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

Simultaneous construction strategy using two types of fluorescent markers for HVT vector vaccine against infectious bursal disease and H9N2 avian influenza virus by NHEJ-CRISPR/Cas9

Recently, herpesvirus of turkeys (HVT), which was initially employed as a vaccine against Marek's disease (MD), has been shown to be a highly effective viral vector for producing recombinant vaccines that can simultaneously express the protective antigens of multiple poultry diseases. Prior to the development of commercial HVT-vectored dual-insert vaccines, the majority of HVT-vectored vaccines in use only contained a single foreign gene and were often generated using time-consuming and inefficient traditional recombination methods. The development of multivalent HVT-vectored vaccines that induce simultaneous protection against several avian diseases is of great value. In particular, efficacy interference between individual recombinant HVT vaccines can be avoided. Herein, we demonstrated the use of CRISPR/Cas9 gene editing technology for the insertion of an IBDV (G2d) VP2 expression cassette into the UL45/46 region of the recombinant rHVT-HA viral genome to generate the dual insert rHVT-VP2-HA recombinant vaccine. The efficacy of this recombinant virus was also evaluated in specific pathogen-free (SPF) chickens. PCR and sequencing results showed that the recombinant virus rHVT-VP2-HA was successfully constructed. Vaccination with rHVT-VP2-HA produced high levels of specific antibodies against IBDV (G2d) and H9N2/Y280. rHVT-VP2-HA can provide 100% protection against challenges with IBDV (G2d) and H9N2/Y280. These results demonstrate that rHVT-VP2-HA is a safe and highly efficacious vaccine for the simultaneous control of IBDV (G2d) and H9N2/Y280.

Advances on adaptive immune responses affected by infectious bursal disease virus in chicken

Infectious bursal disease (IBD) is an acute, highly infectious, and immunosuppressive disease caused by the infectious bursal disease virus (IBDV), which interferes with the immune system, causes hypoimmunity and seriously threatens the healthy development of the poultry industry. Adaptive immune response, an important defense line of host resistance to pathogen infection, is the host-specific immune response mainly mediated by T and B lymphocytes. As an important immunosuppressive pathogen in poultry, IBDV infection is closely related to the injury of the adaptive immune system. In this review, we focus on recent advances in adaptive immune response influenced by IBDV infection, especially the damage on immune organs, as well as the effect on humoral immune response and cellular immune response, hoping to provide a theoretical basis for further exploration of the molecular mechanism of immunosuppression induced by IBDV infection and the establishment of novel prevention and control measures for IBD.

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.

Dynamics of the Emerging Genogroup of Infectious Bursal Disease Virus Infection in Broiler Farms in South Korea: A Nationwide Study

Infectious bursal disease (IBD), caused by IBD virus (IBDV), threatens the health of the poultry industry. Recently, a subtype of genogroup (G) 2 IBDV named G2d has brought a new threat to the poultry industry. To determine the current status of IBDV prevalence in South Korea, active IBDV surveillance on 167 randomly selected broiler farms in South Korea from August 2020 to July 2021 was conducted. The bursas of Fabricius from five chickens from each farm were independently pooled and screened for IBDV using virus-specific RT-PCR. As a result, 86 farms were found to be infected with the G2d variant, 13 farms with G2b, and 2 farms with G3. Current prevalence estimation of IBDV infection in South Korea was determined as 17.8% at the animal level using pooled sampling methods. G2d IBDV was predominant compared to other genogroups, with a potentially high-risk G2d infection area in southwestern South Korea. The impact of IBDV infection on poultry productivity or Escherichia coli infection susceptibility was also confirmed. A comparative pathogenicity test indicated that G2d IBDV caused severe and persistent damage to infected chickens compared with G2b. This study highlights the importance of implementation of regular surveillance programs and poses challenges for the comprehensive prevention of IBDV infections.

Full-Length Genome Sequence of a Novel European Antigenic Variant Strain of Infectious Bursal Disease Virus

We report the full-length genome sequence (compared to reference sequences) of a novel European variant strain of infectious bursal disease virus (IBDV), designated 19P009381 (AxB1). This should help to further identify such viruses in Europe.

