The Over-40-Years-Epidemic of Infectious Bursal Disease Virus in China

Infectious bursal disease (IBD) is an acute, highly contagious, immunosuppressive disease of chickens caused by the virus (IBDV), which critically threatens the development of the global chicken industry and causes huge economic losses. As a large country in the poultry industry, the epidemic history of IBDV in China for more than 40 years has been briefly discussed and summarized for the first time in this report. The first classic strain of IBDV appeared in China in the late 1970s. In the late 1980s and early 1990s, the very virulent IBDV (vvIBDV) rapidly swept across the entirety of China, threatening the healthy development of the poultry industry for more than 30 years. Variants of IBDV, after long-term latent circulation with the accumulation of mutations since the early 1990s, suddenly reappeared as novel variant strains (nVarIBDV) in China in the mid-2010s. Currently, there is a coexistence of various IBDV genotypes; the newly emerging nVarIBDV of A2dB1 and persistently circulating vvIBDV of A3B3 are the two predominant epidemic strains endangering the poultry industry. Continuous epidemiological testing and the development of new prevention and control agents are important and require more attention. This report is of great significance to scientific cognition and the comprehensive prevention and control of the IBDV epidemic.

Pathogenicity and molecular characterization of infectious bursal disease virus in China

Infectious bursal disease virus (IBDV) caused an acute and highly contagious infectious disease, resulting in considerable economic losses in the world poultry industry. Although this disease was well-controlled under the widely use of commercial vaccines, the novel variant IBDV strain emerged due to the highly immunized-selection pressure in the field, posting new threats to poultry industry. Here, we reported the epidemic and pathogenicity of IBDV in Hubei Province from May to August 2020. We isolated 12 IBDV strains from the broiler flocks, including 9 novel variants, 2 very virulent strains and 1 medium virulent strain. Interestingly, we identified a series of changes of amino acid sites in the VP2. Further analysis indicated that the novel variant IBDV strains caused damage to bursa of fabricius and spleen, leading to immunosuppression. Our findings underscore the importance of IBDV surveillance, and provide evidence for understanding the evolution of IBDV.

Relevant SARS-CoV-2 Genome Variation through Six Months of Worldwide Monitoring

Real-time genome monitoring of the SARS-CoV-2 pandemic outbreak is of utmost importance for designing diagnostic tools, guiding antiviral treatment and vaccination strategies. In this study, we present an accurate method for temporal and geographical comparison of mutational events based on GISAID database genome sequencing. Among 42523 SARS-CoV-2 genomes analyzed, we found 23202 variants compared to the reference genome. The Ti/Tv (transition/transversion) ratio was used to filter out possible false-positive errors. Transition mutations generally occurred more frequently than transversions. Our clustering analysis revealed remarkable hotspot mutation patterns for SARS-CoV-2. Mutations were clustered based on how their frequencies changed over time according to each geographical location. We observed some clusters showing a clear variation in mutation frequency and continuously evolving in the world. However, many mutations appeared in specific periods without a clear pattern over time. Various important nonsynonymous mutations were observed, mainly in Oceania and Asia. More than half of these mutations were observed only once. Four hotspot mutations were found in all geographical locations at least once: T265I (NSP2), P314L (NSP12), D614G (S), and Q57H (ORF3a). The current analysis of SARS-CoV-2 genomes provides valuable information on the geographical and temporal mutational evolution of SARS-CoV-2.

Safety and Efficacy Profile of Live, Intermediate Plus Vaccine Against Infectious Bursal Disease Based on Strain G6

Infectious bursal disease (IBD) is an acute, highly contagious, immunosuppressive disease of young chickens that causes considerable economic loss in the poultry industry worldwide. Vaccination with live attenuated vaccines is still the most important method used for the control and prevention of IBD in chickens. Here we present the results of in vitro characterization, as well as efficacy and safety testing of a live, intermediate plus vaccine against IBD based on strain G6. Strain characterization confirmed that G6 strain is an intermediate plus strain, showing a high degree of homology with the existing vaccine strains of the same virulence. Safety studies showed that chickens can be vaccinated from 10 days of age. Onset and duration of immunity in specific pathogen free and maternally derived antibodies (MDA) chickens was proven to be 14 and 35 days after vaccination, respectively. When immunizing MDA-positive chickens, vaccine is capable of breakthrough at a titer of ≤500 ELISA units. The field trial conducted on commercial broilers showed a 95% protection against vvIBDV challenge. Stability of the freeze-dried vaccine after reconstitution was confirmed over a period of 3 h. Overall, IBD G6 vaccine has shown good safety and efficacy profile in accordance with European Pharmacopoeia requirements.

Reverse genetics approaches for live-attenuated vaccine development of infectious bursal disease virus

Infectious bursal disease (IBD), which is caused by infectious bursal disease virus (IBDV) infection, leads to severe immunosuppression in young chickens and results in significant economic losses in the poultry industry. To date, vaccination with live-attenuated vaccine (LAV) is a convenient method to provide effective protection against IBDV infection. Classical attenuated viruses are usually obtained by either passaging virus in cultured cells or natural isolation. However, these empiric attenuation methods, which are time-consuming and not guaranteed, are not reliable for emergent antigenic variant and very virulent IBDV strains. The reverse genetics (RG) system opens a new avenue for the development of IBDV LAV. In this review, we summarize the current knowledge on the biological characteristics of IBDV structure and genome organization, as well as the established RG systems. We also describe the details for the strategies used to develop IBDV LAV based on the RG systems.

Identification and genetic analysis of infectious bursal disease viruses from field outbreaks in Kerala, India

Recurrent infectious bursal disease (IBD) outbreaks were reported in different regions of Kerala, India. This paper reports the comparative genetic analysis of the hypervariable region of the VP2 gene of IBD virus isolates from the field outbreaks in Kerala. In phylogenetic analysis, the obtained field isolates fall into genogroup 1 and 3. In genogroup 3, all vvIBDV isolates shared a common ancestor with other south Indian isolates but isolates 9/CVASP/IBDV, 10/CVASP/IBDV, 12/CVASP/IBDV, 14/CVASP/IBDV and 17/CVASP/IBDV are most recently evolved and are diverged from the south Indian isolates. The amino acid sequence of 22 isolates was analysed, out of which 18 had conserved amino acids which were characteristic of vvIBDV. All the vvIBDV isolates obtained in the study had phenylalanine and valine at the position 240 and 294, respectively, similar to recently evolved Indian IBDV isolate (MDI14). But we observed T269A and S299N mutations in the isolate 6/CVASP/IBDV, and it is the first report of such mutations at these positions in India IBDV isolates. The isolate 11/CVASP/IBDV had a unique mutation of V225A which is not yet reported in IBDV isolates. Two isolates (15/CVASP/IBDV and 18/CVASP/IBDV) were 100% amino acid similar to intermediate plus vaccine strain. The isolates 8/CVASP/IBDV/VP2 and 19/CVASP/IBDV had amino acids unique for the intermediate vaccine with mutations observed at H253Q and V256I in 19/CVASP/IBDV, T270A and novel mutation N279Y in isolate 8/CVASP/IBDV. These two isolates had non-virulent classical heptapeptide sequence 'SWSARGS'; nevertheless, they produce field outbreaks of IBD. This is the first report of genetic characterisation of IBDV in Kerala, India.

