amplified the expected product of 271bp using PCR product of whole E2 region as template DNA.

CONCLUSION

CSF is highly endemic disease in Mizoram. The viral strains circulating in this region are highly virulent. The disease can be diagnosed specifically using RT-PCR.

Classical Swine Fever in Wild Hog: Report of its Prevalence in Northeast India

Classical swine fever virus (CSFV) is the causative agent of a highly contagious disease, hog cholera in pigs. The disease is endemic in many parts of the world and vaccination is the only way to protect the animals from CSFV infection. Wild hogs belong to the species Sus Scrofa Cristatus under the family Suidae are quite susceptible to CSFV infection. The epidemiological role concerning classical swine fever (CSF) in India is largely unknown. We report here the three isolated cases of CSF in wild hogs from three National parks, namely Kaziranga National Park, Manas National Park and Jaldapara National Park, from north-east part of India. The post-mortem and histopathological findings were clearly indicative for CSFV infection. The presence of CSFV genome was demonstrated in several organs and tissues collected from hogs died due to viral infection. In addition, CSF-specific antibodies were detected in two wild hogs as well as in eighteen feral pigs from the same locations. The phylogenetic analysis of the partial E2 protein gene and 5' untranslated region of CSFV isolates from the wild hog showed identities with genotype 2.2 of the Indian isolates. Occurrence of CSF in wild hogs may pose a potent threat in the epidemiology of the virus in Northeast part of India. To the best of our knowledge, the report presented in the manuscript is the first comprehensive report on CSF in wild hogs form Northeast India. The findings reported would help us to understand the epidemiology and biology of CSFV in wild animals.

Predominance of genotype 1.1 and emergence of genotype 2.2 classical swine fever viruses in north-eastern region of India

Classical swine fever (CSF) is a highly contagious and the most important disease of pigs worldwide.CSF is enzootic in pig herds in India and continues to cause huge economic losses to pig farmers. Nearly 40% of the total pig population of India is present in the north-eastern (NE) states where pig husbandry plays an important role in the socio-economic development. Pigs reared in the backyards are the only source of livelihood for a majority of poor tribal population in the region. Hardly any CSF vaccination is currently being undertaken in the unorganized pig farming in the NE region due to economic reasons and vaccine unavailability. A thorough understanding of the current epidemiological status of CSF is essential for the effective control of the disease in the NE region. Hence, we carried out molecular characterization of CSFV isolates from field outbreaks during 2011-2012 in the entire north-eastern region of India to establish the genetic groups of prevalent CSF viruses in the region. A total of 17 CSFV isolates obtained from different parts of the NE region were characterized by comparing the sequences of three partial genomic regions of the virus, that is 150 nt of 5' UTR, 190 nt of E2 and 409 nt of NS5B. Of the 17 CSFV isolates, 15 isolates belonged to 1.1 (88.2%) and two isolates (11.8%) belonged to 2.2 subgenogroup. The genogroup 2.2 CSFV were associated with outbreaks in Arunachal Pradesh that shares international borders with Bhutan, Myanmar and China. Genogroup 2.2 CSFV isolated in the present study shared high level of sequence similarity with 2.2 viruses form China, raising the possibility of virus incursion from this region. In summary, we found a continued predominance of 1.1 subgroup and an emergence of 2.2 subgroup CSFV in NE region of India.

The CSFV DNAChip: a novel diagnostic assay for classical swine fever virus

A novel assay, the CSFV DNAChip, was developed to clearly and rapidly discriminate three genotypes of classical swine fever virus (CSFV). Total RNA was extracted from clinical samples and then subjected to a one-step reverse-transcription polymerase chain reaction (RT-PCR) using Cy3-labeled primers from the 5' non-coding region (NCR) of CSFV. Amplicons were hybridized to the CSFV DNAChip and fluorescence scanning was performed for detection of CSFV. A cut-off fluorescence intensity value of 5000 was determined by two-graph receiver operating curve (TG-ROC) analysis. The limit of detection values for the developed DNA chip assay were 0.313ng/μL for amplicon concentration and 1TCID50/100μL for virus titer. Using the developed DNA chip, 157 field samples (91 CSFV-positive and 66 CSFV-negative) were investigated. The genotypes determined by the CSFV DNAChip agreed completely with those determined by nucleotide sequence analysis of the viral genome. The developed CSFV DNAChip will be helpful in implementing a CSFV eradication strategy, as it provides a rapid and accurate diagnostic assay that can discriminate easily among CSFV genotypes.