Pathogenicity and molecular characterization of infectious bursal disease virus in China

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

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.

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.

Characterization of antigenic variant infectious bursal disease virus strains identified in South Korea

Infectious bursal disease (IBD) is one of the most important immunosuppressive diseases of young chickens, causing considerable economic losses to the poultry industry. More than 30 years ago, an antigenic variant (av) pathotype of the IBD virus (IBDV) was reported to originate in, and subsequently spread among, poultry farms in the USA. Recently, a novel avIBDV lineage was identified in China and was shown to exhibit clear differences in its pathogenicity as well as molecular characteristics compared with the previously isolated variant strains. In this study, we conducted a passive surveillance of chicken carcasses submitted to our research division from June-December 2019, and detected the IBDV strains by reverse transcription PCR. Five avIBDV strains were isolated, and their pathogenicity was determined by necropsy and molecular analysis. Additionally, a coinfection field case involving an avIBDV strain and a very virulent IBDV (vvIBDV) strain was identified. Multiple sequence alignment and phylogenetic analysis of partial viral protein 1 (VP1) and hypervariable region (hv) VP2 genes revealed that those strains originated from two different avIBDV lineages. The co-occurrence of two sub-groups of avIBDVs in South Korea confirms for the first time the evolution of antigenic variant IBDV strains, and highlights the urgency for the development of new strategies for IBDV intervention in South Korea.RESEARCH HIGHLIGHTS Five avIBDV strains were identified in South Korea by passive surveillance test in 2019.A coinfection between two IBDV strains from different genogroups was reported in a field case.By phylogenetic analysis, Korean avIBDVs belonged to two distinct lineages of antigenic variant genogroup.

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.

Novel variants of infectious bursal disease virus can severely damage the bursa of fabricius of immunized chickens

In recent years, atypical infectious bursal disease (IBD) with severe immunosuppression has brought new threats to the poultry industry and has caused considerable economic losses. Novel variant infectious bursal disease virus (IBDV) has been identified as the etiological pathogen and for unknown reasons is widespread in poultry on many chicken farms in China that have been immunized with vaccines against very virulent IBDV (vvIBDV). Using immunoprotection experiments in specific-pathogen-free chickens, we first verified that novel variant IBDV could severely damage the bursa of Fabricius of the important immune organ of immunized chicken in the presence of antibodies induced by three types of vvIBDV vaccines, which is a primary reason for the current epidemic of atypical IBD. Monoclonal antibody reactivity patterns and cross-neutralization assays further confirmed the obvious antigenic mismatch between novel variant IBDV and vvIBDV. Sequence analysis of the genome of novel variant IBDV (SHG19 strain) was performed and the key amino acid residues that might be involved in antigenicity and virulence differences of novel variant IBDV compared to vvIBDV were further analyzed. This study not only determined the primary reason for the atypical IBD epidemic, but also remind us of the urgency for developing new vaccines against novel variant IBDV.

Identificación de genogrupos del virus de la Enfermedad de Gumboro en granjas avícolas en Colombia

