Acute infectious bursal disease in poultry: Immunological and molecular basis of antigenicity of a highly virulent strain

Comparative Pathogenicity of Malaysian variant and very virulent infectious bursal disease viruses in chickens

Variant infectious bursal disease virus (vaIBDV) has been identified in various countries with significant economic losses. Recently, the first identification of variant strain in Malaysia was reported. The pathogenicity of the Malaysian variant, UPM1432/2019 and very virulent infectious bursal disease virus (vvIBDV), UPM1056/2018 strains were comparatively evaluated in specific-pathogen-free (SPF) chickens based on gross and histopathological examinations and viral load. Four-week-old SPF chickens were randomly divided into 3 groups; Group 1 served as the control, while groups 2 and 3 birds were challenged with the vaIBDV and vvIBDV, respectively. Three birds from each group were weighed, euthanised and necropsied at 2-, 3-, 4-, 5-, 7- and 21-days post-challenge (dpc). Unlike UPM1056/2018 group, birds from UPM1432/2019 group did not show clinical signs or death. UPM1056/2018 strain caused 11% mortality rate in the infected chickens. The bursal body index (BBIX) for UPM1432/2019- and UPM1056/2018 groups was < 0.7 from 2 dpc and continued to decrease to 0.49 and 0.45, respectively at 21 dpc. UPM1432/2019 strain was more persistent in the bursa than UPM1056/2018 strain. Both strains induced similar pathological lesions in SPF chicks. These results indicate that the Malaysian vaIBDV severely damaged the immune organs of chickens and was more persistent in bursal tissue than vvIBDV. The study provides insight into the pathogenicity of the variant strain as further study may be required to evaluate the efficacy of the current available IBD vaccines in Malaysia against the strain.

Review of Poultry Recombinant Vector Vaccines

The control of poultry diseases has relied heavily on the use of many live and inactivated vaccines. However, over the last 30 yr, recombinant DNA technology has been used to generate many novel poultry vaccines. Fowlpox virus and turkey herpesvirus are the two main vectors currently used to construct recombinant vaccines for poultry. With the use of these two vectors, more than 15 recombinant viral vector vaccines against Newcastle disease, infectious laryngotracheitis, infectious bursal disease, avian influenza, and Mycoplasma gallisepticum have been developed and are commercially available. This review focuses on current knowledge about the safety and efficacy of recombinant viral vectored vaccines and the mechanisms by which they facilitate the control of multiple diseases. Additionally, the development of new recombinant vaccines with novel vectors will be briefly discussed.

Genetic Diversity of Recent Infectious Bursal Disease Viruses Isolated From Vaccinated Poultry Flocks in Malaysia

Vaccination is an essential component in controlling infectious bursal disease (IBD), however, there is a lack of information on the genetic characteristics of a recent infectious bursal disease virus (IBDV) that was isolated from IBD vaccinated commercial flocks in Malaysia. The present study investigated 11 IBDV isolates that were isolated from commercial poultry farms. The isolates were detected using reverse transcription-polymerase chain reaction (RT-PCR) targeting the hypervariable region (HVR) of VP2. Based on the HVR sequences, five isolates (IBS536/2017, IBS624/2017, UPM766/2018, UPM1056/2018, and UPM1432/2019) were selected for whole-genome sequencing using the MiSeq platform. The nucleotide and amino acid (aa) sequences were compared with the previously characterized IBDV strains. Deduced aa sequences of VP2HVR revealed seven isolates with 94–99% aa identity to very virulent strains (genogroup 3), two isolates with 97–100% aa identity to variant strains (genogroup 2), and two strains with 100% identity to the vaccine strain (genogroup 1) of IBDV. The phylogenetic analysis also showed that the isolates formed clusters with the respective genogroups. The characteristic motifs 222T, 249K, 286I, and 318D are typical of the variant strain and were observed for UPM1219/2019 and UPM1432/2019. In comparison, very virulent residues such as 222A, 249Q, 286T, and 318G were found for the vvIBDV, except for the UPM1056/2018 strain with a A222T substitution. In addition, the isolate has aa substitutions such as D213N, G254D, S315T, S317R, and A321E that are not commonly found in previously reported vvIBDV strains. Unlike the other vvIBDVs characterized in this study, UPM766/2018 lacks the MLSL aa residues in VP5. The aa tripeptides 145/146/147 (TDN) of VP1 were conserved for the vvIBDV, while a different motif, NED, was observed for the Malaysian variant strain. The phylogenetic tree showed that the IBDV variant clustered with the American and Chinese variant viruses and are highly comparable to the novel Chinese variants, with 99.9% identity. Based on the sequences and phylogenetic analyses, this is the first identification of an IBDV variant being reported in Malaysia. Further research is required to determine the pathogenicity of the IBDV variant and the protective efficacy of the current IBD vaccines being used against the virus.