Improved strategy for phylogenetic analysis of classical swine fever virus based on full-length E2 encoding sequences

Molecular epidemiology has proven to be an essential tool in the control of classical swine fever (CSF) and its use has significantly increased during the past two decades. Phylogenetic analysis is a prerequisite for virus tracing and thus allows implementing more effective control measures. So far, fragments of the 5´NTR (150 nucleotides, nt) and the E2 gene (190 nt) have frequently been used for phylogenetic analyses. The short sequence lengths represent a limiting factor for differentiation of closely related isolates and also for confidence levels of proposed CSFV groups and subgroups. In this study, we used a set of 33 CSFV isolates in order to determine the nucleotide sequences of a 3508-3510 nt region within the 5´ terminal third of the viral genome. Including 22 additional sequences from GenBank database different regions of the genome, comprising the formerly used short 5´NTR and E2 fragments as well as the genomic regions encoding the individual viral proteins Npro, C, Erns, E1, and E2, were compared with respect to variability and suitability for phylogenetic analysis. Full-length E2 encoding sequences (1119 nt) proved to be most suitable for reliable and statistically significant phylogeny and analyses revealed results as good as obtained with the much longer entire 5´NTR-E2 sequences. This strategy is therefore recommended by the EU and OIE Reference Laboratory for CSF as it provides a solid and improved basis for CSFV molecular epidemiology. Finally, the power of this method is illustrated by the phylogenetic analysis of closely related CSFV isolates from a recent outbreak in Lithuania.

Disease severity declines over time after a wild boar population has been affected by classical swine fever--legend or actual epidemiological process?

Classical swine fever (CSF) is a severe multi-systemic disease that can affect both domestic pigs and wild boar. Past outbreaks in European wild boar involved high-virulent CSF virus (CSFV) strains and were mostly self-limiting. In these cases, morbidity and mortality rates were high in the affected regions. In contrast, endemic infections have been observed in several European wild boar populations in recent decades. Morbidity and mortality rates were much lower despite the fact that outbreaks were still detected via diseased or fallen animals. The virus strains involved were mostly classified as genotype 2.3 strains of moderate virulence causing age-dependent disease outcomes. The mechanisms leading to the establishment and perpetuation of endemicity are still not fully understood, but the factor "moderate virulence" seems to be of considerable importance. In this study, we aim to clarify whether the perception of declined 'CSF severity' could hypothetically reflect the adaptation of an initially high-virulent virus or whether this might be better explained as a misinterpretation of observations. A mechanistic eco-epidemiological model was employed to follow up a highly virulent strain of CSFV introduced into large connected wild boar populations. In the model, the virulence of the CSF virus is represented by case mortality and life expectancy after lethal infection. Allowing for small stochastic variation, these two characteristics of the virus are passed on with every new simulated infection that occurs. Model analysis revealed a decrease from high to moderate case mortality within a few years of simulated perpetuation of the virus. The resulting mortality corresponded to the level where the population average of the infectious period and the basic reproduction number of the disease were maximal. This shift in virulence was sufficient to prolong virus circulation considerably beyond the epidemic phase of the simulated outbreaks. Alternative mechanistic explanations for the decrease in disease severity in a CSF-affected wild boar population were evaluated in the light of the simulation experiments and the available epidemiological or virological evidence. In conclusion, the current virus isolates of subgroup 2.3 might be the ideally adapted variants of the CSF virus for long-term perpetuation in wildlife and indeed may have evolved (once) during past outbreaks in large populations. A repeated perception of a declining severity of disease pattern during the course of a CSF outbreak, however, favours the explanation based on monitoring and detection biases rather than repeated observation of selection against highly virulent virus during the time of virus perpetuation.

Clustering of classical swine fever virus isolates by codon pair bias

Background

The genetic code consists of non-random usage of synonymous codons for the same amino acids, termed codon bias or codon usage. Codon juxtaposition is also non-random, referred to as codon context bias or codon pair bias. The codon and codon pair bias vary among different organisms, as well as with viruses. Reasons for these differences are not completely understood. For classical swine fever virus (CSFV), it was suggested that the synonymous codon usage does not significantly influence virulence, but the relationship between variations in codon pair usage and CSFV virulence is unknown. Virulence can be related to the fitness of a virus: Differences in codon pair usage influence genome translation efficiency, which may in turn relate to the fitness of a virus. Accordingly, the potential of the codon pair bias for clustering CSFV isolates into classes of different virulence was investigated.