El virus de la enfermedad de Gumboro (IBDV) es un avibirnavirus con genoma dsARN que presenta altas tasas de mutación y recombinación. A pesar del efecto inmunosupresor en aves y la frecuencia con que ocurre la infección por este agente en el país son pocos los estudios que caracterizan los cuadros clínicos y se desconoce cuáles son los genogrupos circulantes. Esta investigación tuvo como objetivo determinar la frecuencia de lesiones histopatológicas en órganos del sistema inmune e identificar los genogrupos del IBDV en aves comerciales de Colombia. Para determinar la frecuencia de presentación de lesiones en órganos del sistema inmune se analizaron 381 casos clínicos de las bases de datos del Laboratorio de Patología Aviar (LPA) de la Universidad Nacional de Colombia, sede Bogotá (periodo 2016-2018). Asimismo, se secuenciaron los productos de RT-PCR del gen que codifica para la proteína viral VP2 provenientes de 35 muestras de bursas de Fabricio. Como resultado se encontró evidencia de lesiones microscópicas compatibles con procesos de inmunodepresión en órganos del sistema inmune (bursa de Fabricio, timo, bazo y médula ósea) en el 25 % (97) de los casos analizados y se identificaron los genogrupos 1, 2 y 4 en la siguiente proporción: genogrupo 1-69 % (virus clásicos), genogrupo 2-25 % (variantes) y genogrupo 4-6 % (identificado en Suramérica). Estos hallazgos demuestran la presencia de lesiones en órganos del sistema inmune y la existencia de los genogrupos 1, 2 y 3 del IBDV circulando en aves comerciales en Colombia. Esta es la primera investigación en el país con este sistema de clasificación que permite evidenciar con mayor precisión los cambios en el genoma del IBDV. Lo anterior señala la necesidad de continuar con este tipo de estudios para tener una mejor comprensión de la infección en campo y orientar el diseño e implementación de estrategias de control.

Phylogeny and evolution of infectious bursal disease virus circulating in Turkish broiler flocks

The emergence of new infectious bursal disease virus (IBDV) variants can threaten poultry health and production all over the world causing significant economic losses. Therefore, this study was performed to determine IBDV molecular epidemilogy, VP2 gene variation, and corresponding pathological lesions in IBDV infected chickens in Turkey. For this, 1855 bursa of Fabricius samples were collected from 371 vaccinated broiler flocks. Atrophia and haemorrhages were seen in the bursa Fabricius of very virulent IBDV (vvIBDV) infected chickens. Partial VP2 gene was sequenced and phylogenetic, recombination, and evolutionary analyses were performed. 1548 (83.5%) out of 1855 of bursa of Fabricius samples were IBDV positive and 1525 of those could be sequenced. The recombination analysis did not detect occurrence of any recombination event among the Turkish strains. Among 1525 sequenced samples, 1380 of them were found to be classical strains. Among 1380 classical strains, 1317 were similar to IBDV 2512, 11 to Faragher 52/70, 40 to 228 E, and 12 to Lukert strain. Out of 1525 reverse transcriptase ploymerase chain reaction positive samples, 144 of them were found to be similar to vvIBDV-VP2 gene reported to GenBank previously. The phylogenetic tree performed on a broad sequence dataset demonstrated grouping of vvIBDV Turkish strains in three different clusters, including sequences collected also from Iraq and Kuwait (Cluster 1), Indian (Cluster 2), and a distinct Turkish-only cluster (Cluster 3). The evolutionary rate estimation on branches/clades including Turkish strain mirrored the expected one for RNA viruses and no significant differences were found among different considered branches. In conclusion, results of this study indicate that vvIBDV strains similar to those circulating in various countries in the Middle East are present and undergoing evolution in chickens from Turkish broiler flocks. This point needs to be taken into account in planning adequate control strategies.

Novel variant strains of infectious bursal disease virus isolated in China

Infectious bursal disease (IBD) is one of the most important immunosuppressive diseases that seriously threaten poultry farming and food safety worldwide. The variant strain of infectious bursal disease virus (IBDV) has been greatly neglected for more than 30 years. Recently, the subclinical infection of suspected IBD, causing considerable economic losses, occurred in the main chicken-farming regions of China. Through RT-PCR, sequencing, and phylogenic analyses, novel variant IBDVs were first identified in six provinces of eastern China. Immunological detection further confirmed the antigenic variation of the Chinese variant IBDVs. The Chinese IBDV variants were obviously different from the American IBDV variants, with less than a 97.7% (VP1) or 98.7% (VP2) amino acid sequence identity. Animal experiments further confirmed the serious threat of the variant IBDVs to chickens, demonstrating irreversible damage to the central immune organ, obvious immunosuppression, and growth retardation. This study not only identified the pandemic nature of the novel variant IBDVs for the first time but also discovered the distinct molecular epidemiological characteristics of these viruses, which will contribute more to the control of the disease.