A unified genotypic classification of infectious bursal disease virus based on both genome segments

Infectious bursal disease virus (IBDV) of chickens is a birnavirus with a bi-segmented double-stranded RNA genome, the segments designated as A and B. We performed phylogenetic analysis using a 366-bp fragment of segment A (nt 785-1150) and a 508-bp fragment of segment B (nt 328-835) of IBDV. A total of 463 segment A and 434 segment B sequences from GenBank, including the sequences of eight recent Bangladeshi isolates, were used in the analysis. The analysis revealed eight genogroups of segment A under serotype 1, designated as A1 (classical), A2 (US antigenic variant), A3 (very virulent), A4 (dIBDV), A5 (atypical Mexican), A6 (atypical Italian), A7 (early Australian) and A8 (Australian variant), and a single genogroup under serotype 2, designated as A0. On the other hand, segment B could be categorized into five genogroups irrespective of serotype, these being B1 (classical-like), B2 (very virulent-like), B3 (early Australian-like), B4 (Polish & Tanzanian) and B5 (Nigerian). Segment B of serotype 2 strains clustered within genogroup B1. With the bi-segmented genome of IBDV, these differences would allow for a total of 45 possible assortments. Based on the combinations of segment A and segment B genogroups observed in 463 IBDV strains, a total of 15 genotypes could be recognized. Recent Bangladeshi IBDV strains, isolated in 2016, appeared to be segment reassortants having segment A of genogroup A3 (very virulent) and segment B of genogroup B3 (early Australian-like). An extended system of nomenclature of IBDV strains is proposed.

Potential reverse spillover of infectious bursal disease virus at the interface of commercial poultry and wild birds

Recently, multiple spillover events between domesticated poultry and wild birds have been reported for several avian viruses. This phenomenon highlights the importance of the livestock-wildlife interface in the possible emergence of novel viruses. The aim of the current study was to investigate the potential spillover and epidemiological links of infectious bursal disease virus (IBDV) between wild birds and domestic poultry. To this end, twenty-eight cloacal swabs were collected from four species of free-living Egyptian wild birds (i.e. mallard duck, bean goose, white-fronted goose and black-billed magpie). Genetic and phylogenetic analysis of three positive isolates revealed that the IBDV/USC-1/2019 strain clustered with previously reported very virulent IBDV (vvIBDV) Egyptian isolates. Interestingly, two other wild bird-origin isolates (i.e. IBDV/USC-2/2019 and IBDV/USC-3/2019) grouped with a vaccine strain that is being used in commercial poultry. In conclusion, our results revealed the molecular detection of vaccine and vvIBDV-like strains in Egyptian wild birds and highlighted the potential role of wild birds in IBDV epidemiology in disease-endemic regions.

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.

Characterization of the highly immunogenic VP2 protrusion domain as a diagnostic antigen for members of Birnaviridae family

Birnaviridae is a family of viruses (birnaviruses) which consists of four genera, members of which cause diseases in fish, birds, mollusks, and insects. The genome of birnaviruses encodes the highly immunogenic VP2 capsid protein. In order to demonstrate that the VP2 protein can be exploited as a diagnostic antigen for birnaviruses, we developed a lateral flow assay based on the surface-exposed VP2 protrusion domain of a representative birnavirus, infectious bursal disease virus (IBDV) of serotype 1 which causes the highly devastating infectious bursal disease in chickens. The biophysical characterization of the purified domain reveals that the domain predominantly consists of β-sheets, exists in a trimeric form, and remains folded at high temperatures, making it suitable for diagnostic purposes. Owing to its highly immunogenic nature and excellent biophysical properties, we employed the VP2 protrusion domain in a gold nanoparticle-based lateral flow assay for rapid detection of anti-IBDV antibodies in serum samples of infected chickens. Our results indicate that the domain binds anti-IBDV antibodies with high specificity during laboratory testing and on-site testing. The lateral flow assay reported here yields comparable results in a qualitative manner as obtained through a commercial enzyme-linked immunosorbent assay (ELISA). As VP2 is a common capsid protein of birnaviruses, the lateral flow assay can be generalized for other birnaviruses, and members of Tetraviridae and Nodaviridae families which contain homologous VP2 capsid proteins.