Origin and global spreading of an ancestral lineage of the infectious bursal disease virus

Infectious bursal disease virus (IBDV) is an economically relevant and widespread pathogen that produces immunosuppression in young chickens. IBDV is genetically classified into seven genogroups (G1-G7), where the traditional classic, variant and very virulent strains correspond to G1, G2 and G3, respectively. The G4 strains, also known as 'distinct' (dIBDV), have recently acquired increased relevance because of their prevalence and notorious impair to the poultry industry in South America. Here, worldwide dIBDV strains were studied using phylogenetic and phylodynamic approaches. The phylogenetic analyses performed using partial and complete sequences of both viral segments (A and B) consistently clustered the dIBDV strains in a monophyletic group. The analyses of the VP5, polyprotein and VP1 coding regions identified amino acid residues that act as markers for the identification of the entire dIBDV group or different sub-populations. The phylodynamic analyses performed using the hypervariable region of VP2 indicated that the dIBDV strains emerged in the early 1930s in Eastern Europe, shortly after the emergence of classic strains (1927) and before variant (1949) and very virulent strains (1967). The analysis of the migration routes indicated that after its emergence, the dIBDV strains spread to Eastern Asia around 1959, to Brazil around 1963, and to Argentina around 1990. These inter-continental migrations resulted in three sub-populations that are currently represented by strains from (a) Brazil, (b) Eastern Asia and Canada, and (c) Eastern Europe, Argentina and Uruguay. Taken together, our results highlight the complex evolutionary history of IBDV and the importance of new phylodynamic data to unravel and nearly follow the different evolutionary pathways taken by this important poultry pathogen.

Phylogenetic analyses and pathogenicity of a variant infectious bursal disease virus strain isolated in China

Infectious bursal disease is an acute, highly contagious, immunosuppressive disease of young chickens caused by infectious bursal disease virus (IBDV). In recent years, there has been a notable increase in the isolation rates of variant IBDV strains in China; however, the pathogenicity of these variants is unclear. In the current study, we characterized variant IBDV strain ZD-2018-1 and assessed its pathogenicity in specific-pathogen-free chickens. Phylogenetic analysis revealed that ZD-2018-1 belonged to the variant IBDV strain, which showed several unique amino acid mutations compared with other previously-isolated variant IBDV strains. Pathogenicity assays showed that ZD-2018-1 was less virulent than very virulent IBDV strain SD-2013-1, and did not cause an obvious symptoms or death. In comparison, strain SD-2013-1 had a high mortality rate and caused severe lesions in various tissues. However, both of the strains induced obvious pathological lesions on the bursa of Fabricius, resulting in severe immunosuppression in the infected chickens. The results of this study present a systematic evaluation of the genetic characteristics, pathogenicity, and immunosuppressive properties of a new variant IBDV strain, and may help in the development of strategies for the prevention and control of IBDV in poultry.

Genetic Characterization of Infectious Bursal Disease Viruses from Field Outbreaks of the North East Region of India

In recent years, acute severe outbreaks of infectious bursal disease (IBD) are frequently observed in commercial chicken populations of the North East Region (NER) of India, resulting in huge economic loses to poultry farmers. Field outbreaks of IBD in 30 different poultry farms in the NER were confirmed by clinicopathologic examination and reverse transcriptase PCR. A total of 10 isolates of IBD virus (IBDV) from these outbreaks were characterized by the genetic analysis of VP1 and the hypervariable region of the VP2 gene. Nucleotide sequences, deduced amino acid sequences, and phylogenetic analysis of both VP2 and VP1 genes revealed two genetically diverse strains of very virulent IBDV (vvIBDV) and one intermediate strain circulating in the NER. These isolates differ at nucleotide and amino acid levels from vvIBDV isolates of mainland India and are clustered in distinctly separate groups in the phylogenetic tree. Six of the isolates revealed a unique combination of vvIBDV amino acid signatures in the VP2 gene (A222, I256, I294), while bearing the non-vvIBDV amino acid signatures of the VP1 gene (146E, 147G, 242D), but they are clearly classified as vvIBDV in a phylogenetic analysis of both genes. Interestingly, one of the isolates showed 99% sequence homology with attenuated vaccine strains in the VP2 gene and clustered together. This study demonstrates the diversity of IBDVs in India and document for the first time the possible involvement of attenuated vaccine strains in the epidemiology of IBD in India.

New oligonucleotide microarray for rapid diagnosis of avian viral diseases

BACKGROUND

We developed a new oligonucleotide microarray comprising 16 identical subarrays for simultaneous rapid detection of avian viruses: avian influenza virus (AIV), Newcastle disease virus (NDV), infection bronchitis virus (IBV), and infectious bursal disease virus (IBDV) in single- and mixed-virus infections. The objective of the study was to develop an oligonucleotide microarray for rapid diagnosis of avian diseases that would be used in the course of mass analysis for routine epidemiological surveillance owing to its ability to test one specimen for several infections.

METHODS AND RESULTS

The paper describes the technique for rapid and simultaneous diagnosis of avian diseases such as avian influenza, Newcastle disease, infectious bronchitis and infectious bursal disease with use of oligonucleotide microarray, conditions for hybridization of fluorescent-labelled viral cDNA on the microarray and its specificity tested with use of AIV, NDV, IBV, IBDV strains as well as biomaterials from poultry. Sensitivity and specificity of the developed microarray was evaluated with use of 122 specimens of biological material: 44 cloacal swabs from sick birds and 78 tissue specimens from dead wild and domestic birds, as well as with use of 15 AIV, NDV, IBV and IBDV strains, different in their origin, epidemiological and biological characteristics (RIBSP Microbial Collection). This microarray demonstrates high diagnostic sensitivity (99.16% within 95% CI limits 97.36-100%) and specificity (100%). Specificity of the developed technique was confirmed by direct sequencing of NP and M (AIV), VP2 (IBDV), S1 (IBV), NP (NDV) gene fragments.

CONCLUSION

Diagnostic effectiveness of the developed DNA microarray is 99.18% and therefore it can be used in mass survey for specific detection of AIV, NDV, IBV and IBDV circulating in the region in the course of epidemiological surveillance. Rather simple method for rapid diagnosis of avian viral diseases that several times shortens duration of assay versus classical diagnostic methods is proposed.

Effects of challenge with very virulent infectious bursal disease virus reassortants in commercial chickens

Pathogenicity and immune responses were characterized in commercial broilers and layers challenged with very virulent infectious bursal disease virus (vvIBDV) reassortants (vvIBDV segment A + serotype 2 segment B and vvIBDV segment A + classic virulent segment B) at 7 days of age. In addition, functional immunosuppression was evaluated after challenge with infectious bronchitis virus (IBV) at 15 days of age. Layers showed higher levels and increased persistence of IBDV- and IBV-specific maternal antibodies than broilers at 1, 13, and 28 days of age. Cytokine gene expression was evaluated, after IBDV challenge, as an indicator of the innate immune function. Similar results were detected between the groups inoculated with vvIBDV reassortants. Interleukin-1β (IL-1β) in the bursa of layers demonstrated down-regulation at 1 day postinfection (DPI; 8 days of age), and no changes at 4 DPI (11 days of age) compared with controls. In broilers, IL-6 expression in the bursa was down-regulated 1 DPI (8 days of age) and up-regulated at 4 DPI (11 days of age). A significant lymphoid depletion was detected at 21 DPI (28 days of age) in broilers exposed to a reassortant of vvIBDV segment A and classic virulent IBDV segment B. Finally, reduced specific antibodies against IBV measured 13 days after challenge were detected in layer and broiler chickens inoculated with a reassortant serotype 2 IBDV in segment B, suggesting functional immunosuppression. These results provide evidence indicating that current IBDV vaccination of breeders does not completely protect progeny chickens from challenge with reassortant vvIBDV.