Results

The complete genomic sequences encoding the viral polyprotein of 52 different CSFV isolates were analyzed. This included 49 sequences from the GenBank database (NCBI) and three newly sequenced genomes. The codon usage did not differ among isolates of different virulence or genotype. In contrast, a clustering of isolates based on their codon pair bias was observed, clearly discriminating highly virulent isolates and vaccine strains on one side from moderately virulent strains on the other side. However, phylogenetic trees based on the codon pair bias and on the primary nucleotide sequence resulted in a very similar genotype distribution.

Conclusion

Clustering of CSFV genomes based on their codon pair bias correlate with the genotype rather than with the virulence of the isolates.

Molecular Characterization of Classical swine fever virus Involved in the Outbreak in Mizoram

Classical swine fever is the most insidious and devastating disease of pigs and wild boars. The virus is closely related to the other members of the genus Pestivirus. The outbreak recorded in Mizoram, India was strategically important as the state shares porous international boundary with East Asian countries. Both immunodiagnostic and molecular techniques were used to confirm the involvement of Classical swine fever virus (CSFV) in this outbreak. Sandwich ELISA and direct FAT could detect CSFV in the tissue samples. RT-nPCR specifically amplified E2 and 5'NTR product of 271 bp. Phylogenetic analysis showed, that the Mizoram isolate (MZ4/69) was very close to the Chinese strain Shimen-HVRI (93.0%) rather than other Indian isolate (CSF-30-03). Present study provides a valuable sequence based molecular data on Indian isolate of CSFV and will be useful in investigation on transmission of such disease from neighbour countries.

Molecular epidemiology of current classical swine fever virus isolates of wild boar in Germany

Classical swine fever (CSF) has caused significant economic losses in industrialized pig production, and is still present in some European countries. Recent CSF outbreaks in Europe were mainly associated with strains of genogroup 2 (subgroup 2.3). Although there are extensive datasets regarding 2.3 strains, there is very little information available on longer fragments or whole classical swine fever virus (CSFV) genomes. Furthermore, there are no detailed analyses of the molecular epidemiology of CSFV wild boar isolates available. Nevertheless, complete genome sequences are supportive in phylogenetic analyses, especially in affected wild boar populations. Here, German CSFV strains of subgroup 2.3 were fully sequenced using two different approaches: (i) a universal panel of CSFV primers that were developed to amplify the complete genome in overlapping fragments for chain-terminator sequencing; and (ii) generation of a single full-length amplicon of the CSFV genome obtained by long-range RT-PCR for deep sequencing with next-generation sequencing technology. In total, five different strains of CSFV subgroup 2.3 were completely sequenced using these newly developed protocols. The approach was used to study virus spread and evolutionary history in German wild boar. For the first time, the results of our study clearly argue for the possibility of a long-term persistence of genotype 2.3 CSFV strains in affected regions at an almost undetectable level, even after long-term oral vaccination campaigns with intensive monitoring. Hence, regional persistence in wild boar populations has to be taken into account as an important factor in the continual outbreaks in affected areas.

Genetic typing of recent classical swine fever isolates from India

Seventeen classical swine fever virus (CSFV) isolates recovered during the period of 3 years (2006-2008) from India were subjected to nucleotide sequencing in the 5' untranslated region (UTR). For genetic typing, 150 nucleotides within this region were used. For better epizootiological understanding, 39 nucleotide sequences of the above region, including 13 Indian CSFV sequences, available either in the Genbank or published literature were also included in the study. Based on the phylogenetic analysis, the Indian isolates could be grouped in to two subgroups, viz., 1.1 and 2.2. The study also revealed predominance of subgroup 1.1 and involvement of viruses of more than one subgroup in an outbreak.