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.

Phylodynamic analyses of Brazilian antigenic variants of infectious bursal disease virus

Infectious bursal disease virus (IBDV) is a very important pathogen to poultry production and it is classified into three main groups: classical virulent (cvIBDV), very virulent (vvIBDV) and antigenic variants (avIBDV). This last group is composed by five different genetic lineages (recently classified in genogroups G2, G4, G5, G6, and G7) distributed in specific regions around the world. Brazil is one of the biggest poultry producers in the world and the present study aimed to investigate the evolutionary history of avIBDVs of the genogroup G4 in Brazil. A total of 5331 IBDV positive bursa samples, from different Brazilian poultry flocks, were genotyped in a period of ten years (2005 to 2014) and 1888 (35.42%) were identified as local avIBDVs. The highly variable region of the viral protein 2 (hvvp2) gene of 28 avIBDVs was sequenced and used in phylogenetic analyses and evaluation of local amino acid signatures. In addition, all complete and partial IBDV vp2 gene sequences, with local and year of collection information available on GenBank, were retrieved. Phylogenetic analyses were carried out based on a maximum likelihood method for the classification of genogroups occurring in Brazil. Based on a Maximum Likelihood (ML) phylogenetic tree, all Brazilian avIBDVs grouped into the genogroup 4. Bayesian phylodynamics analysis demonstrated the ancestor virus of this group was probably introduced in South America in 1968 (1960 to 1974, 95% HPD) and in Brazil in 1974 (1968 to 1977, 95% HPD) and the most likely source was East Europe (Hungary or Poland). All Brazilian avIBDV sequences, as well as the other genogroup 4 sequences, showed a specific pattern of amino acid: S222, T272, P289, I290, and F296. This report brings new insights about the IBDV epidemiology in Brazil and South America.

Antigenicity, pathogenicity and immunosuppressive effect caused by a South American isolate of infectious bursal disease virus belonging to the "distinct" genetic lineage

Infectious bursal disease virus (IBDV) is the causative agent of a highly contagious immunosuppressive disease affecting young chickens. The recently described "distinct IBDV" (dIBDV) genetic lineage encompasses a group of worldwide distributed strains that share conserved genetic characteristics in both genome segments making them unique within IBDV strains. Phenotypic characterization of these strains is scarce and limited to Asiatic and European strains collected more than 15 years ago. The present study aimed to assess the complete and comprehensive phenotypic characterization of a recently collected South American dIBDV strain (1/chicken/URY/1302/16). Genetic analyses of both partial genome segments confirmed that this strain belongs to the dIBDV genetic lineage and that it is not a reassortant. Antigenic analysis with monoclonal antibodies indicated that this strain has a particular antigenic profile, similar to that obtained in a dIBDV strain from Europe (80/GA), which differs from those previously found in the traditional classic, variant and very virulent strains. Chickens infected with the South American dIBDV strain showed subclinical infections but had a marked bursal atrophy. Further analysis using Newcastle disease virus-immunized chickens, previously infected with the South American and European dIBDV strains, demonstrated their severe immunosuppressive effect. These results indicate that dIBDV strains currently circulating in South America can severely impair the immune system of chickens, consequently affecting the local poultry industry. Our study provides new insights into the characteristics and variability of this global genetic lineage and is valuable to determine whether specific control measures are required for the dIBDV lineage. Research Highlights A South American strain of the dIBDV lineage was phenotypically characterized. The strain produced subclinical infections with a marked bursal atrophy. Infected chickens were severely immunosuppressed. The dIBDV strains are antigenically divergent from other IBDV lineages.

Molecular characterization of very virulent infectious bursal disease virus strains circulating in Egypt from 2003 to 2014

In the present study, four very virulent infectious bursal disease virus (vvIBDV) isolates from flocks of chickens with vaccination failure in Egypt in 2003, 2007, 2010 and 2014 were characterized. The four viruses, designated USC2003, USC2007, USC2010 and USC2014, were detected by reverse transcription PCR, subjected to sequencing of both genomic segments (A and B) and compared with geographically and phylogenetically diverse IBDV strains. Phylogenetic analysis of segment A (complete) and B (partial) revealed a close relationship between Egyptian and vvIBDV reference strains of European and Asian origin. The sequences of segments of A and B the current Egyptian isolates were 96.1-98.2% and 96.5-98.7% identical, respectively, to those of other known vvIBDV isolates. The deduced amino acid sequences of VP1, polyprotein (pVP2-VP4-VP3) and VP5 revealed the presence of putative virulence determinants of Egyptian isolates compared with vvIBDV and less virulent (classical and variant) strains. The Egyptian isolates also possess unique amino acids substitutions within the hypervariable region of VP2 that differ from those of other reference IBDV strains. Further studies may be necessary to determine the pathogenic significance of these amino acid substitutions to fully understand the molecular epidemiology and evolution of IBDV.