Genetic characterization of South American infectious bursal disease virus reveals the existence of a distinct worldwide-spread genetic lineage

Infectious bursal disease virus (IBDV) is one of the most concerning health problems for world poultry production. IBDVs comprise four well-defined evolutionary lineages known as classic (c), classic attenuated (ca), variant (va) and very virulent (vv) strains. Here, we characterized IBDVs from South America by the genetic analysis of both segments of the viral genome. Viruses belonging to c, ca and vv strains were unambiguously classified by the presence of molecular markers and phylogenetic analysis of the hypervariable region of the vp2 gene. Notably, the majority of the characterized viruses (9 out of 15) could not be accurately assigned to any of the previously described strains and were then denoted as distinct (d) IBDVs. These dIBDVs constitute an independent evolutionary lineage that also comprises field IBDVs from America, Europe and Asia. The hypervariable VP2 sequence of dIBDVs has a unique and conserved molecular signature (272T, 289P, 290I and 296F) that is a diagnostic character for classification. A discriminant analysis of principal components (DAPC) also identified the dIBDVs as a cluster of genetically related viruses separated from the typical strains. DAPC and genetic distance estimation indicated that the dIBDVs are one of the most genetically divergent IBDV lineages. The vp1 gene of the dIBDVs has non-vvIBDV markers and unique nucleotide and amino acid features that support their divergence in both genomic segments. The present study suggests that the dIBDVs comprise a neglected, highly divergent lineage that has been circulating in world poultry production since the early time of IBDV emergence.

Genetic, antigenic and pathogenic characterization of four infectious bursal disease virus isolates from China suggests continued evolution of very virulent viruses

Infectious bursal disease virus (IBDV) causes an economically significant disease of young chickens worldwide. The emergence of very virulent IBDV (vvIBDV) strains has brought more challenges for effective prevention and control of this disease. The aim of the present study was to characterize four IBDV isolates from various regions of China between late 1990s and recent years and to compare them with previously isolated European IBDV strains. In this study, one Chinese vvIBDV strain isolated in 1999 and three strains isolated between 2005 and 2011 were analyzed at the genetic, antigenic and pathogenic levels. Strain SH99 was closely related and clustered in the same genetic lineage as the typical vvIBDV based on the genomic sequences of segments A and B. However, the three more recent Chinese vvIBDV (HLJ0504, HeB10 and HuN11) showed several genetic changes in both segments and clustered in a distinct lineage from the typical vvIBDV and the previously known Chinese vvIBDV. Based on the binding to a panel of neutralizing monoclonal antibodies in antigen capture enzyme-linked immunosorbent assays, all Chinese vvIBDVs exhibited similar antigenicity with the European typical vvIBDV strains. Nonetheless, the pathogenicity caused by the recent Chinese vvIBDV was higher than that induced by the European typical vvIBDV. This study calls for a sustained surveillance of IBD situation in China in order to support a better prevention and control of the disease.

Molecular epidemiology studies on partial sequences of both genome segments reveal that reassortant infectious bursal disease viruses were dominantly prevalent in southern China during 2000-2012

A molecular epidemiology study of infectious bursal disease viruses (IBDVs) isolated from seven provinces in southern China during the years 2000-2012 was performed based on partial sequences of genome segments A and B, namely the hypervariable region of the A-VP2 gene (A-vVP2) and the b fragment of VP1 gene (B-VP1b) from a total of 91 field isolates. Sequence analysis based on vVP2 revealed that 72 out of 91 isolates had the same characteristic amino acid (aa) sequences as vvIBDV. The mutation of D212N in A-vVP2 has become prevalent in the recent isolates. The origin of the field isolates with vvIBDV characteristic amino acid residues was complex, evidenced by the findings that more than one subgroup of strains prevailed in each province. When B-VP1b was analyzed, there were three lineages among the field isolates, and none of the isolates had a relationship to vvIBDV-related segment B. Phylogenetic analysis of both segments revealed that only a few isolates (13/91) had the same genetic relatives in consensus trees based on segments A and B, whereas the majority of the isolates (85.71%, 78/91) were identified to be naturally reassorted strains. Based on the origin of each segment, at least six types of reassortant IBDVs prevailed in southern China, three of which were shown to be dominant: segment A from vvIBDV and B from attenuated IBDV, segment A of vvIBDV and B from 002-73-like IBDV, and segment A of vvIBDV and B from HLJ0504 or a similar strain. Our findings suggest that both genomic segments of field IBDVs has been evolving, and continuous monitoring of the evolution of field IBDV genome is therefore urgently needed in the control of IBDV.

Enhanced immune responses of chickens to oral vaccination against infectious bursal disease by ginseng stem-leaf saponins

Infectious bursal disease (IBD), caused by infectious bursal disease virus (IBDV), is an immunosuppressive infectious disease of global economic importance in poultry. This study was designed to evaluate the effect of oral administration of ginseng stem-leaf saponins (GSLS) on humoral and gut mucosal immunity in chickens vaccinated with live IBDV vaccine, and furthermore, to test its protective efficacy against virulent IBDV challenge following vaccination. In experiment 1, chickens were orally administered with GSLS at 5 mg/kg of BW for 7 d, and then immunized with live IBDV vaccine via the oral route. Serum was sampled on 0, 1, 2, 3, 4, and 5 wk postvaccination for detecting antibody titers by ELISA, and intestinal tissues were collected on 0, 1, 3, and 5 wk postvaccination for measurement of IgA-positive cells and intestinal intraepithelial lymphocytes by immunohistochemical and hematoxylin-eosin staining, respectively. Result showed that antibody titers, IgA-positive cells and intestinal intraepithelial lymphocytes were significantly higher in chickens drinking GSLS than the control, suggesting an enhanced effect of GSLS on humoral and gut mucosal immune responses. In experiment 2, chickens were delivered with GSLS and then vaccinated in the same way as in experiment 1. The birds were challenged with virulent IBDV at wk 3 postvaccination. Then the birds were weighed, bled, and necropsied at d 3 postchallenge and the bursae were sampled for gross and histopathological examination. Results demonstrated that GSLS provided a better protection against virulent IBDV challenge following vaccination than the control. In conclusion, oral administration of GSLS enhances both humoral and gut mucosal immune responses to IBDV and offers a better protection against virulent IBDV challenge. Considering its immunomodulatory properties to IBDV vaccine, GSLS might be a promising oral adjuvant for vaccination against infectious diseases in poultry.

