Identification and assessment of virulence of a natural reassortant of infectious bursal disease virus

Genotype classification and pathogenicity of infectious bursal disease virus circulating in vaccinated broiler chicken farms

This study investigated the genotype classification and pathogenicity of infectious bursal disease virus (IBDV) circulating in vaccinated broiler chicken farms in Egypt. A total of 150 samples were collected from 30 vaccinated commercial broiler chicken farms and pooled into 30 working samples. IBDV was tested using reverse transcriptase polymerase chain reaction (RT-PCR) amplification of the hypervariable region of the viral protein 2 (hvVP2) and the VP1 gene 5' extremity. Both RT-PCR fragments were sequenced from six samples, and then the obtained nucleotide sequences were analyzed. The IBDV genotypes were identified using nucleotide sequences. Five sequences of the six strains examined were classified as genotype A3B2 for the highly virulent segments A and B (vv-A/vv-B IBDV). Interestingly, this study identified and classified a novel segment-reassortant strain as the A1B2 genotype. Specifically, it involved the segment reassortment of classical virulent segment A (cv-A) with vv-B producing cv-A/vv-B reassortant IBDV. Subsequently, we compared the pathogenicity of reassortant (cv-A/vv-B) IBDV and vvIBDV strains identified in this study. Both strains developed typical IBD clinical signs, postmortem lesions, histopathology, immunohistochemistry, and lesion scores, which were more severe in vvIBDV than reassortant IBDV. In conclusion, this is the first report of the genotype classification based on both genome segments (hvVP2 and VP1) with pathogenicity of IBDV circulating in vaccinated broiler chicken farms and this pathogenicity is more severe in vvIBDV strain than a novel reassortant IBDV strain.

Genetic Sequence and Pathogenicity of Infectious Bursal Disease Virus in Chickens in Egypt During 2017-2021

The continued circulation of infectious bursal disease virus (IBDV) in Egypt, despite the use of various vaccines, is a serious problem that requires continuous detection of IBDV. In the current study, real-time reverse transcriptase polymerase chain reaction testing of 100 diseased chicken flocks during 2017-2021 revealed the presence of very virulent IBDV (vvIBDV) in 67% of the flocks, non-vvIBDV in 11%, and a mixture of both vvIBDV and non-vvIBDV in 4%. Twenty-nine IBDV isolates were submitted for partial sequencing of the viral protein 2 hypervariable region (VP2-HVR), and 27 isolates were confirmed to be genogroup A3 (vvIBDV) with 96.3%-98.5% similarity to the global A3 (vvIBDV) and 88.9%-97% similarity to genogroup A1 vaccine strains. The remaining two isolates were non-vvIBDV and showed 91.1% and 100% identity with classical genogroup A1 strains, respectively. Furthermore, the sequence and phylogenetic analysis of VP1 (amino acids 33-254) of two selected isolates of A3, 5/2017 and 98/2021, clustered them as B2, vvIBDV-like, strains with high similarity (99.5%) to four Egyptian, 99% to Chinese and European, and 97.7% to Chinese and Polish vvIBDV isolates. Experimental infection of commercial broiler chickens with two vvIBDV-A3B2 isolates (5/2017 and 98/2021) showed no mortality despite typical tissue lesions, clear histopathological changes, and strong ELISA antibody response. Isolate 98/2021 was more pathogenic, as confirmed by histopathology, whereas isolate 5/2017 induced a stronger serological response. In conclusion, vvIBDV (A3B2) strains with two amino acid (aa) substitutions in VP1 as V141I and V234I as well as VP2 as Y220F and G254S are still circulating in Egypt.

Efficacy, immunogenicity, and virus shedding in broiler chickens inoculated with live attenuated fowl adenovirus serotype 8b propagated a bioreactor

Background

Fowl adenovirus (FAdV) 8b causes huge economic losses in the poultry industry worldwide. Attenuated FAdV 8b could be useful in preventing FAdV infections globally and scale-up obstacles could be solved by bioreactor technology.

Aim

This study was carried out to attenuate the FAdV 8b isolate, propagate it in a bioreactor, molecularly characterize the passage isolates, and determine the immunogenicity, efficacy, and shedding of the virus of chickens.

Methods

FAdV serotype 8b (UPM11142) isolate was passaged on chicken embryo liver (CEL) cells until attenuation and propagated in a bioreactor (UPM11142P20B1). Hexon and fiber genes of the isolates were sequenced and analyzed. UPM11142P20B1 was administered to 116-day-old broiler chickens divided into four groups, A (control), B (non-booster), C (booster with UPM11142P20B1), and D (booster with inactivated UPM11142P5B1). Eight chickens from each group were challenged. Body weight (BW) and liver weight (LW), liver: BW ratio (LBR), FAdV antibody titer, T lymphocyte sub-populations in the liver, spleen and thymus; and challenge virus load in the liver and shedding in cloaca were measured at weekly intervals.

