Pathogenic Infectious Bursal Disease Virus Research Articles

RNA-seq reveals role of cell-cycle regulating genes in the pathogenicity of a field very virulent infectious bursal disease virus.

Infectious bursal disease virus (IBDV) infection causes highly contagious and immunosuppressive disease in poultry. The thymus, serving as the primary organ for T cell maturation and differentiation, plays an important role in the pathogenicity of IBDV in the infected chickens. However, there are no reports on the molecular pathogenesis of IBDV in the thymus currently. The aim of the study was to elucidate the molecular mechanisms underlying the pathogenicity of a field very virulent (vv) IBDV strain NN1172 in the thymus of SPF chickens using integrative transcriptomic and proteomic analyses. Our results showed that a total of 4,972 Differentially expressed genes (DEGs) in the thymus of NN1172-infected chickens by transcriptomic analysis, with 2,796 up-regulated and 2,176 down-regulated. Meanwhile, the proteomic analysis identified 726 differentially expressed proteins (DEPs) in the infected thymus, with 289 up-regulated and 437 down-regulated. Overall, a total of 359 genes exhibited differentially expression at both mRNA and protein levels, with 134 consistently up-regulated and 198 genes consistently down-regulated, as confirmed through a comparison of the RNA-seq and the proteomic datasets. The gene ontology (GO) analysis unveiled the involvement of both DEGs and DEPs in diverse categories encompassing cellular components, biological processes, and molecular functions in the pathological changes in IBDV-infected thymus. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that the host mainly displayed severely disruption of cell survival/repair, proliferation and metabolism pathway, meanwhile, the infection triggers antiviral immune activation with a potential emphasis on the MDA5 pathway. Network inference analysis identified seven core hub genes, which include CDK1, TYMS, MCM5, KIF11, CCNB2, MAD2L1, and MCM4. These genes are all associated with cell-cycle regulating pathway and are likely key mediators in the pathogenesis induced by NN1172 infection in the thymus. This study discovered dominant pathways and genes which enhanced our understanding of the molecular mechanisms underlying IBDV pathogenesis in the thymus.

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.

Genetic characterization of Infectious Bursal Disease Viruses isolated from the vaccinated broiler chicken flocks in Egypt during 2015-2016.

The present study was conducted to characterize the infectious bursal disease virus (IBDV) circulating in clinically diseased broiler chicken flocks with previous vaccination history during 2015-2016 in Egypt. IBDVs were isolated from 48 out of 63 of the investigated bursae from 10 flocks onto embryonated chicken eggs (ECEs) and verified by reverse transcriptase-poly- merase chain reaction (RT-PCR). Histopathologically, bursae lesions revealed some lymphocytes depletion as well as the presence of vesicles in the lining epithelium. The hyper variable region (HVR) of VP2 and VP1 genes of the 10 isolates (1 isolate/flock) were partially sequenced and subjected to comparative alignment and phyologenetic analysis. Phylogenetically, IBDV isolates were clustered into two distinct genetic lineages: variants of classical virulent (cv) and very viru- lent (vv) IBDV strains based on VP1 and VP2 amino acid (aa) sequences. Alignment analysis of HVR-VP2 aa sequences has demonstrated that the vvIBDV isolates have the conserved residues of the vvIBDV pathotype (A222, I242, and I256), while, the cvIBDV isolates have the same aa sequences of the classical attenuated vaccine strain (D78). Expected single point mutation occurred at position 253 (H253N). All previously characterized isolates were re-subjected to molecular analysis with VP1 protein due to its correlation with virulence and pathogenicity of IBDVs. vvIBDV isolates have the conserved tripeptide (TDN), while, the cvIBDV isolates have aa substitutions at conserved tripeptide including NEG at 145-147 amino acid. The present study has demonstrated that variants of classical virulent and very virulent IBDV circulated among vaccinated flocks in Egypt during 2015-2016.