Genetic characterization of infectious bursal disease viruses associated with gumboro outbreaks in commercial broilers from asyut province, egypt

Ten infectious bursal disease virus (IBDV) field strains were isolated from 15 broiler flocks located in various parts of Asyut, Egypt. Seven strains were subjected to comparative sequencing and phylogenetic analyses to help provide optimal control program for protection against IBDV infection. Sequence analysis of a 530 bp hypervariable region in the VP2 gene revealed that the rate of identity and homology was around 95.6~99.1%. Sequence characterization revealed the 7 strains identified as vvIBDV with the four amino acids residues typical of vvIBDV (242I, 256I, 294I, 299S). The BURSA-VAC vaccine was the nearest vaccine in sequence similarity to the local examined IBDV strains followed by CEVACIBDL then Bursine plus and Nobilis Gumboro indicating its probable success in the face of incoming outbreaks when using these vaccines. Phylogenetic analysis revealed that the presence of three clusters for the examined strains and are grouped with reference very virulent IBDVs of European and Asian origin (Japanese and Hong Kong) strains suggesting the different ancestors of our isolates. The antigenic index showed a number of changes on the major and minor hydrophilic antigenic peaks of the virus surface structures indicating a new genetic evolution of the surface structure epitopes that may lead to vaccination failure and reemergence of the disease.

Pathogenicity of SspI-positive infectious bursal disease virus and molecular characterization of the VP2 hypervariable region

The pathogenicity of four isolates of infectious bursal disease virus (IBDV) that have restriction fragment length polymorphism patterns of very virulent IBDV (vvIBDV), based on the presence of SspI and TaqI sites in the VP2 hypervariable region, was studied in specific pathogen free chickens. Chickens inoculated with isolates 92/04, 94/B551 and 97/61 developed severe clinical signs with a high mortality ranging from 70 to 80%, whereas the 94/273 isolate caused 10% mortality. Regardless of the isolates, significant differences were noted in the bursal lesion scores and bursa:body weight ratio index in the infected groups in comparison with the control groups. However, the presence of lesions in non-bursal tissues, muscles, thymus and at the junction of the proventriculus and gizzard were found only in the 92/04, 97/61 and 94/B551 isolates. Restriction fragment length polymorphism and sequence analysis of the VP2 hypervariable region indicated that all the isolates can be classified as vvIBDV based on the presence of SspI and TaqI sites at nucleotide positions 1011 and 833, respectively. In addition, all the isolates had amino acid substitutions at P222A, V256I and L294I, which are characteristic for vvIBDV isolated from different parts of the world. All the isolates except 94/273 also had a StyI site at nucleotide position 888. The absence of a StyI site in this isolate was associated with amino acid substitution at 254 from G to S. The 94/273 also had an amino acid substitution at position 270 from A to E, which is variable in the STC, Cu1 and OH strains. The presence of amino acid substitutions from G254S andA270E in SspI- and TaqI-positive vvIBDV strains is very uncommon and has not been reported previously. These amino acid variations might have caused the 94/273 to become less virulent in specific pathogen free chickens and resemble a classical virulent IBDV strain.

Rapid detection of Infectious bursal disease virus by reverse transcription loop-mediated isothermal amplification assay

A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for the rapid identification of Infectious bursal disease virus (IBDV). The RT-LAMP assay used a set of 4 primers to amplify the viral protein 2 gene of IBDV for the detection of IBDV, showing not only high efficiency but also analytic specificity. The data demonstrated that the RT-LAMP assay detected 30 different IBDV isolates, had no cross-reaction with 3 other avian viruses (Infectious bronchitis virus, Newcastle disease virus, and Avian influenza virus), and obtained a 95.45% sensitivity in 22 positive clinical samples in reference to virus isolation. Therefore, this rapid, specific, sensitive, and convenient RT-LAMP assay could be applicable to the identification of IBDV in less-equipped laboratories as well as in the field.

Molecular epizootiology of infectious bursal disease (IBD) in Korea

We conducted a molecular epizootiological study of infectious bursal disease (IBD) in Korea by analyzing 85 IBD viruses (IBDVs) obtained from vaccinated or unvaccinated flocks between 1980 and 2007. Phylogenetic analysis of the partial nucleotide sequence of the hypervariable region of the VP2 gene (nucleotides 661-1020) and pathogenicity tests revealed more genetic and phenotypic diversity of IBDV in Korea than has been reported previously. We showed that very virulent IBDVs (vvIBDVs) were already present in Korea in 1986. Moreover, vvIBDVs were repeatedly detected in Korean poultry that had been vaccinated, which casts doubt on the IBD vaccine programs. We also identified novel putative antigenic variant (AV)-like IBDV isolates on the basis of their antigenic indices and the presence of amino acid changes (P222S or P222T-A321D) that are known to affect the antigenicity of VP2. These observations suggest that future studies examining the efficacy of conventional vaccines against atrophy of the bursa of Fabricius and vvIBDV shedding may be useful. Moreover, it will be of interest to determine the prevalence of putative Korean antigenic variants and whether these strains exert immunosuppressive effects in vaccinated birds.

Comparative study of the replication of infectious bursal disease virus in DF-1 cell line and chicken embryo fibroblasts evaluated by a new real-time RT-PCR

A real-time RT-PCR method was developed for the detection of infectious bursal disease virus (IBDV). The VP5 gene of IBDV was chosen as the target binding region for a specific TaqMan probe. The results showed that viral genomic copy number could be quantified accurately ranging from 10(8)copies/microL to 10(1)copies/microL. No positive signal was detected for other avian pathogens in the specificity test. This assay was highly sensitive and could detect as little as 30 copies of viral RNA. Both the coefficients of variation (CVs) of inter- and intra-assay reproducibility were less than 2%. Growth curves of the IBDV Gt strain in chicken embryo fibroblasts (CEF) and DF-1 cells were evaluated by the real-time RT-PCR. The data showed that the cytopathic effects of the virus in CEF and DF-1 cells were similar. However, higher viral titers were detected in the DF-1 cell line. This study indicated that the real-time RT-PCR approach provided a powerful diagnostic tool with high sensitivity and specificity for the identification and quantitation of IBDV. The DF-1 cell line may be a more suitable continuous cell line for the propagation of IBDV compared to CEF.

Molecular characterization of Iranian infectious bursal disease viruses

This study was conducted to characterize nine infectious bursal disease virus (IBDV) isolates from Iran. A reverse transcriptase-polymerase chain reaction (RT-PCR) procedure was used to amplify a 743-bp fragment of the VP2 gene hypervariable region from IBDV field isolates. Amplified VP2 fragments of the nine IBDV isolates were sequenced and compared with published sequences of IBDV strains from Iran and around the world, and their phylogenetic relationships were analyzed. Three isolates demonstrated close relation to classical attenuated strains of IBDVs, and six isolates showed sequences common in European and Asian strains of very virulent IBDVs (vvIBDVs). Four nucleotide changes--802A, 934A, 940A, and 1366A--were common in all Iranian vvIBDVs except in one isolate. Amino acid sequences of three Iranian vvIBDVs were 100% identical and resembled vvIBDV strains from European (UK661), Asian (HK46, GZ96), and Iranian origins (IR01, SDH1). Some unique amino acid substitutions after major hydrophilic peak A in Iranian vvIBDV field isolates were observed: 231S-L, 231S-P, and 233N-K. Phylogenetic analysis showed that the Iranian vvIBDVs were closely related to European and Asian vvIBDVs. Further comprehensive investigations will provide more information on the distribution, variability, and phylogenetic relationships of different IBDVs isolated in Iran and other parts of the world.