Results

The isolate caused typical cytopathic effects on CEL cells typical of FAdV. Novel molecular changes in the genes occurred which could be markers for FAdV 8b attenuation. BW, LW, and LBR were similar among groups throughout the trial but the uninoculated control-challenged group (UCC) had significantly higher LBR than the inoculated and challenged groups at 35 dpi. Non-booster group had higher FAdV antibodies at all time points than the uninoculated control group (UCG); and the challenged booster groups had higher titer at 35 dpi than UCC. T lymphocytes increased at different time-points in the liver of inoculated chickens, and in the spleen and thymus as well, and was higher in the organs of inoculated challenged groups than the UCC. There was a significantly higher challenge virus load in the liver and cloaca of UCC chickens than in the non-booster chickens.

Conclusion

UPM11142P20B1 was safe, efficacious, significantly reduced shedding, and is recommended as a candidate vaccine in the prevention and control of FAdV 8b infections in broiler chickens.

First Detection and Molecular Characterization of Novel Variant Infectious Bursal Disease Virus (Genotype A2dB1b) in Egypt

Infectious bursal disease (IBD) is an immunosuppressive disease causing significant damage to the poultry industry worldwide. Its etiological agent is infectious bursal disease virus (IBDV), a highly resistant RNA virus whose genetic variability considerably affects disease manifestation, diagnosis and control, primarily pursued by vaccination. In Egypt, very virulent strains (genotype A3B2), responsible for typical IBD signs and lesions and high mortality, have historically prevailed. The present molecular survey, however, suggests that a major epidemiological shift might be occurring in the country. Out of twenty-four samples collected in twelve governorates in 2022-2023, seven tested positive for IBDV. Two of them were A3B2 strains related to other very virulent Egyptian isolates, whereas the remaining five were novel variant IBDVs (A2dB1b), reported for the first time outside of Eastern and Southern Asia. This emerging genotype spawned a large-scale epidemic in China during the 2010s, characterized by subclinical IBD with severe bursal atrophy and immunosuppression. Its spread to Egypt is even more alarming considering that, contrary to circulating IBDVs, the protection conferred by available commercial vaccines appears suboptimal. These findings are therefore crucial for guiding monitoring and control efforts and helping to track the spread of novel variant IBDVs, possibly limiting their impact.

A rapid and affordable amplicon-based method for next-generation genome sequencing of the infectious bursal disease virus

The infectious bursal disease virus (IBDV) causes a severe immunosuppressive disorder in young chickens. IBDV evolution resulted in the emergence of strains with divergent genetic, antigenic, and pathogenic characteristics. Genetic classification is typically performed by sequencing the coding region of the most immunogenic region of the viral protein 2 (VP2). Sequencing both double-stranded RNA genome segments is essential to achieve a more comprehensive IBDV classification that can detect recombinants and reassortments. Here, we report the development and standardization of a tiled PCR amplicon protocol for the direct and cost-effective genome sequencing of global IBDV strains using next-generation technology. Primers for tiled PCR were designed with adapters to bypass expensive and time-consuming library preparation steps. Sequencing was performed on Illumina MiniSeq equipment, and fourteen complete genomes of field strains were assembled using reference sequences. The PCR-enrichment step was used to obtain genomes from low-titer biological samples that were difficult to amplify using traditional sequencing. Phylogenetic analyses of the obtained genomes confirmed previous strain classification. By combining the enrichment methodology with massive sequencing, it is possible to obtain IBDV genomic sequences in a fast and affordable manner. This procedure can be a valuable tool to better understand virus epidemiology.

Whole Genome Sequencing of Infectious Bursal Disease Viruses Isolated from a Californian Outbreak Unravels the Underlying Virulence Markers and Highlights Positive Selection Incidence

Outbreaks of the immunosuppressive infectious bursal disease (IBD) are frequently reported worldwide, despite the vaccination regimes. A 2009 Californian IBD outbreak caused by rA and rB isolates was described as very virulent (vv) IBD virus (IBDV); however, molecular factors beyond this virulence were not fully uncovered. Therefore, segments of both isolates were amplified, successfully cloned, whole genome sequenced by Next Generation Sequencing, genotyped, and the leading virulence factors were entirely investigated in terms of phylogenetic and amino acid analysis and protein modeling for positive selection orientation and interaction analysis. rA and rB isolates displayed the highest amino acid identity (97.84-100%) with Genotype 3 strains. Interestingly, rA and rB contained all virulence hallmarks of hypervariable (HVR), including 222A, 242I, 249Q, 256I, 284A, 286T, 294I, 299S, and 318G, as well as the serine-rich heptapeptide sequence. Moreover, we pinpointed the A3B2 genotype of rA and rB, predominant in non-reassortants, and we highlighted the absence of recombination events. Furthermore, gene-wise phylogenetic analysis showed the entire genes of rA and rB clustered with the vvIBDVs and emphasized their share in IBDV virulence. VP5 showed a virulence marker, MLSL (amino acid sequence). VP2 encountered three significant novel mutations apart from the HVR, including G163E in rA and Y173C and V178A in rB, all residing within interacting motifs. VP4 contained 168Y, 173N, 203S, and 239D characteristic for the vv phenotype. A235V mutation was detected at the dsRNA binding domain of VP3. In VP1, the TDN triplet and the mutation (V4I) were detected, characteristic of hypervirulence occurring at the N-terminus responsible for protein priming. Although selection analysis revealed seven sites, codon 222 was the only statistically significant selection site. The VP2 modeling of rA and rB highlighted great structure fitness, with 96.14% Ramachandran favored positioning including the 222A, i.e., not influencing the structure stability. The 222A was found to be non-interface surface residue, associated with no interaction with the attachment-mediated ligand motif. Our findings provide pivotal insights into the evolution and underlying virulence factors and will assist in the development of control strategies via sequence-based continuous monitoring for the early detection of novel vv strains.