Characterization of two recombinant HVT-IBD vaccines by VP2 insert detection and cell-mediated immunity after vaccination of specific pathogen-free chickens

Infectious bursal disease (IBD) is an avian viral disease that causes severe economic losses in the poultry industry worldwide. The live IBD virus (IBDV) has a potential immunosuppressive effect. Currently available IBDV vaccines have shortcomings, prompting the development of safer and more effective vaccination approaches, including the use of the recombinant turkey herpesvirus vaccine expressing the immunogenic structural VP2 protein of IBDV (recombinant HVT (rHVT)-IBD). The objectives of this study were twofold: (i) to develop in vitro assays and molecular tools to detect the VP2 protein and gene and (ii) to evaluate cell-mediated immunity (CMI) induced by rHVT-IBD vaccination of day-old specific pathogen-free chickens. The VP2 protein expressed by rHVT-IBD-infected chicken embryo fibroblasts was detected using the enzyme-linked immunosorbent assay and immunofluorescence. Using molecular techniques, the VP2 gene was detected in various organs, providing a method to monitor vaccine uptake. rHVT-IBD vaccination induced CMI responses in specific pathogen-free chickens at 5 weeks. CMI was detected by measuring chicken interferon-gamma after ex vivo antigenic stimulation of splenocytes. Moreover, our results showed that the enzyme-linked immunospot approach is more sensitive in detecting chicken interferon-gamma than enzyme-linked immunosorbent assay. The tools developed in this study may be useful in the characterization of new-generation recombinant vaccines and the cellular immune response they induce.

Development of an RT-qPCR assay for the specific detection of a distinct genetic lineage of the infectious bursal disease virus

The infectious bursal disease virus (IBDV) is a major health threat to the world's poultry industry despite intensive controls including proper biosafety practices and vaccination. IBDV (Avibirnavirus, Birnaviridae) is a non-enveloped virus with a bisegmented double-stranded RNA genome. The virus is traditionally classified into classic, variant and very virulent strains, each with different epidemiological relevance and clinical implications. Recently, a novel worldwide spread genetic lineage was described and denoted as distinct (d) IBDV. Here, we report the development and validation of a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) assay for the specific detection of dIBDVs in the global poultry industry. The assay employs a TaqMan-MGB probe that hybridizes with a unique molecular signature of dIBDV. The assay successfully detected all the assessed strains belonging to the dIBDV genetic lineage, showing high specificity and absence of cross-reactivity with non-dIBDVs, IBDV-negative samples and other common avian viruses. Using serial dilutions of in vitro-transcribed RNA we obtained acceptable PCR efficiencies and determination coefficients, and relatively small intra- and inter-assay variability. The assay demonstrated a wide dynamic range between 10³ and 10⁸ RNA copies/reaction. This rapid, specific and quantitative assay is expected to improve IBDV surveillance and control worldwide and to increase our understanding of the molecular epidemiology of this economically detrimental poultry pathogen.

Molecular characterization of infectious bursal disease virus (IBDV) isolated in Argentina indicates a regional lineage

In Argentina, classical vaccines are used to control infectious bursal disease virus (IBDV); however, outbreaks of IBDV are frequently observed. This could be due to failures in the vaccination programs or to the emergence of new strains, which would be able to break through the protection given by vaccines. Hence, genetic characterization of the viruses responsible for the outbreaks that occurred in recent years is crucial for the evaluation of the control programs and the understanding of the epidemiology and evolution of IBDV. In this study, we characterized 51 field samples collected in Argentina (previously identified as IBDV positive) through the analysis of previously identified apomorphic sequences. Phylogenetic analysis of regVP2 showed that 42 samples formed a unique cluster (Argentinean lineage), seven samples were typical classical strains (one of them was a vaccine strain), and two belonged to the very virulent lineage (vvIBDV). Interestingly, when the analysis was performed on the regVP1 sequences, the field samples segregated similarly to regVP2; thus, we observed no evidence of a reassortment event in the Argentinean samples. Amino acid sequence analysis of regVP2 showed a particular pattern of residues in the Argentinean lineage, particularly the presence of T272, P289 and F296, which had not been reported before as signature sequences for any IBDV phenotype. Notably, the residue S254, characteristic of the antigenic variant, was not present in any of the Argentinean samples.