A proposed nomenclature for infectious bursal disease virus isolates

Infectious bursal disease virus (IBDV) was initially identified in the USA. For decades, these viruses were not categorized using a typing system because they were considered to be antigenically and pathogenically similar. In the 1980s, a second major serotype, serotype 2, was found in turkeys. Classification of IBDV became more complex with the discovery of antigenic variant strains called "variants" in the United States and a highly virulent strain known as "very virulent" or vvIBDV identified in Europe. To distinguish the IBDV strains identified prior to this time from the antigenic variant viruses, the term "classic viruses" was adopted. Studies over the next three decades produced a wealth of information on the antigenicity, pathogenicity and molecular structure of IBDV isolates. These data made it clear that the descriptive nomenclature used for IBDV was inadequate. For example, not all viruses identified as vvIBDV by genotyping are highly pathogenic; some have reassorted genome segments that result in lower virulence. Furthermore, variant viruses are not an antigenically homogeneous group and the term "classic virus" has been used interchangeably to describe antigenic and pathogenic types of IBDV. These and other issues make the current naming system for strains of IBDV archaic. The lack of uniform testing and standards for antigenicity and pathogenicity makes it difficult to categorize IBDV strains on a global basis. A new nomenclature that includes a genotyping system that can easily be applied worldwide is proposed and serves as a platform to begin discussions on its value to the scientific community.

Detection of different genotypes of infectious bronchitis virus and of infectious bursal disease virus in European broilers during an epidemiological study in 2013 and the consequences for the diagnostic approach

To gather recent data regarding the infectious bronchitis (IB) and infectious bursal disease (IBD) situation in Europe, a large-scale field epidemiological survey using diagnostic samples has been implemented in 2013 for about six months in several European countries: France, Germany, Greece, Italy, the Netherlands, Poland, Portugal, the Republic of Ireland, Spain and the UK. In 234 flocks that were sampled, strains from 10 different IBV genotypes were detected: the 793B genotype was detected most frequently, followed by QX, Massachusetts (Mass) and the Xindadi-like strains. Strains belonging to the Q1, Ark, D274, D1466, Italy-02 and B1648 genotypes were detected as well, although less frequently. The separate sampling of tracheas and kidneys for IBV detection using reverse transcriptase PCR was very useful, as different genotypes or significant differences in sequences of the same genotype were detected between both organs. The data of this survey also provided valuable information about the replication of IBD vaccines and subsequent infectious bursal disease virus (IBDV) antibody responses under field conditions. The detection of five non-vvIBDV field strains of two different genotypes shows the presence of non-vvIBDV non-vaccine strains, which can easily be undetected in Europe due to the focus on sampling of clinically ill birds. Detection of vaccine virus in the bursa and antibody response to the IBD vaccination in flocks that had been vaccinated by the drinking water with a live attenuated vaccine compared to a vaccination in the hatchery using an immune-complex vaccine showed a delayed replication of the vaccines that had been applied by the drinking water, indicating mistakes in the timing and/or application of the vaccines.

Infection with Some Infectious Bursal Disease Virus Pathotypes Produces Virus in Chicken Muscle Tissue and the Role of Humoral Immunity as a Mitigation Strategy