Feed contaminated with classical swine fever vaccine virus (LOM strain) can induce antibodies to the virus in pigs

In November 2004, antibodies to classical swine fever virus (csfv) were detected in finishing pigs during the annual serological surveillance in Jeju Province, Korea. In addition, csf vaccine viruses (lom strain) had recently been isolated from pigs raised on farms known to have csfv antibody-positive pigs. In contrast with mainland Korea, Jeju Province had been csf free and its pigs had not been vaccinated against csf for more than five years. An epidemiological investigation team from the National Veterinary Research and Quarantine Service investigated the current status of csf prevention on the Korean mainland and in Jeju Province to determine possible routes of introduction of the virus into the province. It was concluded that improperly processed blood meals, manufactured on mainland Korea, had been contaminated with the csf vaccine lom strain, and that the lom strain had been transmitted to pigs fed feed or feedstuffs containing the contaminated meal.

Efficacy of intradermally administrated E2 subunit vaccines in reducing horizontal transmission of classical swine fever virus

To investigate if intradermal (ID) vaccination and intramuscular (IM) vaccination result in a comparable reduction of horizontal transmission of classical swine fever virus (CSFV), two registered E2 subunit marker vaccines were examined. Vaccine A was a water-in-oil emulsion containing the E2 glycoprotein originating from the Alfort/Tübingen strain and vaccine B was a water-oil-water emulsion containing the E2 glycoprotein originating from the Brescia strain. Eight groups, of ten pigs each, were vaccinated with either vaccine A or B, intramuscularly (IM) or intradermally (ID). Two different vaccination-challenge intervals were used for each vaccine. Furthermore, one group was vaccinated with a tenfold ID dose of vaccine A and one non-vaccinated group served as a control group. Five pigs from each group were challenged with the moderately virulent CSFV strain Paderborn, while the remaining five pigs served as contacts. Using vaccine A, full transmission to all contact pigs in both ID vaccinated groups occurred. No virus transmission was observed when IM vaccinated pigs were challenged 14 days post-vaccination (14dpv) whereas only one out of five contact pig became infected when they were challenged 10dpv. Using vaccine B no virus transmission was observed when pigs were ID or IM vaccinated and challenged 10dpv. When challenged 3dpv full transmission occurred in the ID vaccinated group, whereas four out of five contact pigs became infected in the IM vaccinated group. This result indicates that ID vaccination does not result in better protection against horizontal CSFV transmission compared to IM vaccination, for the vaccines studied.

Diagnostic methods for detection of Classical swine fever virus—Status quo and new developments

Classical swine fever (CSF) is a highly contagious disease causing major losses in pig populations almost worldwide. The disease occurs in many regions of Asia, Central and South America and parts of Europe and Africa. Some countries have eradicated the disease (Australia, USA, Canada, within the EU), yet it keeps recurring sporadically (South Africa, Germany, Netherlands, England). The causative virus is a member of the genus Pestivirus, family Flaviviridae. The first diagnosis of CSF is based on the recognition of clinical signs by the veterinarian in the field and by post mortem findings. Many signs are not exclusively associated with CSF and they may vary with the strain of virus, age and health status of the pigs. Since clinical signs may be confused with other pig diseases, laboratory diagnosis of CSF is indispensable. Both the Office International des Epizooties (OIE) and the European Union, have approved diagnostic manuals establishing sampling methods and diagnostic procedures for the confirmation of the disease. In this review, experiences with current tests will be analyzed and complemented with new developments, with emphasis on the polymerase chain reaction after reverse transcription of the RNA genome (RT-PCR).

Phylogenetic characterization of classical swine fever viruses isolated in Korea between 1988 and 2003

Twenty-four isolates of classical swine fever (CSF) virus which were obtained from CSF outbreaks during 1988 and 2003 in the Republic of Korea were genetically characterized for partial E2 gene (190 nucleotides) and compared with CSF viruses reported by other countries. Phylogenetic analyses classified Korean field isolates between1988 and 1999 into subgroup 3.2, forming an independent clade distinct from CSF viruses identified in other countries. In contrast, the viruses isolated during 2002-2003 CSF epidemics were classified into a different subgroup (2.1). The 2.1 viruses showed a close genetic relationship (92.1-100% nucleotide similarity) with CSF viruses reported from China and Taiwan in 1998-2001. As no evidence of CSF virus infection was detected in the wild boar (Sus scrofa coreanus) population that inhabits Korea, the results of molecular characterization strongly suggest that CSF epidemic outbreaks in Korean swine populations during 2002-2003 were attributed to the introduction of a new strain or strains, likely from neighboring countries.