Role of naturally occurring genome segment reassortment in the pathogenicity of IBDV field isolates in Three-Yellow chickens

ABSTRACTReassortment among genome segments of infectious bursal disease virus (IBDV) field isolates was reported frequently worldwide, however the pathogenicity of the reassortant field IBDV is poorly understood. In this paper, a pathogenicity study on four representative IBDV field strains isolated from Southern China between 2005 and 2011 was conducted. Twenty-eight-day-old Three-Yellow chickens were divided into four groups and were inoculated intraocularly with one of the four field IBDV strains, namely NN1172, NN1005, GD10111 and JS7, respectively. The mortality and relative weight of bursa and thymus were subsequently determined in the acute phase of infection. In addition, B cells, T cells (CD4+ and CD8+) and virus were quantified in the bursa of Fabricius and thymus, respectively, by flow cytometry and real-time reverse transcription-polymerase chain reaction. The results showed that isolate NN1172, of which parts of segment A and B encoding the hypervariable (v) region of viral protein (VP2) and VP1, respectively, derived from vvIBDV strains, showed the most severe pathogenicity, and caused the most severe bursal B cell depletion as well as CD4+ and CD8+ T cell infiltration in the bursa of Fabricius. However, the virus induced the strongest decrease in CD4+ and CD8+ T cells in the thymus and exhibited the most efficient viral replication in the target organs. Isolate NN1005, whose vVP2 derived from vvIBDV and VP1 from unidentified origin, exhibited relatively lower pathogenicity compared to NN1172. The other two isolates, JS7 and GD10111, of which the vVP2 derived from vvIBDV and intermediate IBDV, and VP1 from 002–73 and attenuated IBDV, respectively, showed the lowest level of virulence. Our results suggest that various IBDV field isolates with different natural segment reassortments exhibit differential pathogenicity after infection of commercial Three-Yellow chickens.

Triplet amino acids located at positions 145/146/147 of the RNA polymerase of very virulent infectious bursal disease virus contribute to viral virulence

Infectious bursal disease virus (IBDV) causes an economically significant disease of chickens worldwide. The emergence of very virulent IBDV (vvIBDV) has brought more challenges for effective prevention of this disease. The molecular basis for the virulence of vvIBDV is not fully understood. In this study, 20 IBDV strains were analysed phylogenically and clustered in three branches based on their full-length B segments. The amino acid triplet located at positions 145/146/147 of VP1 was found highly conserved in branch I non-vvIBDVs as asparagine/glutamic acid/glycine (NEG), in branch II vvIBDVs as threonine/glutamic acid/glycine (TEG) and in branch III vvIBDVs as threonine/aspartic acid/asparagine (TDN). Further studies showed that the three amino acids play a critical role in the replication and pathogenicity of vvIBDV. Substitution of the TDN triplet with TEG or NEG reduced viral replication and pathogenicity of the vvIBDV HuB-1 strain in chickens. However, the replication of the attenuated IBDV Gt strain was reduced in chicken embryo fibroblast cells, whilst it was enhanced in the bursa by substituting NEG with TEG or TDN. The exchange of the three amino acids was also found to be capable of affecting the polymerase activity of VP1. The important role of segment B in the pathogenicity of IBDV was confirmed in this study. These results also provided new insights into the mechanism of the virulence of vvIBDVs and may offer new targets for their attenuation to develop potential vaccines using reverse genetics.