Infectious bursal disease virus (IBDV) causes important economic losses and negatively affects global trade in poultry and poultry products. This study determined the presence of IBDV in primary lymphoid tissues and muscle tissue of infected broilers and the role of vaccination as a mitigation strategy. In the first study, specific-pathogen-free (SPF) broiler chickens were challenged with STC (classical [cIBDV]), Indiana (variant [varIBDV]), rA (very virulent [vvIBDV]), or Ohio (serotype 2, avirulent) IBDV. Infection was confirmed in all groups, but only the cIBDV group experienced morbidity or mortality. Virus was only isolated in low titers from a few breast and/or thigh muscle tissue samples from cIBDV and vvIBDV-infected chickens. For the second study, SPF broilers from three different treatment groups were challenged with IBDV viruses that currently circulate in the United States, varIBDV or vvIBDV: 1) maternal antibody-positive (MAb+), vaccinated with recombinant HVT-IBDV vaccine (Vaxxitek^®, Merial; MAb+/Vax); 2) MAb+, not-vaccinated (MAb+/Unvax); and 3) maternal antibody-negative, not-vaccinated chickens (MAb-/Unvax). MAb+/Vax and MAb+/Unvax chickens had significantly lower virus titers in primary lymphoid tissues compared to MAb-/Unvax chickens. No virus was detected in muscle tissues from any of the groups challenged with varIBDV, confirming the results of the first experiment. Only 1 of 36 (MAb+/Vax) and 2 of 36 (MAb+/Unvax) muscle samples were positive at minimal amounts (10^1.97 EID₅₀/ml) in vvIBDV challenge, compared to the 9 of 36 muscle samples that were positive in the MAb-/Unvax group. This study indicates that only cIBDV and vvIBDV strains can be found in muscle at low titers of SPF meat chickens and that the breeder vaccination with MAb transfer to progeny with or without accompanying progeny vaccination, as practiced in the United States, was an effective mitigation strategy for vvIBDV-challenged birds.

A molecular epidemiology study based on VP2 gene sequences reveals that a new genotype of infectious bursal disease virus is dominantly prevalent in Italy

A distinctive infectious bursal disease (IBD) virus genotype (ITA) was detected in IBD-live vaccinated broilers in Italy without clinical signs of IBD. It was isolated in specific-pathogen-free eggs and molecularly characterized in the hypervariable region of the virus protein (VP) 2. Phylogenetic analysis showed that ITA strains clustered separately from other homologous reference sequences of IBDVs, either classical or very virulent, retrieved from GenBank or previously reported in Italy, and from vaccine strains. The new genotype shows peculiar molecular characteristics in key positions of the VP2 hypervariable region, which affect charged or potentially glycosylated amino acids virtually associated with important changes in virus properties. Characterization of 41 IBDV strains detected in Italy between 2013 and 2014 showed that ITA is emergent in densely populated poultry areas of Italy, being 68% of the IBDV detections made during routine diagnostic activity over a two-year period, in spite of the immunity induced by large-scale vaccination. Four very virulent strains (DV86) and one classical strain (HPR2), together with eight vaccine strains, were also detected. The currently available epidemiological and clinical data do not allow the degree of pathogenicity of the ITA genotype to be defined. Only in vivo experimental pathogenicity studies conducted in secure isolation conditions, through the evaluation of clinical signs and macro/microscopic lesions, will clarify conclusively the virulence of the new Italian genotype.

Persistence and tissue distribution of infectious bursal disease virus in experimentally infected SPF and commercial broiler chickens

This study was initiated to determine the persistence, distribution, and quantification of infectious bursal disease virus (IBDV) in lymphoid and nonlymphoid tissues of specific-pathogen-free (SPF) and commercial broiler chickens. Two serotype 1 strains, STC classic and IN variant, were independently used in the experiments. Five separate experiments were conducted using 2- and 4-wk-old SPF chickens, 2- and 4-wk-old in ovo-vaccinated commercial broilers, and 2-wk-old commercial broilers having maternally derived anti-IBDV antibodies. Pooled data from five experiments revealed that SPF chickens had a significantly higher incidence of IBDV-positive reverse transcriptase PCR (RT-PCR) results than commercial chickens (multivariable logistic regression, adjusted odds ratio = 15.28; 95% confidence limits [CL] = 9.53, 24.51, P < 0.0001). In many cases, the viral RNA (vRNA) persisted longer in in ovo-vaccinated commercial broilers bearing maternally derived antibodies compared with similar broilers not vaccinated in ovo. The STC strain was more frequently detected in tissues than the IN strain (chi-square P < 0.0001). In lymphoid tissues, STC and IN strains were detected for the longest duration in bursal tissues followed by spleen, thymus, and bone marrow. In nonlymphoid tissues, STC and IN strains were detected the longest in cecum followed by liver, kidney, pancreas, lungs, thigh, and breast muscles. Compared with bursal tissues, muscle and bone marrow tissues were significantly less likely to yield an IBDV-positive RT-PCR result (P < 0.0001). Although STC vRNA was detected up to 42 days postinoculation (DPI) in bursal homogenates of SPF chickens, virus isolation from bursal homogenates using embryonated chicken eggs was only possible up to 28 DPI. Similarly, STC vRNA was detected up to 42 DPI in bursal tissues of commercial broilers, but infectious virus could be isolated only up to 21 DPI. The IN strain was isolated up to 10 DPI from bursal homogenates of SPF chickens and broilers, but vRNA was detected up to 35 DPI in SPF chickens and 21 DPI in broilers. This study emphasizes that the detection ofvRNA is not indicative of the presence of infectious virus, and virus isolation has to be performed to prove the presence of infectious virus.