Epidemiological survey of viral diseases of pigs in the Mekong delta of Vietnam between 1999 and 2003

In the Mekong delta, backyard pig rearing plays an integral role in recycling nutrients in farming systems and generating valuable cash income. However, development has been hampered by fatal epizootics of piglets and reproductive failure of sows. Diseases are named by symptoms and blindly treated with antibiotics. As antibiotics are often ineffectual, involvement of viral diseases are suspected. To identify the causative agent, we first sero-surveyed porcine reproductive and respiratory syndrome (PRRS) and pseudorabies with 478 sera from non-vaccinated pigs collected from backyard farms, state farms and slaughterhouses in Can Tho province between 1999 and 2002. Antibodies for PRRS were first detected in 2002 in backyard farms and at high prevalence in state farms with increased piglet mortality. A few backyard breeder pigs had antibodies for pseudorabies in 2000 and 2002. With compulsory classical swine fever (CSF) vaccination, we examined the relationship between vaccination and antibodies in 70 serum samples. Seventy-nine percent of vaccinated breeders had CSF antibodies-higher than expected with irregular vaccination. Since circulation of CSF virus was suspected, isolation was attempted at 10 farms with fatal epizootics between 2002 and 2003. The viruses were detected at all farms and clustered within genogroup 2, despite vaccines corresponding to genogroup 1. This study demonstrated virologically/serologically the existence of PRRS, pseudorabies and CSF viruses in the Mekong delta of Vietnam. We also identified CSF as a cause of piglet mortality that disastrously affected backyard farming. Vaccine standardization and proper instructions are needed to simplify diagnosis and complement established simultaneous vaccination of sows with piglets.

Phylogenetic analysis of recent isolates of classical swine fever virus from Colombia

The ability to discriminate between different classical Swine fever virus (CSFV) isolates is a prerequisite for identifying the possible origin of an outbreak. To determine the relatedness between Colombian isolates from different geographical regions, genetic sequences of the glycoprotein E2 and the 5'UTR of CSFV were amplified by PCR, sequenced and compared with reference strains of different genetic grouping. The viruses originated from classical swine fever (CSF) outbreaks in Colombia during 1998-2002. All viruses characterized belonged to genogroup 1 and were members of the subgroup 1.1. The results indicate that the outbreaks from the year 2002 are caused by a strain related to the virus CSF/Santander, isolated in 1980, suggesting that the current CSF outbreaks are the consequence of a single strain that continues to circulate in the field. For the first time, an association between isolates from outbreaks in Colombia in the 1990s was established with a virus isolate from Brazil, indicating a possible origin of the virus causing the outbreak.

Origin and evolution of viruses causing classical swine fever in Cuba

We have analyzed the origin and evolution of viruses from the classical swine fever (CSF) epidemic that affects Cuba since 2001 by nucleotide sequencing of regions within the E2 glycoprotein and the NS5B (polymerase) genes. The sequence of 190 nucleotides from E2 gene was determined for 10 CSF viruses isolated at different locations of the island, and used for phylogenetic analyses, including sequences from viruses of the 1993--1997 epizootic, previously determined, as well as those from representatives of the different CSFV genotypes. The phylogenetic tree obtained indicates that viruses circulating at present belong to the subgroup 1.2 and are closely related to those isolated during the 1993--1997 epizootic, including the strain Margarita used for vaccine potency tests in Cuba. However, the pattern of evolution revealed by these analyses was different than that observed previously, in which western isolates were almost identical to Margarita strain, while eastern isolates showed a higher level of genetic diversification. In this case, all the viruses analyzed grouped in an independent, define cluster that is closely related, albeit distinguishable, from that of Margarita-related viruses that previously circulated in the western part of Cuba. In addition, the 2001--2003 viruses showed a branched pattern with a level of sequence diversification similar to that observed in the eastern 1993--1997 viruses. Interestingly, a significant fraction (about 54%) of the mutations found in the E2 sequence led to amino acid replacements. This high rate of non-synonymous mutations was not found in the previous Cuban epizootic and has not been reported for other CSF outbreaks. In spite of these amino acid replacements, no antigenic changes were observed in the reactivity of different isolates with CSFV-specific MAbs and polyclonal sera. The phylogenetic tree derived from 409 nucleotides of NS5B gene of seven isolates and Margarita strain, was consistent with that obtained from E2 sequences. In this region, encoding a non-structural protein, a low level of fixation of non-synonymous mutations was observed. The results obtained suggests that epidemiological factors affecting CSFV spread during the current epizootic in Cuba can favour the fixation of non-synonymous mutation in the E2 gene, which could be associated with a lower severity in the clinical signs developed by most of the affected animals.