Different Domains of the RNA Polymerase of Infectious Bursal Disease Virus Contribute to Virulence

BackgroundInfectious bursal disease virus (IBDV) is a pathogen of worldwide significance to the poultry industry. IBDV has a bi-segmented double-stranded RNA genome. Segments A and B encode the capsid, ribonucleoprotein and non-structural proteins, or the virus polymerase (RdRp), respectively. Since the late eighties, very virulent (vv) IBDV strains have emerged in Europe inducing up to 60% mortality. Although some progress has been made in understanding the molecular biology of IBDV, the molecular basis for the pathogenicity of vvIBDV is still not fully understood.Methodology, Principal FindingsStrain 88180 belongs to a lineage of pathogenic IBDV phylogenetically related to vvIBDV. By reverse genetics, we rescued a molecular clone (mc88180), as pathogenic as its parent strain. To study the molecular basis for 88180 pathogenicity, we constructed and characterized in vivo reassortant or mosaic recombinant viruses derived from the 88180 and the attenuated Cu-1 IBDV strains. The reassortant virus rescued from segments A of 88180 (A88) and B of Cu-1 (BCU1) was milder than mc88180 showing that segment B is involved in 88180 pathogenicity. Next, the exchange of different regions of BCU1 with their counterparts in B88 in association with A88 did not fully restore a virulence equivalent to mc88180. This demonstrated that several regions if not the whole B88 are essential for the in vivo pathogenicity of 88180.Conclusion, SignificanceThe present results show that different domains of the RdRp, are essential for the in vivo pathogenicity of IBDV, independently of the replication efficiency of the mosaic viruses.

Detection and characterization of infectious bursal disease viruses in broilers at processing

The presence of infectious bursal disease virus (IBDV) in broilers entering processing plants was examined. The dissemination of IBDV and the introduction of non-native strains for example very virulent (vv) IBDV have had a negative economic impact on poultry production in many countries. Restrictions have been placed on the import and export of poultry products by some countries. There is a perceived risk that IBDV can be spread through transportation and contamination of processing equipment, poultry protein products and processing plant personnel. This risk, however, is fundamentally unknown because scientific studies have not been conducted on the presence of IBDV in birds entering processing plants or the variables that may affect this risk during and post-harvest. The goal of this study was to determine if infectious IBDV was present in broilers entering processing plants. A total of 47 pooled bursa samples from 26 processing plants in the Eastern U.S. were examined. Molecular testing indicated that an IBDV specific RT-PCR was positive in 12 (25.5%) of the samples from 11 different processing plants. Nucleotide sequence analysis was conducted on the 12 RT-PCR positive samples and indicated the IBDV was not commercially available attenuated vaccine strains. Most of the sequences were unique and a phylogenic analysis indicated they were most closely related to variant strains of IBDV. Five RT-PCR positive samples were selected at random for testing in specific-pathogen-free chickens. All five samples contained infectious IBDV as evidenced by macroscopic lesions and bursa/body weight ratios that were significantly lower in infected birds than in the non-inoculated controls. The five viruses were re-identified in bursa tissue from chickens in their respective groups at necropsy using RT-PCR and nucleotide sequencing. The results indicate that infectious and pathogenic IBDV are entering processing plants in this geographic region of the U.S. Additional studies are needed on post-harvest variables that could increase or decrease the risk that these viruses are being disseminated during this process.

Detecção do virus da anemia das galinhas em coinfecção com o vírus doença infecciosa bursal em frangos

Este estudo investigou a manifestação do vírus da Anemia Infecciosa das Aves (VAIA) em lotes de frangos que apresentavam retardo no crescimento e aumento da mortalidade observado a partir do quarto dia de idade. Clinicamente, as aves apresentavam depresão, palidez, despigmentação e retardo de crescimento. À necropsia, as aves apresentavam lesões compatíveis com a infecção pelo vírus da Anemia infecciosa das aves (VAIA). Amostras de fígado, baço e timo foram examinadas por PCR que amplifica um frangmento de 675 pb do gene VP-1 do VAIA. Todos os órgãos examinados foram positivos para o vírus da Anemia Infecciosa das Aves. Os demais patógenos, como adenovírus, reovírus, astrovírus, vírus da doença infecciosa bursal e coronavírus aviário não foram detectados pelas diferentes técnicas laboratoriais, como sorologia, PCR ou PAGE. Os resultados mostraram que o vírus da Anemia Infecciosa das Aves (VAIA) pode manifestar-se clinicamente nos primeiros dias de vida dos frangos - um fato ainda não reportado - associado ao vírus vacinal da doença infecciosa bursal (DIB) cepa forte pode induzir um persistente retardo de crescimento, por várias semanas, em frangos.