Genetic Insight into the Interaction of IBDV with Host-A Clue to the Development of Novel IBDV Vaccines

Infectious bursal disease virus (IBDV) is an immunosuppressive pathogen causing enormous economic losses to the poultry industry across the globe. As a double-stranded RNA virus, IBDV undergoes genetic mutation or recombination in replication during circulation among flocks, leading to the generation and spread of variant or recombinant strains. In particular, the recent emergence of variant IBDV causes severe immunosuppression in chickens, affecting the efficacy of other vaccines. It seems that the genetic mutation of IBDV during the battle against host response is an effective strategy to help itself to survive. Therefore, a comprehensive understanding of the viral genome diversity will definitely help to develop effective measures for prevention and control of infectious bursal disease (IBD). In recent years, considerable progress has been made in understanding the relation of genetic mutation and genomic recombination of IBDV to its pathogenesis using the reverse genetic technique. Therefore, this review focuses on our current genetic insight into the IBDV's genetic typing and viral genomic variation.

Infectious bursal disease virus in Western Europe: the rise of reassortant strains as the dominant field threat

Infectious bursal disease virus (IBDV) is a highly contagious birnavirus causing a burdensome immunosuppressive disease in chickens. IBDV features a remarkable antigenic, pathogenic and genetic heterogeneity, with significant implications on disease manifestation, control measures and diagnostic approaches. The recent proposals of comprehensive phylogenetic classification systems offered the ideal platform for large-scale molecular surveys, which are crucial to gather epidemiological data and inform control efforts. In this study, the IBDV scenario was investigated in most of Western Europe by considering the results of diagnostic activities performed internationally throughout 2021. In total, 470 bursal samples from nine different countries were analysed by RT-PCR targeting the VP2. When a field virus was identified, the VP1 was also characterized. Most of the 132 detected field viruses were highly homologous reassortants featuring a very virulent-like VP2 and a classical-like VP1 (genotype A3B1). Despite emerging recently, these reassortants were already signalled in several countries in North-Western Europe and associated with subclinical infections. Here, we report their further spread in the region, where they currently represent the dominant field threat. Two other IBDV types were found, one in Italy, where all the identified viruses clustered in a clade of the A3B1 genotype previously reported only in Russia and the Middle East, and the other in Portugal, where the recently characterized A9B1 genotype was confirmed to circulate. The obtained data suggest the recent occurrence of a major shift in the Western European epidemiological landscape of IBDV, stressing the importance of steady monitoring and sharing of information among different countries and laboratories.RESEARCH HIGHLIGHTS The IBDV scenario in Western Europe seems to have radically changed in recent years.IBDV reassortants were found to be the dominant field type in the region.Local circulation of two other IBDV types was detected in Italy and Portugal.

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.

Genetics and Pathogenicity of Natural Reassortant of Infectious Bursal Disease Virus Emerging in Latvia

Infectious bursal disease virus is an immunosuppressive pathogen that, despite applied vaccination, is affecting the poultry industry worldwide. This report presents the genetic and pathotypic characterization of a natural reassortant emerging in Europe (Latvia). Genetic characterization showed that strain 25/11/Latvia/2011 represents genotype A3B1, whose segment A is derived from very virulent strains, while segment B is from the classic-like genogroup. Phylogenetic maximum likelihood inference of the B-segment sequence clustered the reassortant strain together with the US antigenic variant E strain. However, the obtained full-length sequence of 25/11/Latvia/2011 revealed that not only reassortment but also dozens of mutations shaped the unique genetic makeup. Phenotypic characterization showed no mortality and no clinical signs of disease but a severe bursa of Fabricius atrophy and splenomegaly in the convalescent birds at 10 days post infection. The results obtained indicate that the acquired genetic constellation contributed to a decrease in virulence; nevertheless, the infection causes severe damage to lymphoid organs, which can lead to impaired immune responses.

Research Note: “Hidden” infectious bursal disease virus infections in Central Europe

Infectious bursal disease virus (IBDV) is a major threat to the poultry industry globally, represented by a variety of genetic, pathogenic, and antigenic variants. The recognition of the infection may be challenging due to several factors, as the virulence of the strain, age, and immune status of the birds at infection, to name the most important ones.