Current status of vaccines against infectious bursal disease

Infectious bursal disease virus (IBDV) is the aetiological agent of the acute and highly contagious infectious bursal disease (IBD) or "Gumboro disease". IBD is one of the economically most important diseases that affects commercially produced chickens worldwide. Along with strict hygiene management of poultry farms, vaccination programmes with inactivated and live attenuated viruses have been used to prevent IBD. Live vaccines show a different degree of attenuation; many of them may cause bursal atrophy and thus immunosuppression with poor immune response to vaccination against other pathogens and an increase in vulnerability to various types of infections as possible consequences. Depending on their intrinsic characteristics or on the vaccination procedures, some of the vaccines may not induce full protection against the very virulent IBDV strains and antigenic variants observed in the last three decades. As chickens are most susceptible to IBDV in their first weeks of life, active immunity to the virus has to be induced early after hatching. However, maternally derived IBDV-specific antibodies may interfere with early vaccination with live vaccines. Thus new technologies and second-generation vaccines including rationally designed and subunit vaccines have been developed. Recently, live viral vector vaccines have been licensed in several countries and are reaching the market. Here, the current status of IBD vaccines is discussed.

Differential modulation of cytokine, chemokine and Toll like receptor expression in chickens infected with classical and variant infectious bursal disease virus

Infectious bursal disease (IBD) is an important immunosuppressive disease of chickens. The causative agent, infectious bursal disease virus (IBDV), consists of two serotypes, 1 and 2. Serotype 1 consists of classic IBDV (cIBDV) and variant IBDV (vIBDV). Both of these strains vary in antigenicity and pathogenesis. The goal of this study was to compare the immunopathogenesis of cIBDV and vIBDV. Three-week-old specific pathogen free chickens were inoculated intraocularly with standard challenge strain (STC) (cIBDV) and a variant strain Indiana (IN) (vIBDV). The cIBDV produced more pronounced bursal damage, inflammatory response and infiltration of T cells as compared to vIBDV. There were significant differences in the expression of innate (IFN-α and IFN-β), proinflammatory cytokine and mediator (IL-6 and iNOS) in cIBDV- and vIBDV-infected bursas. The expression of chemokines genes, IL-8 and MIP-α was also higher in cIBDV-infected chickens during the early phase of infection. The expression of Toll like receptor 3 (TLR3) was downregulated at post inoculation days (PIDs) 3, 5, and 7 in the bursas of vIBDV-infected chickens whereas TLR3 was upregulated at PIDs 3 and 5 in cIBDV-infected bursas. In vIBDV-infected bursa, TLR7 expression was downregulated at PIDs 3 and 5 and upregulated at PID 7. However, TLR7 was upregulated at PIDs 3 and 7 in cIBDV-infected bursas. The expression of MyD88 was downregulated whereas TRIF gene expression was upregulated in cIBDV- and vIBDV-infected bursa. These findings demonstrate the critical differences in bursal lesions, infiltration of T cells, expression of cytokines, chemokines and TLRs in the bursa of cIBDV-and vIBDV-infected chickens.