VP5-deficient mutant virus induced protection against challenge with very virulent infectious bursal disease virus of chickens

A mutant infectious bursal disease virus (IBDV) deficient in expressing VP5, rGx-F9VP2ΔVP5, was generated using reverse genetics technology. In comparison to the characteristics of rGx-F9VP2 virus in vitro, the mutant virus demonstrated lower viral titer and cytopathogenicity. To understand the role of VP5 in the pathogenicity of IBDV in vivo, animal experiments were conducted. rGx-F9VP2ΔVP5 caused reduced bursal lesion of SPF chickens compared to rGx-F9VP2. Although rGx-F9VP2ΔVP5 induced lower serum antibody than rGx-F9VP2 did, both inoculated groups were fully protected against vvIBDV challenge 4 weeks post-inoculation. In addition, immunosuppression induced by VP5-deficient virus was studied in 2-week-old SPF chickens immunized with AIV inactivated vaccine. And there was reduced immunosuppression as shown in our experimental results. The results showed that AIV hemagglutination inhibition (HI) antibodies of the rGx-F9VP2ΔVP5 inoculated group were similar to those of the mock-inoculated group, however, they were higher than those of the rGx-F9VP2 inoculated group, indicating that deficiency of VP5 decreased the immunosuppression of rGx-F9VP2ΔVP5 in chickens. All data indicated that VP5 played an important role in viral replication and pathogenesis both in vitro and in vivo. The VP5-deficient mutant virus could be a good candidate as a marked vaccine.

Antibody to VP4 protein is an indicator discriminating pathogenic and nonpathogenic IBDV infection

Infectious bursal disease virus (IBDV) causes an acute, highly contagious immunosuppressive disease—infectious bursal disease (IBD) in poultry. It is known that VP4 is a non-structural protein and a viral protease encoded by IBDV. Currently, little is known about VP4 characteristics during pathogenic and nonpathogenic IBDV infection. Here, we investigated the expression profiles of VP4 during pathogenic and nonpathogenic IBDV infection. By IFA and ELISA, using VP4 protein respectively expressed in Vero cells transfected with VP4 gene and in Escherichia coli as antigens, we firstly confirmed serum anti-VP4 antibodies in pathogenic IBDV-infected rather than nonpathogenic IBDV-infected chickens. Kinetic analysis of anti-IBDV antibody shows that in the pathogenic IBDV-infected chickens, the antibody to VP4 was later detectable than anti-VP3 antibody and virus neutralizing antibody. Immunohistochemistry further demonstrates that VP4 antigen can be detected mainly in the cortex of lymphoid follicles of bursa of Fabricius infected with pathogenic IBDV. These data first suggest that VP4 antibody is an indicator discriminating pathogenic and nonpathogenic IBDV infection in chickens.

Protection Conferred by Coarse Spray Vaccination Against Challenge with Infectious Bursal Disease Virus in Commercial Broilers

The efficacy of coarse spray vaccination against pathogenic infectious bursal disease virus (IBDV) in commercial broilers was evaluated. Different coarse spray vaccination schedules using a commercial 2512 strain vaccine were compared with single or double drinking water application at 1 and/or 10 days of age. At 29 days of age, the chickens were challenged with the virulent Edgar strain of IBDV. Seven days postchallenge, severe gross bursal atrophy was observed in the unvaccinated-challenged birds. After challenge and regardless of the method of vaccination used, moderate-to-severe lymphoid depletion was observed, indicating challenge virus replication, later confirmed by reverse transcriptase-polymerase chain reaction and restriction fragment length polymorphism analysis. Coarse spray and drinking water vaccination induced protection against body weight loss. Significant differences (P < 0.05) were observed between the unvaccinated-challenged group (1483 g) and the birds vaccinated at 10 days of age by coarse spray (1812 g). The coarse spray vaccination also induced protection against challenge-induced gross bursal atrophy, as determined by bursal index values. After challenge, significant bursal atrophy was observed in the birds orally vaccinated at 1 day (0.61), 10 days (0.66), and 1 and 10 days (0.63) as well as the unvaccinated-challenged birds (0.62), but not in the coarse-spray-vaccinated groups that exhibited bursal indexes above 0.70 and did not differ from the unvaccinated-unchallenged control group. These results suggest that coarse spray vaccination can be considered as another tool to control IBDV in the field.