Phylogenetic analysis of classical swine fever virus isolated from Taiwan

By analyzing the E2 sequences of classical swine fever virus from field outbreaks in Taiwan during 1993-2001, three virus populations with distinct genotypes were determined including one historical (subgroup 3.4) and two exotic (subgroup 2.1) strains. The first subgroup 2.1 virus was isolated in 1994 and further sporadic outbreaks occurred after 1996. Phylogenetic analysis using the E2 region has segregated the Taiwanese strains of 2.1 virus into two different genotypes (termed 2.1a and 2.1b). The 2.1b viruses were only isolated in 2001 and shared approximately 94.8% nucleotide identities to the 2.1a viruses in the total genomic sequences. The results suggest that the 2.1a and 2.1b viruses may be introduced from different origins.

Genetic typing of recent classical swine fever virus isolates from Croatia

During a period of 5 years (1997-2001) several outbreaks of classical swine fever (CSF) were recorded in Croatia. For genetic typing, fragments of 150 nucleotides within the 5'-non-translated region (5'-NTR) and 190 nucleotides within the E2 glycoprotein coding gene of nine field isolates that were derived from domestic pigs and wild boars were used. For better epizootiological understanding, isolates from other European countries were included in the study. The results show that the isolates belong to subgroups 2.1 and 2.3 of CSF virus. Isolates from subgroup 2.1 were collected from domestic pigs during sporadic outbreaks in June 1997 and are genetically closely related. A genomic similarity between these isolates and CSF virus isolates from pigs in other European countries from the same year could also be confirmed. In contrast, the isolate from October 1997 was found to be a member of subgroup 2.3, and is closely related to European CSF virus isolates from outbreaks in the last decade in Western and Central European countries. These results show that two different sources of CSF virus caused outbreaks in Croatia during the same year. Furthermore, a close relationship was found between an isolate from a domestic pig in 1999 and isolates of subgroup 2.3 that originated from Croatian wild boars.

Molecular epidemiology of classical swine fever in the Russian Federation

The ability to discriminate between various classical swine fever virus (CSFV) strains and isolates is a prerequisite for following the spread of the virus after an outbreak. To determine the relatedness between Russian CSFV isolates from different geographical regions, three fragments of the viral genome (5' NTR, the variable region of the E2 gene and a fragment of the NS5B gene) were sequenced and used for genetic typing. Thirty-one field isolates were obtained from CSF outbreaks which occurred between 1994 and 1999. In addition, three attenuated strains were included in the study, namely the LK and CS vaccine strains, and the moderately virulent 238H isolate. The vaccine strains have been used in Russia for more than 30 years. Our results showed that all field isolates are in subgroup 1.1 together with Alfort 187 and with the highly virulent strain Shimen. In contrast, the CS and LK vaccine strains belong to subgroup 1.2. While there is no evidence for the reversion of the two vaccine strains to wild type, it is feasible that the highly virulent Shimen strain, which has been used as a challenge strain for many years, contributed to field strain generation. The Russian field isolates from the 1990s can be distinguished from the CSF virus isolates which occurred in the EU Member States in the same decade, as here all outbreaks were caused by CSF viruses belonging to subgroup 2.

Construction of an infectious chimeric classical swine fever virus containing the 5′UTR of bovine viral diarrhea virus, and its application as a universal internal positive control in real-time RT-PCR