Sequence analysis of both genome segments of three Croatian infectious bursal disease field viruses

In order to determine the mutations responsible for virulence, three Croatian field infectious bursal disease viruses (IBDV), designated Cro-Ig/02, Cro-Po/00, and Cro-Pa/98 were characterized. Coding regions of both genomic segments were sequenced, and the nucleotide and deduced amino acid sequences were compared with previously reported full-length sequenced IBDV strains. Phylogenetic analysis, based on the nucleotide and deduced amino acid sequences of polyprotein and VP1, was performed. Eight characteristic amino acid residues, that were common to very virulent (vv) IBDV, were detected on polyprotein: 222A, 256I, 294I, 451L, 685N, 715S, 751D, and 1005A. All eight were found in Cro-Ig/02 and Cro-Po/00. C-Pa/98 had all the characteristics of an attenuated strain, except for glutamine on residue 253, which is common for vv, classical virulent, and variant strains. Between less virulent and vvIBDV, three substitutions were found on VP5: 49 G --> R, 79 --> F, and 137 R --> W. In VP1, there were nine characteristic amino acid residues common to vvwIBDV: 146D, 147N, 242E, 390M, 393D, 511S, 562P, 687P, and 695R. All nine residues were found in A-Ig/02, and eight were found in B-Po/00, which had isoleucine on residue 390. Based on our analyses, isolates Cro-Ig/02 and Cro-Po/00 were classified with vv IBDV strains. C-Pa/98 shared all characteristic amino acid residues with attenuated and classical virulence strains, so it was classified with those.

Sequence analysis of the VP2 hypervariable region of eight very virulent infectious bursal disease virus isolates from the northeast of China

Nucleotide sequences of the VP2 gene of eight infectious bursal disease viruses isolated from vaccinated chicken flocks in the northeast of China were determined. The sequence analysis showed that all of the isolates were also characterized by the vvIBDV conserved amino acid residues: 222A, 256I, 294I, and 299S. Four of them had one amino acid change (D-->N) at position 212 in VP2 major hydrophilic peak A, while two of the four isolates had another one (A-->V) at position 321 in major hydrophilic peak B. The other isolates were similar to the UK661 strain. Our findings demonstrated that the vvlBDV strains in the northeast of China could be diverse.

Molecular characterisation of infectious bursal disease viruses isolated in recent acute outbreaks in Slovenia

In 2004 and then in 2006 several outbreaks of infectious bursal disease (IBD) were reported in broiler and broiler breeder flocks in Slovenia. In this report ten recently emerged IBD viruses (IBDV) were characterised by sequence analysis of the VP2 hypervariable region and compared to previous Slovene IBDV strains from 1995/1996 and to some representative serotype 1 IBDV strains of different pathotypes. On the basis of nucleotide and amino acid identities, phylogenetic analyses and the presence of very virulent IBDV (vvIBDV) conserved amino acid substitutions, all Slovene isolates from recent outbreaks were identified as vvIBDV. Although some unique nucleotide exchanges and amino acid substitutions have been observed, the results of this study indicated that recent vvIBDV isolates are closely related with those from outbreaks in the 1990s. However, acute IBD has not been reported in commercial flocks in Slovenia for some years. This could lead to the conclusion that poor biosecurity and relaxed vaccination could be responsible for the re-emergence of vvIBDV.

Comparison of four infectious bursal disease viruses isolated from different bird species

Four isolates of infectious bursal disease virus (IBDV), isolated from chicken, duck, goose and sparrow in Jiangsu province of China in 2002, were compared. The viruses were stable to the treatments of 60 degrees C for 1 h, pH 2.0 and lipid solvents. Their antigenic relatedness values (R) were from 0.76 to 0.78. Chickens infected with the chicken isolate showed severe clinical symptoms of IBD and the mortality rate was 33.3% (2/6). Chickens infected with the other three viruses survived but their bursas were damaged and the bursa/body-weight ratios were lower than those of the uninfected control (p< 0.01). The titers of anti-IBDV antibody in infected chicken sera reached up to 1600 by virus neutralization and 6400 by ELISA at 10 days post infection. The sequences of the variable region of VP2 were aligned and compared, showing nucleotide variations ranging from 1.5 to 6.7% and deduced aminoacid variations from 0.8 to 2.2%. All had the same heptapeptide, S-W-S-A-S-G-S, Asp279, and Ala284. The four viruses clustered on a phylogenetic tree and were distant from the STC strain. These findings suggested that different bird species naturally infected with IBDV could serve as carriers or reservoirs in IBDV transmission and might play a role in the emergence of variant IBDV.

Relative quantification and detection of different types of infectious bursal disease virus in bursa of Fabricius and cloacal swabs using real time RT-PCR SYBR green technology

In present study, different types of infectious bursal disease virus (IBDV), virulent strain DK01, classic strain F52/70 and vaccine strain D78 were quantified and detected in infected bursa of Fabricius (BF) and cloacal swabs using quantitative real time RT-PCR with SYBR green dye. For selection of a suitable internal control gene, real time PCR parameters were evaluated for three candidate genes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), 28S rRNA and beta-actin to IBDVs. Based on this beta-actin was selected as an internal control for quantification of IBDVs in BF. All BF samples with D78, DK01 or F52/70 inoculation were detected as virus positive at day 1 post inoculation (p.i.). The D78 viral load peaked at day 4 and day 8 p.i., while the DK01 and F52/70 viral load showed relatively high levels at day 2 p.i. In cloacal swabs, viruses detectable were at day 2 p.i. for DK01 and F52/70, day 8 p.i. for D78. Importantly, the primers set were specific as the D78 primer set gave no amplification of F52/70 and DK01 and the DK01 primer set gave no amplification of D78, thus DK01 and D78 could be quantified simultaneously in dually infected chickens by use of these two set of primers. The method described here is robust and may sever as a useful tool with high capacity for diagnostics as well as in viral pathogenesis studies.

Effects of DDA, CpG-ODN, and plasmid-encoded chicken IFN-gamma on protective immunity by a DNA vaccine against IBDV in chickens

This study examined the adjuvant effects of dimethyl dioctadecyl ammonium bromide (DDA), CpG oligodeoxynucleotides (CpG-ODN), and chicken interferon-γ (ChIFN-γ) on a DNA vaccine (pcDNA-VP243) against the infectious bursal disease virus (IBDV). A plasmid encoding chicken IFN-ã was constructed. Twice at 2-week intervals, two-week-old chickens were injected intramuscularly and intraperitoneally with either a DNA vaccine alone or a DNA vaccine together with the respective adjuvants. On week 2 after the second immunization, the chickens were orally challenged with the highly virulent IBDV. The groups that received the DNA vaccines plus either DDA or CpG-ODN showed significantly lower survival rates than the group that received the DNA vaccine alone. However, the survival rates for the DNA vaccine alone and for the DNA vaccine plus ChIFN-γ were similar. The chickens had no detectable antibodies to the IBDV before the challenge but all the surviving chickens in all groups except for the normal control group showed the induction of antibodies to the IBDV at day 10 after the challenge. As judged by the lymphocyte proliferation assays using the a WST-8 solution performed on the peripheral blood and splenic lymphocytes, the stimulation indices (SI) of the peripheral blood lymphocytes in all groups except for the normal control group were similar immediately before the challenge. At 10 days post-challenge, the SI for DNA vaccine plus either CpG-ODN or ChIFN-γ was similar to that of the DNA vaccine control group. For splenic lymphocytes, the SI in the DNA vaccine plus CpG-ODN and DNA vaccine plus ChIFN-γ groups were higher than for the DNA vaccine control. These results suggest that DDA actually compromises the protection against the IBDV by DNA vaccine, and CpG-ODN and IFN-γ had no significant effect.