Chicken B Lymphoma DT40 Cells as a Useful Tool for in vitro Analysis of Pathogenic Infectious Bursal Disease Virus

Susceptibility of DT40 cells to pathogenic field strains of infectious bursal disease virus (IBDV) including very virulent and classical virulent strains were studied. After the first and second passage of the virus in DT40 cells, IBDV-specific antigen was readily detected in DT40 cells inoculated with the pathogenic field strain infected bursal homogenates. Nucleotide sequence analysis in the VP2 hypervariable domain, which is critical for the virulence of IBDV, revealed no common amino acid substitutions among the pathogenic IBDVs in accordance with the propagation in DT40 cells. These results indicate that DT40 cells are a useful tool for rapid isolation of pathogenic field strains and successive in vitro analysis of IBDV.

The Use of Gamma Irradiation for the Inactivation of Infectious Bursal Disease Viruses

The maximum dosage of gamma irradiation approved by the U.S. Food and Drug Administration (FDA) for poultry is 3.0 kiloGrays (kGy). This treatment is designed to reduce bacterial contamination on uncooked poultry carcasses and meat products. The possible presence of infectious bursal disease virus (IBDV) on poultry postharvest has prompted some countries to study the risk associated with introducing nonnative strains of the virus from imported commodities. The goal of this study was to determine if this risk could be reduced using gamma irradiation to inactivate IBDV. At the dosage approved by the FDA, the titers of IBDV vaccine strains were reduced between 0 and 1 log10. Titers of the pathogenic IBDV strains tested were not reduced after the 3.0 kGy exposure. Furthermore, titers of pathogenic viral strains were not reduced following exposure up to 5.0 kGy. As the exposure to gamma irradiation increased, the titers of the vaccine strains decreased. At the maximum dosage tested (10 kGy), the 89/03 variant virus vaccine was completely inactivated. Titers of the three classic IBDV vaccine strains were reduced between 1.6-2.0 logs after the 10 kGy exposure; however, these viruses remained viable after this treatment. Gamma irradiation is not an effective intervention to reduce the risk of IBDV introduction via processed poultry.

Alteration of amino acids in VP2 of very virulent infectious bursal disease virus results in tissue culture adaptation and attenuation in chickens.

Reverse genetics technology offers the possibility to study the influence of particular amino acids of infectious bursal disease virus (IBDV) on adaptation to tissue culture. Genomic segments A and B of the very virulent (vv) IBDV field strain UK661 were completely cloned and sequenced, and the strain was rescued from full-length cDNA copies of both segments (UK661rev). Using site-directed mutagenesis, alteration of a single amino acid in the segment A-encoded VP2 (A284T) resulted in a limited capacity of UK661 to replicate in tissue culture. Additional alteration of a second amino acid (Q253H) increased replication efficiency in tissue culture. The second mutant (UK661-Q253H-A284T) was used to infect chickens and results were compared with UK661 and UK661rev. Whereas UK661 and UK661rev induced 100% morbidity and 50-80% mortality, UK661-Q253H-A284T proved to be strikingly attenuated, producing neither morbidity nor mortality. Moreover, UK661-Q253H-A284T-infected animals were protected from challenge infection. Thus, alteration of two specific amino acids in the VP2 region of IBDV resulted in tissue culture adaptation and attenuation in chickens of vvIBDV. The data demonstrate that VP2 plays a decisive role in pathogenicity of IBDV.