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.

Characteristics of very virulent infectious bursal disease viruses isolated from Chinese broiler chickens (2012-2013)

The objective of this study was to characterize the infectious bursal disease viruses (IBDVs) circulating in broiler chicken farms in China between 2012 and 2013. The VP2 gene sequences of nine newly isolated IBDVs, obtained using reverse transcriptase polymerase chain reaction, were determined and compared with worldwide reference isolates, which have been previously well characterized. Phylogenetic analysis revealed that the nine broiler IBDV isolates are closely related to very virulent IBDV (vvIBDV) strains. Analysis of the predicted amino acid sequences of VP2 from the nine vvIBDVs isolated from the broilers revealed that they share 99.2 to 100% sequence similarity. Additionally, amino acids A222, I242, I256, I294 and S299 of VP2 that are conserved among previously characterized vvIBDV strains are also encoded by the nine isolates. This study confirms the circulation of vvIBDVs in Chinese broiler chicken farms experienced slow evolution and was relatively stable in China.

Overexpression of recombinant infectious bursal disease virus (IBDV) capsid protein VP2 in the middle silk gland of transgenic silkworm

Infectious bursal disease virus (IBDV) is the causative agent of a highly contagious disease affecting young chickens and causes serious economic losses to the poultry industry worldwide. Development of subunit vaccine using its major caspid protein, VP2, is one of the promising strategies to protect against IBDV. This study aim to test the feasibility of using silkworm to produce recombinant VP2 protein (rVP2) derived from a very virulent strain of IBDV (vvIBDV). A total of 16 transgenic silkworm lines harboring a codon-optimized VP2 gene driven by the sericin1 promoter were generated and analyzed. The results showed that the rVP2 was synthesized in the middle silk gland of all lines and secreted into their cocoons. The content of rVP2 in the cocoon of each line was ranged from 0.07 to 16.10 % of the total soluble proteins. The rVP2 was purified from 30 g cocoon powders with a yield of 3.33 mg and a purity >90 %. Further analysis indicated that the rVP2 was able to tolerate high temperatures up to 80 °C, and exhibited specific immunogenic activity in mice. To our knowledge, this is the first report of overexpressing rVP2 in the middle silk gland of transgenic silkworm, which demonstrates the capability of silkworm as an efficient tool to produce recombinant immunogens for use in new vaccines against animal diseases.

Characterization of field and vaccine infectious bursal disease viruses from Nigeria revealing possible virulence and regional markers in the VP2 minor hydrophilic peaks

Outbreaks of infectious bursal disease in vaccinated chicken flocks are frequent in Nigeria. For the control of infectious bursal disease, live vaccines based on foreign infectious bursal disease virus (IBDV) strains are used. The present study investigated the phylogenetic relationship between field and vaccine IBDV strains from northwestern Nigeria. Thirty field IBDV strains and three commercial vaccines strains were characterized through sequencing the VP2 hypervariable region. In addition, the complete genome segment A coding region for two vaccines and two field strains was sequenced. The deduced amino acid sequences (position 212 to 331) of IBDV strains from Nigeria and other regions of the world were aligned and possible regional and virulence markers were identified associated with VP2 minor hydrophilic peaks. Reversion to virulence of a vaccine strain with a Q to L mutation at position 253 was observed. Phylogenetic analyses revealed a unique cluster of northwest Nigerian field IBDV strains alone or related to imported characterized classical and very virulent IBDV vaccines. The results suggest that when IBDV strains spread from their region of origin to a different region they mutate alongside indigenous field strains but may retain their identity on the VP2 region.