Here we report about the molecular typing of IBDVs detected over the recent years in Central Europe. The results revealed the diversity of IBDV in the region, that is, very virulent strains being present in all four involved countries, the successive detection of a recently described reassortant variant in the Czech Republic, and the “rediscovery” of a subclinical pathotype virus in Hungary. These findings highlight the need for monitoring the flocks regularly not only by evaluating the production parameters but to look specifically for the occurrence of IBDV and adjust the control measures according to the results.

Differential Modulation of Innate Antiviral Profiles in the Intestinal Lamina Propria Cells of Chickens Infected with Infectious Bursal Disease Viruses of Different Virulence

Infectious bursal disease virus (IBDV) is one of the most important infectious diseases of poultry around the world. Gut-associated lymphoid tissues (GALT) are the first line of defense of the host against the infection. The purpose of this study was to investigate the role of innate immune antiviral signaling triggered by Toll-like receptor 3 (TLR3), as well as macrophage activation and cytokine response in the intestinal lamina propria (ILP) cells after the oral challenge of IBDV in relation to IBDV virulence and disease pathogenesis. The results showed that the expression levels of TLR3, IRF7, IFN-α/β and the corresponding downstream antiviral factors OAS, PKR and Mx were all upregulated in the SPF chicken ILP cells at 8 h post-infection (hpi) and 12 hpi. Similarly, macrophages were activated, with the initial macrophage M1 activation observed at 8 hpi, but then it rapidly shifted to a non-protective M2-type. Both Th1 (IFN-γ, TNF-α, IL-12) and Th2 (IL-4 and IL-10) types of cytokines were differentially upregulated during the early stage of infection; however, the Th1 cytokines exhibited stronger activation before 8 hpi compared to those of the Th2 cytokines. Interestingly, differential regulations of gene expression induced by different IBDV strains with different virulence were detected. The HLJ0504-like very virulent (vv) IBDV strain NN1172 induced stronger activation of TLR3-IFN-α/β pathway, macrophages and the Th1/2 cytokines’ expression, compared to those induced by the attenuated strain B87 at 8 hpi and 12 hpi in the ILP cells. In conclusion, the innate antiviral response mediated by the TLR3-IRF7 pathway, macrophage activation and cytokine expression in the GALT cells at the early stage of IBDV infection was differentially modulated, and the HLJ0504-like vvIBDV strain triggered stronger activation than the attenuated vaccine strain, and that may play an important role in the progression of disease.

Genotyping and Molecular Characterization of Infectious Bursal Disease Virus Identified in Important Poultry-Raising Areas of China During 2019 and 2020

Infectious bursal disease (IBD) is an acute and highly contagious immunosuppressive disease caused by the infectious bursal disease virus (IBDV), which seriously threatens the healthy development of the poultry industry. Since its spread to China in the early 1990s, the very virulent IBDV (vvIBDV) characterized by high lethality, has been the focus of prevention and control. However, the novel variant IBDV (nVarIBDV), which has been widely prevalent in China since 2017, has brought a new threat to the poultry industry. In this study, the prevalence of IBDV in the important poultry-raising areas of China from 2019 to 2020 was detected. Of these, 45.1% (101/224) of the samples and 61.9% (26/42) of the chicken flocks were shown to be positive for IBDV. For 50 IBDVs, the sequences of the hypervariable region of the VP2 gene in segment A and of the B-marker of the VP1 gene in segment B were analyzed. The results revealed the coexistence of a number of different IBDV genotypes, including A2dB1 (nVar, 26/50, 52.0%), A3B3 (HLJ0504-like, 15/50, 30.0%), A1B1 (classical, 1/50, 2.0%), and A8B1 (attenuated, 1/50, 2.0%). This indicated that the newly emerging nVarIBDV of A2dB1 and the persistently circulating HLJ0504-like vvIBDV of A3B3 are the two important epidemic strains. Furthermore, we established that segment reassortment has occurred among these circulating strains. This study is the first to reveal the novel epidemic characteristics of IBDV since the report of the emerging nVarIBDV of A2dB1 in China.

Characterization and pathogenicity of a naturally reassortant and recombinant infectious bursal disease virus in China

Infectious bursal disease virus (IBDV), an Avibirnavirus, is the pathogen of infectious bursal disease, which is a severely immunosuppressive disease in 3-15-week-old chickens. Different phenotypes of IBDV, including classical, variant, very virulent (vv) and attenuated IBDV, have been reported in many chicken-rearing countries worldwide. Here, we isolated and identified a naturally reassortant and recombinant IBDV (designated GXB02) from 20-day-old chickens with clinicopathological changes of infectious bursal disease (IBD) in Guangxi Province, China. Whole genomic sequencing showed that the strain GXB02 simultaneously has both reassortant and recombinant characteristics with segments A and B being derived from recombinant intermediate vaccine strain and classic strains of IBDV. Segment A of strain GXB02 was incorporated into the skeleton of an intermediate IBDV vaccine strain (W2512), where the breakpoints of two recombinant events located at nucleotide positions 1468 and 1648 were replaced by reassortant vvIBDV (PK2) and vvIBDV (D6948) of segment A, respectively. We used this GXB02 strain to inoculate 21-day-old specific-pathogen-free chickens to evaluate its pathogenicity. Strain GXB02 has clinicopathologic characteristics of IBD with severe bursal lesions, as evidenced by necrosis, depletion of lymphocytes, and follicle atrophy, indicating that reassortment with classical strains in segment B or/and recombination with very virulent strains increased pathogenicity of the strain GXB02 in chickens. These findings provide important insights into the genetic exchange between classic and attenuated strains of IBDV with two recombinant events occurring at the intermediate derivative segment A with vvIBDV strains, thereby increasing the difficulty of prevention and control of IBD due to novel reassortant-recombinant strains.