Pathogenicity of Czech Isolates of Infectious Bursal Disease Virus

Juranova, R., Nguyen Thi Nga, L. Kulikova, V. Jurajda: Pathogenicity of Czech Isolates of Infectious Bursal Disease Virus. Acta Vet. Brno, 2001, 70: 425-431. An epidemics of infectious bursal disease (IBD) affected chicken farms in the Czech Republic in the mid nineties. Characteristics of six strains of infectious bursal disease virus (IBDV), isolated from broiler chickens, lying hybrid pullets, and chickens kept on small farms, were compared using standard methods of assessment of lymphatic tissue (cloacal bursa, thymus, spleen) index and morphological lesions indicative of immunosuppression, and haematological tests. The results of the investigations confirmed the presence of pathogenic IBDV strains in the local chicken population; four of the isolates induced in susceptible chickens clinical IBD manifested by inflammatory and degenerative lesions in cloacal bursa, thymus and spleen. Irrespective of the severity of the clinical disease, all the isolates induced significant (p < 0.05) bursal atrophy on postinfection days 3 to 6 and a serious damage to the two central lymphatic organs. The effect of infection on haematopoietic tissues was manifested by shifts in the leukogram and in percentages of immature and mature lymphocytes. Infection by some of the isolates resulted in early predominance of heterophils and an increase in the number of mature lymphocytes. Increased number of lymphocytes and a higher percentage of immature lymphocytes were found in chickens recovered from the infection. Although the isolates differed from each other in the indexes of pathogenicity, cloacal bursa, and thymus, they showed similar immunosuppressive activity in regard to the primary lymphatic tissues. The IBDV isolates can be classified as standard strains differing in virulence. None of the isolates showed characteristics of a variant IBDV strains.

Comparative study of the resistance or susceptibility of local Nigerian and exotic chickens to infectious bursal disease

Local or village chickens are the major source of poultry products in less developed countries, but information is limited on their susceptibility or resistance to diseases. In this work, 6‐week‐old local Nigerian chickens, pullets and broilers were used. One day before infection the mean body weight of the broilers was more than twice that of the pullets and local chickens. The bursa: body weight (B:BW) indices of the pullets and local chickens were similar, but were significantly higher than those of broilers. Following inoculation intraocularly with pathogenic infectious bursal disease virus (IBDV) drowsiness, drop in feed and water consumption, and diarrhoea were observed among the local chickens and pullets on day 2 post inoculation (p.i.) and in the broilers on day 3 p.i. Body weight losses from 1 to 11 days p.i. were significantly higher in the local chickens and pullets than in the broilers, total mortalities being 17.6, 50.0 and 61.51% in broilers, pullets and local chickens, respectively. Necropsy showed that dehydration and proventricular haemorrhages occurred most frequent in the local chickens. The response correlated with the B:BW indices on day 14 p.i. This showed that local Nigerian chickens are more susceptible to IBD than broilers and slightly more susceptible than pullets, explaining earlier reports that mortality due to IBD was higher in pullets or light breeds of chicken than in broilers or heavy breeds.

Modified activity of a VP2-located neutralizing epitope on various vaccine, pathogenic and hypervirulent strains of infectious bursal disease virus.