Molecular characterization of two Bangladeshi infectious bursal disease virus isolates using the hypervariable sequence of VP2 as a genetic marker

Two Bangladeshi infectious bursal disease virus (IBDV) isolates collected in 2007, termed GB1 and GB3, were subjected to comparative sequencing and phylogenetic analyses. Sequence analysis of a 474-bp hypervariable region in the VP2 gene revealed that among four major amino acid substitutions observed in the strains, two were unique to GB1 and GB3 (Ser217Leu and Ala270Thr) while one substitution was only found in GB1 (Asn299Ser). Among IBDVs from Bangladesh including GB1 and GB3, the rate of identity and homology was around 97~99%. The amino acid sequences of GB1 and GB3 differ from those of previous Bangladeshi IBDV isolates and contain amino acid substitutions Pro222Ala and Asn299Ser (in GB3 only). Phylogenetic analysis revealed that GB1 and GB3 are grouped with other very virulent IBDVs of European and American origin in contrast to two previously isolated Bangladeshi IBDV strains (GenBank accession Nos. AF362776 and AF260317), which belong to the Asian group. It was concluded that GB1 and GB3 belong to a very virulent group of IBDVs. However, amino acid sequences of GB1 and GB3 differ from those of the other Bangladeshi IBDVs by one or two amino acids encoded in the hypervariable region of the VP2 gene.

Transient expression of VP2 in Nicotiana benthamiana and its use as a plant-based vaccine against infectious bursal disease virus

Infectious Bursal Disease Virus (IBDV) is the etiological agent of an immunosuppressive and highly contagious disease that affects young birds. This disease causes important economic losses in the poultry industry worldwide. The VP2 protein has been used for the development of subunit vaccines in a variety of heterologous platforms. In this context, the aim of this study was to investigate VP2 expression and immunogenicity using an experimental plant-based vaccine against IBDV. We determined that the agroinfiltration of N. benthamiana leaves allowed the production of VP2 with no apparent change on its conformational epitopes. Chickens intramuscularly immunized in a dose/boost scheme with crude concentrated extracts developed a specific humoral response with viral neutralizing ability. Given these results, it seems plausible for a plant-based vaccine to have a niche in the veterinary field. Thus, plants can be an adequate system of choice to produce immunogens against IBDV.

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.

Phylogenetic evidence for homologous recombination within the family Birnaviridae

Birnaviruses are bi-segmented double-stranded RNA (dsRNA) viruses infecting insects, avian species and a wide range of aquatic species. Although homologous recombination is a common phenomenon in positive-sense RNA viruses, recombination in dsRNA viruses is rarely reported. Here we performed a comprehensive survey on homologous recombination in all available sequences (>1800) of the family Birnaviridae based on phylogenetic incongruence. Although inter-species recombination was not evident, potential intra-species recombination events were detected in aquabirnaviruses and infectious bursal disease virus (IBDV). Eight potential recombination events were identified and the possibility that these events were non-naturally occurring was assessed case by case. Five of the eight events were identified in IBDVs and all of these five events involved live attenuated vaccine strains. This finding suggests that homologous recombination between vaccine and wild-type IBDV strains may have occurred; the potential risk of mass vaccination using live vaccines is discussed. This is the first report of evidence for homologous recombination within the family Birnaviridae.

Molecular characteristics of infectious bursal disease viruses from asymptomatic broiler flocks in Europe

Infectious bursal disease virus (IBDV) exists in several different antigenic and pathogenic forms. The immune suppression caused by this virus in young chickens is not always associated with clinical signs of disease. The antigenic variant viruses originally described in the United States typically do not cause clinical signs of disease but can cause a marked immune suppression via the destruction of B lymphocytes. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay we conducted a survey of asymptomatic broiler chicken flocks in Europe for IBDV. Restriction fragment length polymorphisms in the viral protein 2 (VP2) gene of four isolates from Spain and four isolates from France indicated they may be different from the classic and very virulent (vv) IBDV strains found throughout Europe. Nucleotide sequence and phylogenetic analysis of the hypervariable region of the VP2 gene indicated that all eight viruses were more similar to U.S. variant viruses than classic viruses. In two viruses, one from France and one from Spain, threonine was observed at amino acid position 222 and serine was found at position 254. These two substitution mutations are characteristic of Delaware variant viruses. In addition, all eight viruses had mutated amino acid position 318 from glycine to aspartic acid, another substitution mutation commonly found in U.S. variant viruses. Although importation restrictions prevented us from directly testing the antigenicity of these viruses, their nucleotide and predicted amino acid sequences suggest they could be antigenically distinctive compared to classic and vvIBDV commonly found in Europe. Confirmation of the presence of antigenic variant IBDV strains in Europe requires additional immunologic studies to elucidate the exact nature of the viral epitopes. Our data support the need for these immunologic studies.