Detection of Very Virulent Strains of Infectious Bursal Disease Virus (vvIBDV) in Commercial Broilers from Uruguay

Bursal samples were collected from commercial broiler flocks exhibiting clinical signs suggestive of infectious bursal disease (IBD). The presence of IBD virus (IBDV) was confirmed by partial amplification of the VP2 and VP1 genes by reverse transcription and polymerase chain reaction. The Uruguayan viruses were identified as very virulent strains of IBDV (vvIBDV) by nucleotide and amino acid sequence analysis. The comparison of the VP2 nucleotide sequences among the Uruguayan samples revealed the presence of single-nucleotide polymorphisms suggestive of different viral subpopulations or quasispecies in the same flock. The comparative analysis indicated that these Uruguayan viruses were genetically close to the European strain UK661 and to the vvIBDVs previously detected in Venezuela. Our analyses provided new information about the distribution, variability, and evolutionary trends of vvIBDV strains in the Americas.

Chicken recombinant antibodies specific for very virulent infectious bursal disease virus

A phage-displayed single chain variable fragment (scFv) antibody library was constructed from the immune spleen cells of chickens immunized with very virulent infectious bursal disease virus (vvIBDV) strain CS89. A library consisting of around 9.2 x 10(7) clones was subjected to 3 rounds of panning against captured CS89 virus. Analysis of individual clones by nucleotide sequencing revealed at least 22 unique scFv antibodies binding to vvIBDV in ELISA. Testing of the scFv antibody panel in ELISA against classical, variant or vaccine strains and a wide variety of vvIBDV isolates from the UK, China, France, Belgium, Africa, Brazil, Indonesia and the Netherlands identified one antibody, termed chicken recombinant antibody 88 (CRAb 88) that was specific for vvIBDV. CRAb 88 was capable of recognizing all vvIBDV strains tested regardless of their country of origin and showed no reactivity with classical, variant or vaccine strains, lending support to the use of this scFv as a powerful diagnostic tool for the differentiation of vvIBDV strains. Immunoprecipitation studies revealed that CRAb 88 was directed towards a highly conformational epitope located within the major neutralizing protein VP2. Sequence analysis of the hypervariable region of VP2 of the IBDV strains tested indicate that Ile(256) and Ile(294) may play roles in binding of CRAb 88. This is the first reagent of its type capable of positively distinguishing vvIBDV from other IBDV strains.

Molecular studies on suspect very virulent infectious bursal disease virus genomic RNA samples

Infectious bursal disease (IBD) associated with high mortality was first observed in Europe in the mid-1980s. The viruses identified in those outbreaks were described as being very virulent infectious bursal disease virus (vvIBDV) strains. These viruses have spread to nearly every continent but have not yet been identified in North America, Australia, and New Zealand. There is a real and immediate concern that the very virulent form of IBDV will continue to spread until it is present on every continent. Genomic RNA samples from IBDV strains suspected of being very virulent were submitted to our laboratory for molecular analysis. Nucleotide sequences of the VP2 gene hypervariable sequence region were determined for 18 of these viruses. A comparison with published vvIBDV sequences indicated that all but one sample (Thai 4) had nucleotide and predicted amino acid sequences consistent with vvIBDV strains. Published sequences and the nucleotide sequences of our 17 putative vvIBDV strains were used to identify unique nucleotides in the VP2 gene. Probe pairs for a real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay were designed based on these unique sequences and then used to test the 17 genomic samples that were identified by nucleotide sequencing to be consistent with vvIBDV, plus the one Thai 4 sample that was not consistent with vvIBDV. Using melting temperature (Tm) analysis following real-time RT-PCR, two probe pairs (vv232 and vv256) successfully identified the 17 putative vvIBDV strains and distinguished them from the Thai 4 sample. An additional 26 genomic RNA samples submitted as suspect vvIBDV strains were then tested using the vv232 and vv256 probes. Based on the melting point analysis of these two probes, all 26 samples contained nucleotide sequences consistent with vvIBDV strains. The specificity of the vv232 and vv256 probe pairs was evaluated using 19 non-vvIBDV strains. In every case, the probes distinguished the 19 classic and variant (non-vvIBDV) strains from the putative vvIBDV strains. Diagnostic assays that can reliably identify vvIBDV strains are needed for surveillance programs designed to monitor the spread of these viruses.

Vaccination against very virulent infectious bursal disease virus using recombinant T4 bacteriophage displaying viral protein VP2

In order to develop a desirable inexpensive, effective and safe vaccine against the very virulent infectious bursal disease virus (vvIBDV), we tried to take advantage of the emerging T4 bacteriophage surface protein display system. The major immunogen protein VP2 from the vvIBDV strain HK46 was fused to the nonessential T4 phage surface capsid protein, a small outer capsid (SOC) protein, resulting in the 49 kDa SOC-VP2 fusion protein, which was verified by sodium dodecylsulfate polyacrylamide gel electrophoresis and Western blot. Immunoelectromicroscopy showed that the recombinant VP2 protein was successfully displayed on the surface of the T4 phage. The recombinant VP2 protein is antigenic and showed reactivities to various monoclonal antibodies (mAbs) against IBDV, whereas the wild-type phage T4 could not react to any mAb. In addition, the recombinant VP2 protein is immunogenic and elicited specific antibodies in immunized specific pathogen free (SPF) chickens. More significantly, immunization of SPF chickens with the recombinant T4-VP2 phage protected them from infection by the vvIBDV strain HK46. When challenged with the vvIBDV strain HK46 at a dose of 100 of 50% lethal dose (LD50) per chicken 4 weeks after the booster was given, the group vaccinated with the T4-VP2 recombinant phage showed no clinical signs of disease or death, whereas the unvaccinated group and the group vaccinated with the wild-type T4 phage exhibited 100% clinical signs of disease and bursal damages, and 30%-40% mortality. Collectively, the data herein showed that the T4-displayed VP2 protein might be an inexpensive, effective and safe vaccine candidate against vvIBDV.

Development of recombinant VP2 vaccine for the prevention of infectious bursal disease of chickens

The objective of the present study was to investigate the feasibility of a subunit vaccine and a live bacteria vaccine to protect chickens against infectious bursal disease virus (IBDV) infection. The gene for VP2 of a new wild-type very virulent IBDV (vvIBDV) strain was cloned into an Escherichia coli expression system. Following expression, the recombinant VP2 and the induced expression bacteria were used to vaccinate chickens against virulent IBDV (vIBDV). Three weeks after the vaccination, chickens were inoculated with IBDV strain BC 6/85 by intranasal route or eyedrop route, prior to challenge anti-IBDV serum antibody was detected by AGP. All chickens vaccinated with recombinant VP2 could be detected anti-IBDV antibody. The subunit vaccine of recombinant VP2 conferred protection for 90--100% chickens, live bacteria vaccine of recombinant VP2 conferred protection for 85.7% chickens. The results indicate that E. coli BL 21/pET 28 a-VP2 could be used to develop recombinant VP2 vaccine against infectious bursal disease in chickens.