Identification and Pathogenicity Evaluation of a Novel Reassortant Infectious Bursal Disease Virus (Genotype A2dB3)

Infectious bursal disease virus (IBDV) is a non-enveloped, bi-segmented double-stranded RNA virus and the causative agent of a poultry immunosuppressive disease known as infectious bursal disease (IBD). The novel variant IBDV (nVarIBDV) recently posed a great threat to the development of the poultry industry. In this study, we identified a novel segment-reassortant IBDV strain, IBDV-JS19-14701 (Genotype A2dB3). Phylogenic analysis showed that Segments A and B of IBDV-JS19-14701 were derived from emerging nVarIBDV (Genotype A2dB1) and long-prevalent HLJ0504-like strains (Genotype A3B3) in China, respectively. The pathogenicity of IBDV-JS19-14701 was further evaluated via animal experiments. IBDV-JS19-14701 exhibited a similar virulence to chickens with the nVarIBDV. The identification of this reassortment event is beneficial for understanding the epidemiology of nVarIBDV and will contribute to the efficient prevention and control of IBD.

Infectious bursal disease in Nigeria: continuous circulation of reassortant viruses

Outbreaks of infectious bursal disease (IBD), a highly contagious immunosuppressive disease of young chickens, are still reported globally despite vaccination efforts. This study investigated the genetic characteristics of infectious bursal disease virus (IBDV) from 26 reported outbreaks in 2019 in Nigeria. Nucleotide sequences of VP2 hypervariable (hvVP2) region (n=26) and VP1 (n=23) of Nigerian IBDVs were determined. Our results revealed the detection of reassortant strains with segment A related to very virulent IBDV (vvIBDV) having virulence marker (222A, 242I, 256I, 294I and 299S), whereas their segment B were closely related to previously detected IBDV strains having QEG substitution at positions 145-147. Phylogenetic analysis of the hvVP2 region revealed that all the Nigerian IBDV clustered with vvIBDV (genogroup 3) and were independent of the Asian/European lineage. Interestingly, in the hvVP2, all the viruses had a G-S substitution at residue 254. Additionally, one isolate had an A321T substitution at the P_HI loop, which has been suggested to play a key role in antigenicity. Four of the viruses (Bauchi=3 and Plateau=1) had a unique A-T substitution at residue 144 on the VP1 region. We also observed a T174S substitution in nine of the Nigerian viruses from Bauchi and Plateau state that were not found in any outbreak viruses from Oyo and Akwa Ibom. This report demonstrates the circulation of reassortant strains in commercial and backyard poultry farms in Nigeria despite sustained vaccination efforts. Our data suggest that the Nigerian outbreak viruses have mutations that may affect antigenicity and contribute to antigenic drift.

The Novel Genetic Background of Infectious Bursal Disease Virus Strains Emerging from the Action of Positive Selection

The circulation in Europe of novel reassortant strains of infectious bursal disease virus (IBDV), containing a unique genetic background composition, represents a serious problem for animal health. Since the emergence of this novel IBDV mosaic was first described in Poland, this scenario has become particularly attractive to uncover the evolutionary forces driving the genetic diversity of IBDV populations. This study additionally addressed the phenotypic characterization of these emergent strains, as well as the main features affecting the viral fitness during the competition process of IBDV lineages in the field. Our results showed how different evolutionary mechanisms modulate the genetic diversity of co-existent IBDV lineages, leading to the error catastrophe effect, Muller ratchet effect, or prevalence, depending on their genetic compositions. We also determined that the action of the positive selection pressure, depending on the genomic segment on which it is acting, can drive two main phenotypes for IBDV: immune-escaping strains from the selection on segment A or strains with functional advantages from the selection on segment B. This last group seems to possess an increased fitness landscape in the viral quasispecies composition, presenting better adaptability to dissimilar environmental conditions and likely becoming the dominant population. The reassortant strains also exhibited a lower mortality rate compared with the well-known vvIBDV strains, which can facilitate their spreading.

Genogrouping of Infectious Bursal Disease Viruses Circulating in Ethiopian Chickens: Proposal for Assigning Very Virulent Strains in the Country into New Sub Genogroup 3d

Introduction

In 2017 infectious bursal disease viruses (IBDVs) were reclassified into genogroups based on nature of clustering on a phylogenetic tree constructed using VP2 gene sequence data rather than according to their pathotype and/or antigenic types. Ethiopian IBD viruses were not reclassified according to the proposed genogrouping.