Characterization of Infectious Bursal Disease Viruses from Argentina

Infectious bursal disease (IBD) viruses detected in commercial flocks of different regions of Argentina were analyzed by reverse transcription-polymerase chain reaction-restriction fragment length polymorphism (RFLP) of a VP2 gene fragment, followed by sequence analysis. Two out of eight IBD viruses presented an SspI restriction site, typical of the very virulent phenotype. Three IBD viruses presented a SacI restriction site, typical of classic virulent strains, and one isolate presented restriction sites for both enzymes. The Argentine IBD viruses showed three different molecular patterns by RFLP with the restriction endonuclease BstNI and five different patterns with MboI. By comparison of nucleotide and deduced amino acid sequences of the hypervariable region of the VP2 protein, four Argentine viruses were found to be closely related to Brazilian subclinical strains and two isolates were found to be related to vaccine IBDV strains in use in Argentina. Strain LD9569 was genetically characterized as a very virulent strain and was found to be closely related to international and regional vvIBDV strains. This is the first report on variability of IBDV strains circulating in Argentina.

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 analysis of two epitope regions encoded by the VP2 gene of infectious bursal disease viruses

Infectious bursal disease virus (IBDV) causes an immunosuppressive disease in chickens and leads to severe economic losses in the poultry industry. Vaccination may not be effective if there is exposure of the vaccinated flock to a different antigenic subtype, which reinforces the importance of identification of new IBDV variants. The virus outer capsid is constituted of VP2, in which the major neutralizing epitopes are located. Forty-eight bursa samples collected from IBDV infected commercial broiler flocks in the US were analyzed by real-time RT-PCR using probes designed for two epitope regions of VP2 denominated minor peak 1 and peak B. It was observed that 23, 48 and 44 samples tested with the minor peak probes Del-E, STC and F15, respectively, had a lower melting temperature (Tm) than expected. Furthermore, 44, 41 and 48 samples tested with the Del-E, STC and F15 peak B probes, respectively, had a lower Tm compared to the control, which indicates the presence of one or more nucleotide mutations in the samples. This fact was confirmed by nucleotide sequencing which also demonstrated that most mutations resulted in amino acid substitutions. Real-time RT-PCR can be a useful tool to assist in the development of more effective vaccination strategies.

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.

Differentiation of infectious bursal disease viruses isolated in Croatia

Infectious bursal disease viruses (IBDVs) in 26 IBDV-positive bursa samples collected in Croatia during the period 1996-2000 and in two commercially available vaccines were differentiated by the presence or absence of the CfoI, SacI, SspI, StuI, and TaqI restriction sites in the 422-bp fragment of segment A of the VP2 gene (nt 732-1153). The fragments from 14 (54%) field isolates were TaqI+ StuI+ SspI+ and SacI- CfoI-, indicating their very virulent (vv) character. The presence of CfoI restriction site in 10 (38%) field isolates is uncommon for vvIBDV strains. It was detected in only the 88180 vvIBDV strain. Nevertheless, these isolates can be classified as vv strains according to TaqI+ StuI+ SspI+ SacI- restrictions. Two SacI+ StuI+ CfoI+ TaqI- SspI- field isolates (8%) could be classified as non-vvIBDVs. The StuI+ restriction is common to vvIBDV strains. However, the StuI recognition sequence is present in the F52/70 classic European and 002-73 attenuated strains as well. The SacI+ CfoI+ StuI- SspI- restrictions and the lack of the TaqI restriction at nt position 832 show that the IBDV in GUMBOKAL IM-SPF vaccine corresponds to the attenuated and/or vaccine strains. The TaqI restriction at nt position 875 suggests that the IBDV in GUMBOKAL SPF vaccine could belong to the mild strains.

Very virulent infectious bursal disease virus in southeastern Europe

Clinical outbreaks of severe acute infectious burial disease (IBD) were recorded since the mid- and late 1990s in several countries in the southeastern part of Europe. Epidemiologic data showed that both infectious bursal disease virus (IBDV)-vaccinated and IBDV-nonvaccinated chickens were affected with acute IBD and mortality up to 50% independent of the IBDV vaccination status of the appropriate parent flocks. For investigation of the causative agent of acute IBD, the variable region of VP2 was amplified, cloned, and sequenced. Nucleotide sequence analysis of polymerase chain reaction fragments showed several silent nucleotide exchanges in comparison with the sequence of the very virulent (vv) IBDV strain UK661. Also, restriction enzyme cleavage sites proposed specific for vvIBDV were present in all investigated strains. On the basis of clinical signs in affected flocks, recorded epidemiologic data, and sequence analysis, it is very likely the IBD-causing strains were of the vv phenotype.