RT-PCR is used widely as a diagnostic method to detect and differentiate pestiviruses. The construction of two chimeric classical swine fever virus (CSFV) recombinants based on a marker virus constructed previously [J. Virol. 72 (1998) 5318-5322] is described. These viruses, termed vA187CAT_5UTRBVD and vA187CAT_IRESBVD, contain the entire 5' untranslated region (5'UTR) or the internal ribosome entry site (IRES) of bovine viral diarrhea virus (BVDV), respectively. Both chimeric viruses proved to be infectious in cell culture. Hence, the 5'UTR as well as the IRES element only of BVDV can substitute for the corresponding genome region of CSFV. Next, two sets of primers and corresponding dual-labeled TaqMan probes were designed; one detecting specifically a conserved but CSFV-specific area within the 5'UTR of wild-type CSFV, the other one targeting the CAT gene inserted in vA187CAT_5UTRBVD. The two primer/probe sets were combined in a closed-tube multiplex one-step RT-PCR. To monitor the entire extraction and detection process limited amounts of vA187CAT_5UTRBVD were added directly to clinical samples before RNA extraction. The multiplex RT-PCR proved to be as sensitive as the single primer/probe set method, but allowed the validation of each sample tested individually, based on the detection of the CAT marker gene. vA187CAT_5UTRBVD was also used successfully for foot-and-mouth disease virus (FMDV) TaqMan RT-PCR. Therefore, it is considered a universal internal positive control for RT-PCR assays to exclude loss of RNA during extraction, or failure of amplification due to inhibitory substances present in the sample.

Virulence of recent and former classical swine fever virus isolates evaluated by their clinical and pathological signs

The clinical diagnosis of classical swine fever (CSF) still caused problems to the veterinarians during the last decade. The primary CSF outbreak was often detected too late and, meanwhile, the virus had spread. Consequently, the recent classical swine fever virus isolates (CSFV) were suspected to be of low virulence. The purpose of the study was to quantify the virulence of four recent CSFV by evaluating the clinical and pathological signs caused by different CSFV. Pigs of the same breed and age group were inoculated intranasally with CSFV from recent epidemics in European Union (EU) member states. The CSFV used are registered in the data base of the EU Reference Laboratory for CSF and belong to different genotypes: 2.1, 2.2 and 2.3 respectively. Clinical signs of CSF were evaluated by using a score system suggested previously (Mittelholzer et al., 2000: Vet. Microbiol. 74, 293). For the evaluation of pathological lesions, a new pathological score was introduced. The four CSFV tested here were classified as moderately virulent in general, although one CSFV may cause different clinical courses, ranging from highly virulent to avirulent. This indicates the importance of additional factors in the host animal for virulence. Differences in the clinical and pathological signs between these four recent CSFV were rather minor, emphasizing that the genetic typing of CSFV is absolutely essential. Differences towards former CSFV (e.g. reference virus strain Alfort 187) were more pronounced, especially regarding the onset and duration of the disease, the occurrence of skin haemorrhages and pathological lesions of kidney, subcutis and serosae. It is concluded that clinical diagnosis of CSF is rather difficult in pigs up to 14 days post-CSFV infection using these four CSFV, emphasizing the need for careful differential diagnosis and the laboratory investigation for CSF at an early stage.

Clinical signs and epidemiology of classical swine fever: a review of new knowledge

Although classical swine fever (CSF) has been well known for decades and epidemics still occur, clinical diagnosis continues to cause problems for veterinary practitioners. This is due to the extensive differential diagnosis, further complicated by the emergence of new diseases such as porcine reproductive and respiratory syndrome (PRRS) and porcine dermatitis and nephropathy syndrome (PDNS). In addition, acute, chronic and prenatal courses of CSF have to be distinguished. As a cause of considerable economical losses within the EU, control of CSF requires knowledge of the primary outbreaks and spread of the disease. Genetic typing of CSF virus isolates has proved to be a potent method of supporting epidemiological investigations. Phylogenetic analysis of CSF virus strains and isolates originating from different continents has allowed three genetic groups and several subgroups within these groups to be distinguished. Whereas isolates belonging to group 3 seem to occur solely in Asia, all CSF virus isolates of the 1990s isolated in the EU belonged to one of the subgroups within group 2 (2.1, 2.2, or 2.3) and were clearly distinct from former CSF reference viruses, which belong to group 1. Within the EU, different strategies are followed for the eradication of CSF in domestic pigs and in wild boar. While a strict non-vaccination policy is followed for domestic pigs, eradication of the disease in wild boar is more complex, and oral immunisation together with special hunting strategies have been applied. Recently, marker vaccines with a companion discriminatory test designed to allow differentiation between vaccinated animals and animals having recovered from field virus infection have been developed. Preliminary studies indicated that the discriminatory tests had a reduced sensitivity and specificity. Further improvements are therefore necessary before marker vaccines can be considered for emergency use in EU Member States. Prevention of CSF remains the main objective within the EU.

