Characterization of the antigenic, immunogenic, and pathogenic variation of infectious bursal disease virus due to propagation in different host systems (bursa, embryo, and cell culture). II. Antigenicity at the epitope level

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.

A Comparative Study of Pathology and Host Immune Response Induced by Very Virulent Infectious Bursal Disease Virus in Experimentally Infected Chickens of Aseel and White Leghorn Breeds

Indigenous breeds of young chickens in India are believed to be resistant to the classical strain of infectious bursal disease virus (IBDV). However, the mechanism underlying this resistance is obscure. Innate immunity is a key factor in defining the clinical course and pathology of microbial infections. The present study is aimed to compare the pathology of very virulent IBDV (vvIBDV) and immunological host response in experimentally infected - vaccinated and unvaccinated indigenous Aseel and commercial White Leghorn chickens. The viral loads and innate immune gene expression profiles of MDA-5, Mx, IFN-α, and IFN-β in different lymphoid organs were analyzed by quantitative PCR. The histopathological scores in Aseel birds were lower than in White Leghorns despite comparable viral loads. The degrees of histopathological lesions were fewer in vaccinated birds than in unvaccinated birds of both breeds. Analysis of innate immune response genes revealed that the cytoplasmic pattern recognition receptor MDA-5 gene was overexpressed mainly in the cecal tonsils of both vaccinated and nonvaccinated White Leghorn chickens. An increase in the expression of the IFN-α gene was seen in the cecal tonsils of Aseels, and an increase in IFN-β gene expression was seen in the thymuses of White Leghorns following vvIBDV challenge both in vaccinated and nonvaccinated birds. In addition, we observed that the Mx gene plays a minimal role, if any, in vvIBDV infection of the breeds under study. It remains interesting and important that although vvIBDV causes disease in indigenous Aseel birds, the faster clearance and reduced pathology of the virus in Aseel birds compared to White Leghorn chicken indicate some unidentified innate immune factors that are limiting IBDV in this breed. Further studies will be required to correlate kinetics of humoral and cellular immune response in relation to the virus load in different organs to illuminate the mechanism of genetic resistance in native breeds of chicken.

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

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

Preparation and evaluation of an experimental iscom-based infectious bursal disease vaccine

The present study was conducted to develop and evaluate an experimental ISCOM-based infectious bursal disease (IBD) vaccine. The indigenous very virulent infectious bursal disease virus (vvIBDV) already attenuated and adapted to Vero cell line was used. After denaturation of viral proteins with sodium dodecyl sulphate (SDS), an IBD-ISCOM was constructed. The non-incorporated viral components were separated from ISCOM by centrifugation of dialysate. The pathogenicity and immunogenicity trials were conducted in 3-week-old broiler chicken. A commercial oil-emulsified vaccine (CEVAC IBD K) was used for comparison. There were no clinical signs of disease, gross or microscopic lesions in bursa of Fabricius in group G1 vaccinated with ISCOM-based vaccine and bursa to body weight ratio were comparable to un-vaccinated control group (G3). The virus-neutralizing antibody titers were significantly (P<0.05) higher in group G1 as compared with group G2 which was vaccinated with commercial vaccine. On challenge with vvIBDV, 100%, 75% and 0.00% protection was achieved in G1, G2 and G3, respectively. The results indicated that ISCOM-based IBD vaccine is safe and immunogenic.

Characterization of the antigenic, immunogenic, and pathogenic variation of infectious bursal disease virus due to propagation in different host systems (bursa, embryo, and cell culture). III. Pathogenicity

Differences in the relative pathogenicity of variant (1084 E and GLS) and standard (Edgar and STC) infectious bursal disease virus (IBDV) strains were observed after propagation in the bursa of Fabricius, embryos, or cell cultures. Bursa-derived IBDV induced the most severe lesions in the bursa of Fabricius when compared with strains propagated in embryos or cell cultures. Embryo-derived IBDV induced moderate gross bursal lesions, whereas cell culture-derived IBDV did not damage the bursa grossly. A high frequency of virus re-isolations was obtained from bursal, spleen, and thymic samples collected from birds inoculated with bursa-derived or embryo-derived IBDV. Virus re-isolation occurred much less frequently from birds inoculated with cell culture-adapted IBDV. Serological evaluations demonstrated that bursa-derived IBDV strains induced a higher neutralizing antibody response than did embryo-derived or cell culture-derived strains. These results document that the relative pathogenicity and immunogenicity of IBDV is reduced following propagation in embryos or cell cultures.