Methods

In order to genogroup the Ethiopian IBDVs, available VP2 gene sequences data together with reference strain sequences were retrieved from GenBank and genogrouped as recently recommended based on evolutionary tree reconstruction and determination of their clustering on the phylogenetic tree.

Results

The Ethiopian IBDVs were grouped into genogroups 1 and 3 that antigenically represent classically virulent and very virulent IBDVs, respectively. The genogroup 1 IBDVs were clustered with the vaccine strain while the genogroup 3 viruses were clustered with four known viruses belonging to sub-genogroup 3a and sub-genogroup 3b. Almost half of the Ethiopian IBDVs reported did not cluster with the specific sub-groups of genogroup 3; rather, the isolates were clustered differently suggesting they deserve a different sub-genogroup tentatively proposed as 3d. The two genogroups observed based on clustering on a phylogenetic tree were supported by corresponding deduced amino acid changes in similar positions in VP2 sequences. In addition, virulence marker amino acid genes coupled with second major hydrophilic region (amino acid positions 314–325) were predicted in these sequences that could be responsible for the occurrence of IBD outbreaks.

Conclusion

A new sub-genogroup of IBDVs, 3d, were observed in the sequences that could be one of the reasons for the frequent occurrence of IBD outbreaks and questions the protective potential of the existing vaccine. To institute disease control in the country, the effectiveness of the vaccine in use needs to be assessed in vivo against both genogroups 1 and 3 viruses and all three sub-genogroup 3 viruses circulating in the country.

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.

The Full Region of N-Terminal in Polymerase of IBDV Plays an Important Role in Viral Replication and Pathogenicity: Either Partial Region or Single Amino Acid V4I Substitution Does Not Completely Lead to the Virus Attenuation to Three-Yellow Chickens

Infectious Bursal Disease Virus (IBDV) has haunted the poultry industry with severe, prolonged immunosuppression of chickens when infected at an early age and can easily lead to other secondary infections. Understanding the pathogenic mechanisms could lead to effective prevention and control of Infectious Bursal Disease (IBD). Evidence suggests that the N-terminal domain of polymerase in segment B plays an important role, but it is not clear which part or residual is crucial for the pathogenicity. Using a reverse genetics technique, a molecular clone (rNN1172) of the parental vvIBDV strain NN1172 was generated, and its pathogenicity was found to be the same as the parental virus. Then, three recombinant chimeric viruses were rescued based on the rNN1172 and substituted with the counterparts in the N-terminal domain of the attenuated vaccine strain B87: the rNN1172-B87VP1a (substituting the full region of the 1–167 aa residuals), the rNN1172-B87VP1a∆4 (substituting the region of the 5–167 aa residuals), and the rNN1172-VP1∆4 (one single aa residual substitution V4I), to better explore the role of the N-terminal domain of polymerase on the viral pathogenicity. Interestingly, all these substitutions played different roles in the viral pathogenicity: the mortality of the rNN1172-B87VP1a-challenged chickens was significantly reduced from 30% to 0%. No obvious lesion was found in the histopathological examination, and the lowest viral genome copy number was also detected in the bursa when compared to the parental and two other recombinant viruses. The mortalities caused by rNN1172-B87VP1a∆4 and rNN1172-B87VP1∆4, respectively, were all reduced to 10% and had a delayed onset of death. Our results also revealed that the pathogenicity of the IBDV was consistent with the viral replication efficiency in vivo (bursae). This study demonstrated that the full region of the N-terminal of polymerase plays an important role in viral replication and pathogenicity, but the substitutions of its partial region or a single residual do not completely lead to the virus attenuation to Three-Yellow chickens, although that significantly reduces its pathogenicity.

Full-length genome sequencing of a very virulent infectious bursal disease virus isolated in Tunisia

Infectious bursal disease (IBD), an acute, highly contagious, and immunosuppressive avian disease, is caused by infectious bursal disease virus (IBDV) and constitutes one of the main threats to the poultry industry, worldwide. This study was performed to isolate and characterize IBDV isolates circulating in Tunisia. Eleven collected bird samples were identified using an SYBR Green–based one-step real-time reverse transcriptase polymerase chain reaction. The full-length genome sequencing of 7 of the 11 IBDV isolates has been realized. VP2 gene data showed limited sequence variations for all the 7 tested samples. The few nucleotide changes were silent and the deduced amino acid sequences were identical with the exception of a unique and characteristic nonsilent mutation (C₁₂₀₃) detected for the TN37/19 isolate, with a change of amino acid (L) to (F) at position 401. In addition, the serine-rich heptapeptide SWSASGS, characteristic of virulent IBDV, as well the amino acid residues, conserved in most very virulent IBDV (vvIBDV) strains, were detected in all the Tunisian tested isolates. Nucleotide sequences of VP5 gene revealed the presence of 5 substitutions leading to changes in the amino acid sequences of the virus. Two of these mutations were unique and characteristic of the Tunisian isolates. Besides, the alternative AUG start codon, characteristic of vvIBDV, was observed in all obtained VP5 gene sequences. The Tunisian protein sequences of VP1 showed E242 and the TDN triplet at positions 145, 146, and 147, a motif specific of vvIBDV. Phylogenetic analyses of the 5 genes confirmed the sequence alignment results and showed that the Tunisian strains are closely related to the very virulent Algerian IBDV strains.