Viral Genotyping of Infectious Bursal Disease Viruses Isolated from the 2002 Acute Outbreak in Spain and Comparison with Previous Isolates

An infectious bursal disease (IBD) outbreak occurred in the east region of Spain in the spring of 2002 and rapidly spread thorough the whole country, although proper vaccination programs were applied. In this report, 33 infectious bursal disease viruses (IBDVs) isolated from this outbreak were characterized by nucleotide sequencing of the VP2 gene hypervariable region and were compared with reference IBD strains and the 1990s Spanish IBDVs in order to determine possible emergence of IBDV isolates with modified antigenic or virulent properties. Moreover, histopathologic and immunohistochemical studies of those cases where bursal tissues were available were carried out. Of the 33 isolates, 23 were identified as very virulent IBDVs (vvIBDVs), whereas the other 10 isolates were classified as attenuated or intermediate virulence classical strains and could possibly be IBDV live vaccine strains used in the immunization of these chickens. Results of this study indicate that wIBDV isolates from the 2002 Spanish outbreak are closely related with those from the 1990s outbreak. However, acute IBD cases have not been reported in Spain during these 10 yr. Genetic, management, and environmental factors likely related with IBD reemergence in Spain are discussed. Moreover, our results indicate that good correlation exists between the IBDV subtype present in the field and the degree of lesions in bursa tissue, as well as the immunohistochemistry staining.

Real-time RT-PCR differentiation and quantitation of infectious bursal disease virus strains using dual-labeled fluorescent probes

A real-time RT-PCR assay was developed utilizing dual-labeled fluorescent probes binding to VP4 sequence that are specific to the classical (Cl), variant (V) and very virulent (vv) strains of infectious bursal disease virus (IBDV). The assay was highly sensitive and could detect as little as 3 x 10(2) to 3 x 10(3) copies of viral template. Viral genomic copy number could be accurately assayed over a broad range of 7-8 logs of viral genome. The variant sequence-specific probe was found to be highly specific in detecting isolates classified as variant A, D, E, G and GLS-5, and did not react with classical strains. A total of 130 field and experimental variant strain isolates were tested using this assay. The classical sequence-specific probe also demonstrated high sensitivity and specificity, and positively detected a total of 87 STC isolates, both field and experimental isolates, while differentiating between isolates that were variant and classical strains. The very virulent sequence-specific probe detected positively the Holland vvIBDV isolate and did not react with classical or variant strains. Rapid identification of viral strain is a primary concern to poultry flock health programs to ensure administered vaccines will protect against current strains of virus circulating in the flock. The ability to quantify virus concurrently is also of assistance in identifying the progression of disease outbreaks within the flock.

A very virulent genotype of infectious bursal disease virus predominantly associated with recurrent infectious bursal disease outbreaks in Tunisian vaccinated flocks

Outbreaks of infectious bursal disease (IBD) still continue to afflict the Tunisian poultry industry even in those flocks where the vaccination program is strictly applied. To characterize the viruses that circumvent protection provided by vaccination, field isolates of infectious bursal disease virus (IBDV) obtained from vaccinated flocks that have repeatedly experienced IBDV outbreak episodes were analyzed from bursal samples by reverse transcription coupled with polymerase chain reaction and dideoxynucleotide sequencing of the VP2 hypervariable region. Although sequence data were obtained from samples collected from three distinct flocks over a period of 3 years, only limited sequence variation has been observed. The few nucleotide changes were silent and the deduced amino acid sequences were identical. Thus, the virus population that predominates in the field seems to represent a homogeneous antigenic pool. Compared with the VP2 sequences of several IBDV strains, this predominant pool was found to be closely related to the very virulent (vv) IBDV viruses described in Europe and Asia. Sequence and phylogenetic analysis of the precursor polyprotein coding sequence of a representative Tunisian isolate further confirmed its assignment to the vv genotype. The deduced amino acid sequence of the whole polyprotein of the Tunisian isolate was found to be identical to a South Korean IBDV strain. Alignment of the polyprotein amino acid sequence of 35 IBDV strains identified additional mutations outside the VP2 variable domain and which occur frequently in vv strains. Based on this comparative analysis, the set of amino acid residues that should represent a typical vv profile involves Ala222, Ile242, Ile256, Ile294, Leu451, Tyr680, N685, Ser715, Asp751, Val990, and Ala1005. Such a combination of amino acid changes was observed for the majority of vvIBDV strains that define a distinct phylogroup.

Assessment of genetic, antigenic and pathotypic criteria for the characterization of IBDV strains

The aim of this work was the selection and comparison of representative infectious bursal disease virus (IBDV) strains. Nine strains of IBDV, isolated at different times and from different geographic regions of Europe and China, were characterized. Batches of all strains were prepared following standardized protocols and checked for the absence of contaminating viruses. Criteria used for their characterization were: (i) the nucleotide sequence of the VP2 variable region, (ii) binding to a panel of neutralizing monoclonal antibodies in antigen capture enzyme-linked immunosorbent assays, and (iii) virulence in specific pathogen free chickens after infection with a standardized number of median embryo infective doses. Based on the first two criteria, two of nine strains were classified as classical virulent (cv) IBDV (F52/70, Cu-1wt), and five as very virulent (vv) IBDV (849VB, 96108, HK46, GX, Harbin). Remarkably, although a clear-cut difference was demonstrable between European cvIBDV (F52/70 and Cu-1wt) and vvIBDV (849VB and 96108) strains, there was a continuum in the pathogenicity of Chinese vvIBDVs. Our results indicate the probable existence of differences in virulence within IBDV lineages determined on the basis of antigenic typing using monoclonal antibodies and the alignment of the VP2 sequences. This indicates limitations in the analysis of IBDV pathotypes based on the VP2 variable region and emphasizes that these criteria may not be sufficient for the classification of IBDV strains.

Genetic characterization of very virulent infectious bursal disease viruses from Latin America

Very virulent infectious bursal disease viruses (vvIBDVs) were detected in phenol inactivated bursal samples obtained from Brazil, the Dominican Republic, and Venezuela. After nucleotide sequence analysis of the hypervariable region of VP2 gene, the vvIBDVs from Brazil and Venezuela exhibited all of the 14 nucleotide changes that are conserved in the European UK-661 and most other vvIBDV strains. However, the vvIBDV from the Dominican Republic presented 11 nucleotide changes that are conserved in vvIBDV strains. After phylogenetic analysis, the Latin American strains were found to be related to other vvIBDV strains from Europe, Asia, and Africa. However, Brazilian and Dominican vvIBDVs clustered in two separate subgroups, while the vvIBDVs from Venezuela were closely related to other strains from other parts of the world. By deduced amino acid sequence, the three conserved amino acid residues in vvIBDV strains (222 Ala, 256 Ile, and 294 Ile) were confirmed in the Latin American viruses, and one amino acid change (300 Ala) was unique to all vvIBDVs from the Dominican Republic. The occurrence of this change in the Dominican vvIBDVs may have an impact in their antigenic makeup. Results of this study indicate that the vvIBDVs detected in Latin America are genetically similar to IBDV strains from other parts of the world. However, vvIBDVs from Venezuela were more similar to the vvIBDV strains from Europe and Asia. Of all the samples analyzed, vvIBDVs from Brazil and the Dominican Republic exhibited more genetic changes. These changes may have emerged as a result of the different management practices and environmental conditions present in each particular geographic area.