Antigenic and molecular characterization of recent infectious bursal disease virus isolates in China

Eleven infectious bursal disease virus (IBDV) strains isolated recently from China were compared with the early classical virulent strain CJ801, the chicken embryo fibroblast-adapted (CEF) variant strain GZ902, and the attenuated vaccine strains BJ836, BK912, and LM to discern the evolutionary characteristics of IBDV in China at both antigenic and genetic levels. Virus neutralization (VN) assay showed that all ten very virulent (vv) IBDV strains belong to the same subtype as attenuated strains, whereas the other variant isolate strain BX could be attributed to other subtype of the variant strain GZ902. Antigen-capture ELISA (AC-ELISA) determined by a panel of monoclonal antibodies (Mabs) against classical and variant strains showed further that among these vv strains, nine strains except for strain NC had no reaction with neutralizing Mab B69. The vv strains SC and YV had no reaction with non-neutralizing Mabs 2B8 and 2C4, respectively, whose epitopes were located in classical IBDV strains. On the other hand, there is no alteration in antigenic epitopes located in the variant strain BX as that of the variant GZ902. Sequence comparison of the highly variable region (HVR) of the VP2 proteins showed that these vv strains had 98.6-100.0% identities to European and Asian vv strains at amino acid level. For the vv strains NC, SC, and YV, all had one amino acid substitution at the major hydrophilic domains, indicating that new vv strains are evolving. In addition, the vv strains DMS and NC had amino acid residue 279N as well, showing that the substitution of amino acid at this position might not be related to the virulence of IBDV. The variant strain BX had one amino acid substitution in the two major hydrophilic domains and two unique amino acids 249K and 254S as the other early variant strains, and shared 97.3% of amino acid identity to the variant strain VarE. Phylogenetic analysis suggests that the recent Chinese vvIBDVs and the previous European and Asian vv strains still belong to a genetic group and the variant strain BX to the other genetic group, which is more closely related to the European classical virulent strain F52/70 and the American classical virulent strain STC than to the early Chinese classical virulent strain CJ801, showing that the recent vv and variant strains that spread widely in the country might be derived from Europe and America than from early Chinese classical virulent strains.

Molecular and antigenic characterization of Bangladeshi isolates of infectious bursal disease virus demonstrate their similarities with recent European, Asian and African very virulent strains

Three isolates of infectious bursal disease virus (IBDV), obtained from chickens in Bangladesh in 1999 and designated as BD 1/99, BD 2/99 and BD 3/99, were characterized. In an antigen capture enzyme-linked immunosorbent assay using a panel of VP2-specific neutralizing monoclonal antibodies (mAb), all three isolates showed a mAb-binding profile similar to that of very virulent IBDV (vvIBDV) strains. In contrast to the classical virulent strains, they did not react with mAb 3 and mAb 4. Molecular characterization was performed by direct sequencing of a 677-base pair cDNA corresponding to the VP2 variable domain of the polyprotein gene, synthesized by a reverse transcription-polymerase chain reaction. In comparison to the classical virulent strains, the Bangladeshi isolates were found to have five amino acid substitutions in this region. Four of these changes, Pro222Ala, Val256Ile, Leu294Ile and Asn299Ser, were also observed in other vvIBDV strains. The fifth substitution, Glu300Ala, was similar to that in some African strains of IBDV. The results support the observation that antigenically and genetically similar vvIBDV strains, first observed in Europe in the late 1980s, have spread to most parts of the world in a short period of time.

Sequence comparison of the VP2 variable region of infectious bursal disease virus isolates from Vietnam

The variable region in the VP2 gene of twenty-three infectious bursal disease virus (IBDV) isolates, collected in Vietnam in 1997 and 1998, was amplified as cDNA by using the reverse transcription-polymerase chain reaction and sequenced. Analysis of amino acid substitutions and phylogenetic relationships of the deduced amino acid sequences (residues 206-350) showed that the nineteen Vietnamese vv IBDVs clustered with the European vv IBDVs, Japanese vv IBDVs and Chinese vv strains, and that the four vietnamese virulent strains were closely related to European virulent strain 52/70. These results suggest that Vietnamese vv IBDVs, European vv IBDVs, Japanese vv IBDVs and Chinese vv strains have the same origin.