Occurrence and spread of a reassortant very virulent genotype of infectious bursal disease virus with altered VP2 amino acid profile and pathogenicity in some European countries

Reassortant strains of Infectious Bursal Disease Virus (IBDV) were detected in commercial broiler flocks in the Netherlands, Belgium, Denmark, Czech Republic and Germany and in layers and organic broilers in Sweden in the period of 2017-19. Genetic analysis, based on hypervariable region of VP2 gene showed grouping together with very virulent IBDV strains (vvIBDV, Genogroup 3), but these recent viruses formed a separate cluster, which was most closely related to Latvian IBDV strains from 2010-13. VP1 gene of these isolates was most closely related to D78 attenuated IBDV strain. The recently described reassortant IBDV strain (Bpop/03/PL) from Poland with similar genomic constellation (segment A from vvIBDV, segment B from attenuated strain) retained its pathogenicity (80 % mortality in SPF chickens). Infection with the North-West European reassortant IBDVs described in this study showed subclinical feature in the field (without complicating agents) and when tested under standardized pathogenicity test in SPF layer chickens (no mortality or clinical signs, but marked bursa atrophy was observed). Although these recent North-West European reassortant strains had all amino acid residues in their VP2 gene which are considered as markers of vvIBDV strains, they exhibited typical amino acid changes compared to vvIBDV reference strains that should contribute to the determination of pathogenicity. Diagnostic investigations indicated that co-infection with fowl adenovirus or chicken infectious anaemia virus exaggerated the outcome of the IBDV infection (10-20 % mortality). Widespread presence of this reassortant IBDV group in clinically healthy flocks draws attention to the importance of active surveillance.

Identification and assessment of pathogenicity of a naturally reassorted infectious bursal disease virus from Henan, China

Infectious bursal disease virus (IBDV) is still a vital etiological agent in poultry farms. IBDV outbreaks occasionally occur due to the presence of very virulent, reassortment or variant strains. Vaccine immunization has played crucial roles in IBD control for decades. However, survival pressure of IBDV from the vaccine immunization also increases the reassortments of circulating viruses. In this study, an IBDV strain was isolated from several broiler farms in Henan Province, central part of China, and named IBDV HN strain. Based on the results of RT-PCR, sequencing and phylogenic analyses of VP1 and VP2 genes, the IBDV HN strain is a novel reassortment strain in the Henan region. Segment A of this strain appears to originate from the very virulent IBDV strain, while segment B comes from the other field reassortment strains. This may be the result of natural reassortant of virus circulating in the field. About 60% (6/10) of experimentally infected specific pathogen-free chickens died after 3 to 5 d post-infection with typical symptom and pathological lesions. The IBDV HN strain was prone to horizontal transmission, which poses a serious threat to the chicken industry. Further investigation on the prevalence, virulence, and evolution of HN strain IBDV will provide a foundation for the prevention and control of the disease in this region.

Emergence and expansion of novel pathogenic reassortant strains of infectious bursal disease virus causing acute outbreaks of the disease in Europe

Infectious bursal disease virus (IBDV) is the aetiological agent of a highly contagious chicken immunodeficiency disorder known as Gumboro disease, which cause severe economic loses to the poultry worldwide. The emergence of very virulent IBDV strains (vvIBDV) during the late 80s resulted in drastic changes to the epidemiology of IBDV with a dramatic increase in the mortality of the animals affected. Molecular studies determined that the emergence of the vvIBDV was a consequence of a genomic reorganization of IBDV known as reassortant event by which the virus combined two emergent genetic background vvIBDV for segment A and vvIBDV for segment B. In the current study, a retrospective analysis was conducted, and samples collected during acute outbreaks of Gumboro disease in Poland during 1992-2015 were submitted to sequencing and further molecular and phylogenetic analyses. The results obtained not only revealed a high genetic diversity for Polish IBDV strains but a new population of IBDV was identified. These novel reassortant strains with a unique genetic background contain the segment A from very virulent strains and segment B from an unidentified source, phylogenetically segregated and classified as 'transitional lineage'. The results obtained also showed the presence of this new lineage in Finland, evidencing the expansion of this new genomic reorganized viral strain in Europe representing an additional threat to the global situation of IBDV.