Nine monoclonal antibodies (Mabs) to a vaccine strain of infectious bursal disease virus (IBDV) of intermediate virulence were characterized in Western-blot, radioimmunoprecipitation, ELISA additivity, and neutralization assays. At least two distinct serotype 1-specific conformation-dependent overlapping neutralizing antigenic domains were shown to be present on IBDV-VP2, and were respectively probed by Mabs 3 and 4, and by Mabs 6 and 7. Ten serotype 1 vaccine or pathogenic IBDV strains were tested for neutralization. Most mild or intermediate vaccine strains were efficiently neutralized by all Mabs, whereas US variant A, European pathogenic strain Faragher 52/70 and French hypervirulent isolate 89163 were not neutralized by Mabs 3 and 4. In addition, these two Mabs were shown to bind to the Faragher 52/70 strain, but not to the 89163 isolate, in an antigen-capture ELISA. These results suggest that a neutralizing epitope is possibly modified in European pathogenic IBDV strains, and that Mabs 3 and 4 may prove useful for antigenic differentiation between European classical and hypervirulent isolates.

Association between Pathogenicity of Infectious Bursal Disease Virus and Viral Antigen Distribution Detected by Immunohistochemistry

Highly pathogenic infectious bursal disease virus (IBDV) strains (Ehime/91, DV86) and a moderately pathogenic strain (J1) were compared in order to clarify the association between the pathogenicity of IBDV and viral antigen distribution. Virus target cells in the bursa, thymus, spleen, and bone marrow were examined by transmission electron microscopy. Although all strains caused similar bursal atrophy, the highly pathogenic strains brought about a greater decrease in the thymic weight index and more severe lesions in the cecal tonsil, thymus, spleen, and bone marrow. Immunohistochemical detection of IBDV antigen in tissues from chickens infected with Ehime/91 and DV86 strains showed a higher frequency of antigen-positive cells in the spleen and bone marrow. Transmission electron microscopy indicated the presence of viral particles in the cytoplasm of epithelial reticular cells in the thymus and monocytes in the bone marrow. The results show that pathogenicity of field strains of IBDV correlates with lesion production in non-bursal lymphopoietic organs. The results also suggest that pathogenicity of IBDV may be associated with virus antigen distribution in non-bursal lymphopoietic organs.

Embryo Vaccination of Specific-Pathogen-Free Chickens with Infectious Bursal Disease Virus: Tissue Distribution of the Vaccine Virus and Protection of Hatched Chickens against Disease

Vaccination of specific-pathogen-free chickens as 18-day embryos with the BVM isolate of infectious bursal disease virus (IBDV) resulted in extensive replication of the vaccine virus in the embryonic tissues. The virus was recovered from lung, thymus, proventriculus, liver, kidney, and spleen of embryos 1 day postvaccination, and recoverable virus persisted for at least 7 days. Replication and spread of the vaccine virus in chickens vaccinated as 18-day embryos was compared with that in chickens vaccinated at hatch. Distribution of the virus in tissues was more extensive, virus levels in tissues were generally higher, and detectable virus persisted longer in chickens vaccinated as 18-day embryos than in those vaccinated at hatch. Effective vaccine response could be initiated with 6.2 median embryo lethal doses, the lowest dose tested. Chickens immunized as embryos developed neutralizing antibody against IBDV and resisted challenge with pathogenic IBDV at 4, 6, 8, and 10 weeks of age.

Antibody to the 32K structural protein of infectious bursal disease virus neutralizes viral infectivity in vitro and confers protection on young chickens.

Chicken sera containing IgG antibodies specific for the 32 000 (32K) mol. wt. structural polypeptide of infectious bursal disease (IBD) virus, as assessed by Western blotting, neutralized the in vitro infectivity of tissue culture-adapted IBD virus. When injected into young chickens, the serum passively protected them from challenge with pathogenic IBD virus. Chickens immunized with the 32K structural polypeptide of IBD virus, prepared by electroelution from SDS-PAGE gels, produced antibody detectable by ELISA and the virus neutralization assay, while chickens immunized with the 37K or 41.5K viral polypeptides synthesized antibody detectable by ELISA, but only very low levels of virus-neutralizing antibody. The immunoglobulin fraction of sera obtained from chickens immunized with the 32K polypeptide, but not the 41.5K polypeptide, passively protected chickens from infection with IBD virus. It is concluded that the 32K polypeptide is a major protective immunogen of IBD virus.