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.

Detection of infectious bursal disease virus isolates with unknown antigenic properties by reverse genetics

Infectious bursal disease virus (IBDV) serotype 1 is the causative agent of a highly contagious immunosuppressive disease of young chickens. In the past, a number of antigenic, as well as pathogenic, subtypes have been described. The determination of the antigenic makeup of circulating strains is of vital interest to the poultry industry because changes in the antigenicity of circulating field strains have an impact on the use of vaccines. To obtain a more comprehensive overview of the relationship between the nucleotide and amino acid sequence and the antigenic makeup of field isolates, a system based on reverse genetics of IBDV was established. Using this approach, a database for field isolates from three different states in the United States (Georgia, Alabama, and Louisiana), consisting of nucleotide sequence, amino acid sequence, and a reaction pattern based on a panel of monoclonal antibodies, was established. The obtained results showed that phylogenic analysis, which is based on the similarity of sequences, would lead to false conclusions regarding a possible antigenic makeup of the particular isolate. Sequences of field samples were divided into three groups: 1) those that grouped with variant strain E/Del sequences but were antigenically different, 2) those that did not group with sequences of E/Del but were similar in their antigenic makeup, and 3) those that did not group with E/Del sequences and were antigenically different. In addition, using the reverse-genetics approach, a number of field isolates showed no reactivity with any of the used monoclonal antibodies, indicating that an unknown, antigenic subtype of IBDV serotype 1 is circulating in the field.

Molecular and structural bases for the antigenicity of VP2 of infectious bursal disease virus

Infectious bursal disease virus (IBDV), a member of the family Birnaviridae, is responsible for a highly contagious and economically important disease causing immunosuppression in chickens. IBDV variants isolated in the United States exhibit antigenic drift affecting neutralizing epitopes in the capsid protein VP2. To understand antigenic determinants of the virus, we have used a reverse-genetics approach to introduce selected amino acid changes-individually or in combination-into the VP2 gene of the classical IBDV strain D78. We thus generated a total of 42 mutants with changes in 8 amino acids selected by sequence comparison and their locations on loops P(BC) and P(HI) at the tip of the VP2 spikes, as shown by the crystal structure of the virion. The antibody reactivities of the mutants generated were assessed using a panel of five monoclonal antibodies (MAbs). Our results show that a few amino acids of the projecting domain of VP2 control the reactivity pattern. Indeed, the binding of four out of the five MAbs analyzed here is affected by mutations in these loops. Furthermore, their importance is highlighted by the fact that some of the engineered mutants display identical reactivity patterns but have different growth phenotypes. Finally, this analysis shows that a new field strain isolated from a chicken flock in Belgium (Bel-IBDV) represents an IBDV variant with a hitherto unobserved antigenic profile, involving one change (P222S) in the P(BC) loop. Overall, our data provide important new insights for devising efficient vaccines that protect against circulating IBDV strains.

Genetic characteristics of infectious bursal disease viruses from four continents

Following the initial discovery of very virulent infectious bursal disease virus (vvIBDV) strains in Europe, these viruses spread to many parts of the world. In this study, we examined the phylogenetic relationship of never-before-published IBDV from 18 countries on four continents. All the samples were collected between 1997 and 2005 and were reported to be from broiler flocks experiencing higher than expected mortality which is often associated with acute very virulent infectious bursal disease. A total of 113 samples were imported into the U.S. and viral genetic material was used to determine the nucleotide sequence of the VP2 gene hypervariable region. Although all the samples were reported to be associated clinically with high mortality, genetic analysis suggests that some were not vvIBDV strains. Two viruses from South Africa were genetically similar to U.S. variant viruses. A majority (71/113) of the viruses examined had the amino acid Alanine at position 222 and sixty-seven of these suspect vvIBDV also had amino acids I242, I256, I294 and S299 which are highly conserved among vvIBDV strains. Phylogenetic analysis placed putative vvIBDV strains from many different countries and geographic regions in a single clade with some minor non-significant branching.