Current state-of-the-art in the use of plants for the production of recombinant vaccines against infectious bursal disease virus

Infectious bursal disease is a widely spread threatening contagious viral infection of chickens that induces major damages to the Bursa of Fabricius and leads to severe immunosuppression in young birds causing significant economic losses for poultry farming. The etiological agent is the infectious bursal disease virus (IBDV), a non-enveloped virus belonging the family of Birnaviridae. At present, the treatment against the spread of this virus is represented by vaccination schedules mainly based on inactivated or live-attenuated viruses. However, these conventional vaccines present several drawbacks such as insufficient protection against very virulent strains and the impossibility to differentiate vaccinated animals from infected ones. To overcome these limitations, in the last years, several studies have explored the potentiality of recombinant subunit vaccines to provide an effective protection against IBDV infection. In this review, we will give an overview of these novel types of vaccines with special emphasis on current state-of-the-art in the use of plants as "biofactories" (plant molecular farming). In fact, plants have been thoroughly and successfully characterized as heterologous expression systems for the production of recombinant proteins for different applications showing several advantages compared with traditional expression systems (Escherichia coli, yeasts and insect cells) such as absence of animal pathogens in the production process, improved product quality and safety, reduction of manufacturing costs, and simplified scale-up.

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.

Continuous circulation of an antigenically modified very virulent infectious bursal disease virus for fifteen years in Egypt

Infectious bursal disease virus (IBDV), the agent of an immunosuppressive and sometimes lethal disease in chickens, is causing recurrent outbreaks in broiler chickens in Egypt. In particular, an antigenically modified isolate of very virulent IBDV (vvIBDV) called 99323 was detected in Egypt nearly twenty years ago; this isolate was shown to be experimentally controlled by an antigenically classical live vaccine. However, acute IBD is still reported, even in vaccinated flocks, and little is known about the genetic and antigenic properties of viruses currently circulating in Egypt. In the present study, ten samples collected in Egyptian broiler farms in 2015 as well as five samples collected in 2001 were analyzed. Genetic analyses of partial VP2 sequences revealed that 8 isolates clustered with vvIBDV strains, and 5 with tissue culture adapted and vaccine strains. Similar results were observed for partial VP1 sequences with the exception of isolate 160019, for which VP2 clustered with the vaccine strain Bursine while VP1 clustered with vvIBDV, suggesting reassortment. For isolates genetically related to vvIBDV, antigenic profiling revealed two patterns: while some isolates exhibited typical European vvIBDV reactivity with lack of binding of mAbs 5, other revealed extensive antigenic modifications, with lack of binding of mAbs 3, 5, 6, 8 and 9, similar to isolate 99323. These different patterns were associated with a single amino acid mutation at position 321 of VP2 that is located within peak P_HI. Full genome sequencing was performed for three isolates, among which two were representative of the two antigenic patterns observed for vvIBDV as well as the reassortant isolate 160019. This study highlights the co-circulation of both antigenically typical and modified vvIBDV during the last fifteen years in Egypt.

Infectious bursal disease virus in chickens: prevalence, impact, and management strategies

Infectious bursal disease (IBD), also known as Gumboro disease, is a highly contagious, immunosuppressive disease of young chickens. Although first observed about 60 years ago, to date, the disease is responsible for major economic losses in the poultry industry worldwide. IBD virus (IBDV), a double-stranded RNA virus, exists as two serotypes with only serotype 1 causing the disease in young chickens. The virus infects the bursa of Fabricius of particularly the actively dividing and differentiating lymphocytes of the B-cells lineage of immature chickens, resulting in morbidity, mortality, and immunosuppression. Immunosuppression enhances the susceptibility of chickens to other infections and interferes with vaccination against other diseases. Immunization is the most important measure to control IBD; however, rampant usage of live vaccines has resulted in the evolution of new strains. Although the immunosuppression caused by IBDV is more directed toward the B lymphocytes, the protective immunity in birds depends on inducement of both humoral and cell-mediated immune responses. The interference with the inactivated vaccine induced maternally derived antibodies in young chicks has become a hurdle in controlling the disease, thus necessitating the development of newer vaccines with improved efficacy. The present review illustrates the overall dynamics of the virus and the disease, and the recent developments in the field of virus diagnosis and vaccine research.

Detection of infectious bursal disease virus (IBDV) antibodies using chimeric plant virus-like particles

The aim of the present study is to use Physalis mottle virus (PhMV) coat protein (CP) as a scaffold to display the neutralizing epitopes of Infectious bursal disease virus (IBDV) VP2. For this, three different chimeric constructs were synthesized by replacing the N-terminus of PhMV CP with tandem repeats of neutralizing epitopes of IBDV VP2 and expressed in Escherichia coli. Expression analysis revealed that all the three recombinant chimeric coat protein subunits are soluble in nature and self-assembled into virus-like particles (VLPs) as evidenced through sucrose density gradient ultracentrifugation. The chimeric VLPs were characterized by various biochemical and biophysical techniques and found that they are stable and structurally sound. When the chimeric VLPs were used as coating antigen, they were able to detect IBDV antibodies. These results indicated that the chimeric VLPs can be used as potential vaccine candidates for the control of IBDV, which needs to be further evaluated in animal models.