Extensive antigenic changes in an atypical isolate of very virulent infectious bursal disease virus and experimental clinical control of this virus with an antigenically classical live vaccine

The 99323 Egyptian isolate of infectious bursal disease (IBD) virus (IBDV) was identified during an international survey of acute IBD cases. Its unique antigenicity was characterized by a markedly reduced binding of neutralizing monoclonal antibodies 3, 4, 5, 6, 8 and 9 in an antigen-capture enzyme-linked immunosorbent assay. Nucleotide sequencing of the genome region encoding the VP2 major immunogenic domain in 99323 revealed amino acid changes occurring at positions critical for antigenicity, but phylogenetic analysis demonstrated that 99323 was related to typical, very virulent IBDV (e.g. isolate 89163). Protection experimentally afforded by an antigenically classical live IBD vaccine was investigated in specific pathogen free chickens challenged with 99323 or 89163. Both viruses were similarly controlled, as evaluated by clinical signs, growth retardation, bursa-to-body weight ratios and histological lesions of the bursa after challenge. These results document that an active antibody response to a classical live antigen may clinically control infection by an antigenically atypical very virulent IBDV.

Heteroduplex Mobility Assay for Genotyping Infectious Bursal Disease Virus

A heteroduplex mobility assay (HMA) was developed to genotype infectious bursal disease virus (IBDV). This method analyzed 390-base pair (bp) polymerase chain reaction (PCR) products, encompassing the hypervariable region of the VP2 gene. IBDV strains from the United States and other countries were analyzed. The HMA was able to differentiate standard, antigenic variants and very virulent strains of IBDV. Minor differences between different strains from the same subtype were also detected. Close relationships between field IBDV with vaccines prepared with Delaware E strain were determined by HMA. The results obtained by HMA were confirmed by restriction fragment length polymorphism (RFLP) and phylogenetic analysis of nucleotide sequences. The HMA proved to be a useful technique to rapidly genotype different field strains of IBDV and should prove to be a useful tool in epidemiologic studies.

Changes in VP2 gene during the attenuation of very virulent infectious bursal disease virus strain Gx isolated in China

Very virulent (vv) infectious bursal disease virus (IBDV) Gx strain with high pathogenicity was attenuated through replication in specific-pathogen-free (SPF) chicken embryos and in chicken embryo fibroblast (CEF) cell cultures. The changes in VP2 nucleotide and the deduced amino acid sequences were obtained during attenuation of vvIBDV in CEF culture. Sequence analysis of selected passages from numbers 0 to 20 in CEFs (designated here Gx to CEF-20) showed that no changes were detectable in the VP2 gene before CEF-7. There were a few changes in the nucleotide sequence of the VP2 gene but no amino acid substitutions at CEF-8. The virus of CEF-9 was an intermediate with some amino acid changes that possibly were related to virulence. CEF-10 virus had become similar to CU-1 strain. The VP2 gene sequence remained the same from CEF-10 to CEF-20. The results of pathogenicity tests showed that the mortalities of Gx, CEF-5, CEF-8, and CEF-9 in 4-wk-old SPF chickens were 64%, 60%, 60%, and 32%, respectively; whereas CEF-10, CEF-15, and CEF-20 were nonpathogenic. Virus neutralization tests with Gx strain showed that the antigenicities are similar from Gx to CEF-20.

Research on infectious bursal disease--the past, the present and the future

Infectious bursal disease (IBD) virus (IBDV) is the etiological agent of "Gumboro disease". Although first observed about 40 years ago, this disease continues to pose an important threat to the commercial poultry industry. The emergence of antigenic variant as well as very virulent strains in vaccinated flocks considerably stimulated research efforts on both, IBD and IBDV. In this review, some of the recent advances in the understanding of the structure, morphogenesis and molecular biology of the virus as well as in development of new diagnostic approaches and new strategies for vaccination against IBD are briefly summarized.

Plasmid DNA encoding antigens of infectious bursal disease viruses induce protective immune responses in chickens: factors influencing efficacy

The complete polyprotein (VP2/4/3) and VP2 genes of two infectious bursal disease viruses (IBDVs) (one attenuated strain JD1 and one virulent strain ZJ2000) were amplified by long and accurate polymerase chain reaction (LA-PCR), cloned, sequenced and inserted into plasmids pCI and pcDNA3 under the control of human cytomegalovirus (hCMV) immediate early enhancer and promoter. A series of DNA vaccine preparations were made using liposome as the adjuvant to examine their immunogenicity. Although VP2 is the main protective immunogen of IBDV, DNA encoding VP2 initiated a very low level of neutralizing antibody and only protected chickens from clinical outbreak and morality, but not bursal damage. In contrast, DNA encoding VP2/4/3 induced neutralizing antibody and satisfactory protection against virulent IBDV. Recombinant plasmids encoding the polyprotein gene of strain ZJ2000 were more efficient at inducing an immune response than that of strain JD1. Polyprotein expressed by the pCI vector induced better immune response than that expressed by the pcDNA3. Delivery of DNA through intramuscular and/or intradermal routes elicited much higher protective responses than that of oral and eyedrop routes. Most of the chickens vaccinated with high doses of DNA were protected from challenge. Additionally, the immune response to the DNA vaccine was significantly enhanced by a liposome adjuvant. These results indicate that the source of the target genes (from different IBDV strains), the eukaryotic expression vector, the adjuvant, the delivery route and the dosage might play a role of varying degree in influencing the efficacy of the DNA vaccine against IBDV.

Characterization of Egyptian Field Strains of Infectious Bursal Disease Virus

The reverse transcriptase-polymerase chain reaction/restriction fragment length polymorphism (RT-PCR/RFLP) technique was used for identification and characterization of Egyptian field strains of infectious bursal disease viruses (IBDVs) that caused severe outbreaks with (30%-60%) mortality rate. Twenty-four bursal samples collected from 24 field outbreaks in commercially reared chicken flocks experiencing signs typical of infectious bursal disease (IBD) were used. Ten of the bursal samples examined were determined to contain IBDV as evidenced by amplification of a 743-bp region of the VP2 gene of IBDV by RT-PCR. The RT-PCR products of the detected viruses were characterized by digestion with three restriction enzymes, BstN, MboI, and SpI. Three different RT-PCR/RFLP profiles were observed. Seven of the detected viruses had RFLP profiles identical to the very virulent European strains of IBDV (vvIBDVs). One virus had a RFLP profile identical to the U.S. classic vaccine strain, and one virus had a unique RFLP profile. The clinical history of the outbreaks and the presence of the SspI site in the 743-bp RT-PCR fragment were the criteria for designating the viruses as belonging to the very virulent (vv) phenotype.