Swine fever: classical swine fever and African swine fever

Because of the clinical and pathologic similarity to common endemic diseases, introduction of CSFV or ASFV strains of moderate to low virulence represents the greatest risk to North American swine herds. Producers, veterinarians, and diagnosticians should increase their awareness of these devastating diseases and request specific diagnostic testing whenever they are suspected. Production practices that improve biosecurity will reduce the risk of introduction of CSF and ASF and limit the spread if an incursion occurs. Additional resources. The following Web sites contain excellent color photographs that will assist producers and practitioners in identifying clinical signs and gross lesions associated with CSFV and ASFV: http://www.vet.uga.edu/vpp/gray_book/FAD and http://www.pighealth.com. The latter Web site and the OIE Web site (http://www.oie.int) offer updated information on current worldwide epizootics of ASF and CSF and other swine diseases. Details of biosecurity procedures can be found at http://www.agebb.missouri.edu; see publication G2340.

Classical swine fever (CSF) marker vaccine. Trial I. Challenge studies in weaner pigs

Two commercial marker vaccines against classical swine fever virus (CSFV) and companion diagnostic tests were examined in 160 conventional pigs. To test the vaccines in a "worst case scenario", group of 10 weaners were vaccinated using a single dose of an E2 (gp55) based vaccine at days -21, -14, -10 or -7, and subsequently challenged at day 0. The challenge virus was CSFV 277, originating from a recent outbreak of classical swine fever (CSF) in Germany. In all groups, only 5 out of 10 pigs were challenged; the remaining 5 pigs served as vaccinated contact controls. Also, three control groups, each consisting of 10 non-vaccinated pigs, were challenged in parallel to the vaccinated animals. CSFV could be isolated from all non-vaccinated pigs. Among these pigs 40% displayed a chronic course of the infection (virus positive for more than 10 days). Pigs vaccinated 21 or 14 days before challenge displayed no clinical signs of CSFV after challenge. However, they were still able to replicate CSFV when challenged, as measured by reisolation of CSFV from leukocytes of the directly challenged pigs. CSFV could be isolated from the leucocytes of 25% of the pigs vaccinated 21 days before challenge and 50% of the pigs vaccinated 14 days before challenge. Chronic infection was not observed, but transmission to one vaccinated contact pig occurred. From all pigs vaccinated 10 or 7 days before challenge, CSFV could be reisolated. We observed a chronic course of infection in 5% of pigs vaccinated 10 days before challenge and in 30% of pigs vaccinated 7 days before challenge. The mortality rate was 20% in the pigs vaccinated 10 days before challenge, and varied between 20 and 80% in pigs vaccinated 7 days prior to challenge. The contact animals had lower mortality (0-20%) than directly challenged pigs, probably mirroring the delayed time point of infection. There was thus some protection against clinical illness by both marker vaccines, but not a solid protection against infection and virus shedding. The efficacy of the vaccine was best if used 3 weeks before challenge and a clear correlation between time interval from vaccination to challenge and the level of virus shedding was observed. Each vaccine had its own accompanying discriminatory ELISA, but 18% of the virus positive pigs never seroconverted in these tests.

Classical swine fever (CSF) marker vaccine. Trial III. Evaluation of discriminatory ELISAs

The objective of the marker vaccine trial was to test the two available CSF marker vaccines in scenarios which are likely to occur in the field and to evaluate the reliability of the discriminatory tests. The evaluation of the discriminatory tests was of special importance because there is no requirement for formal data concerning their performance by the European Medicinal Products Evaluation Agency (EMEA) in London. EMEA is responsible for the licensing procedure of the marker vaccines within the EU. Sixteen National Swine Fever Laboratories (NSFL) participated in testing the discriminatory ELISAs. They were tested for sensitivity, specificity, reproducibility and practicability. Reference sera (CSFV and BVDV antibody positive) and field sera were used as well as sera from the weaner and sow experiments produced during the marker vaccine trial. Both discriminatory ELISAs were less sensitive than conventional CSF antibody ELISAs, although there was considerable variation between them. One discriminatory ELISA was less specific than the other, but more sensitive, and vice versa. Neither discriminatory ELISA consistently detected the marker-vaccinated, CSF-challenged weaner pigs correctly as 'CSF positive', although CSF-challenged pregnant sows were identified correctly. The limitations of the discriminatory ELISAs used in the trial was the major factor that would prevent the use of these two marker vaccines under emergency field conditions.