Attenuation of very virulent infectious bursal disease virus and comparison of full sequences of virulent and attenuated strains. Avian Pathol
Migal, South Industrial Area, Kiryat Shmona, Israel.Avian Pathology (Impact Factor: 1.64). 05/2008; 37(2):151-9. DOI: 10.1080/03079450801910206
A very virulent strain of infectious bursal disease virus (IBDVks) was isolated from the bursae of Fabricius of IBDV-affected broiler chickens. Following 43 serial passages in specific pathogen-free embryonated eggs, an attenuated strain was established (IBDVmb). Dosages of IBDVmb in the range 10(2) to 10(4) embryo infective dose of 50% were found to be safe and protective for commercial chicks. Chickens vaccinated with live vaccine containing IBDVmb responded with precipitating and type-specific neutralizing antibodies, and were immune to subsequent challenge with a very virulent IBDV. IBDVmb has been used as an attenuated vaccine throughout the world since 1993. A comparison of the full sequences of the virulent and attenuated strains (IBDVks and IBDVmb, respectively) revealed seven nucleotides that were different, four of them leading to changes in the amino-acid sequence. Comparison of the protein sequence of these strains and published sequences of very virulent and attenuated phenotypes lead us to suggest that the novel difference responsible for virulence of the Israeli strains are: residue 272 (VP2, very conserved site) and residue 527 (VP4), both in segment A, and in segment B (VP1) residues 96 and 161 (both conserved). Our study strengthens the possibility that more than one protein is involved in IBDV attenuation. In all reports, including ours, virulence was reduced without affecting antigenicity of the neutralizing epitopes in VP2. This could have practical implications for attenuated-vaccine development.
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ABSTRACT: The very virulent infectious bursal disease virus (vvIBDV) Gx strain causes over 60% mortality in chickens but cannot replicate in CEF cultures. The attenuated Gt strain, however, is not virulent in chickens and replicates well in CEF cultures. The two strains display differences in 6 amino acids in VP4 and 4 amino acids in VP3. To determine whether VP4 and VP3 are involved in the virulence and replication of IBDV, three chimeric viruses, in which the VP4/VP3/3'UTR, VP3/3'UTR or VP4 region of Gt were replaced by the corresponding region of Gx, were constructed and characterized in vitro and in vivo. The substituted regions in VP4 or VP3 did not affect virulence of Gt. While the substituted region in VP4 had no effect on viral replication of Gt in CEF cultures, substitution of the VP3/3'UTR region did reduce the replicative capacity of the virus. Through site-directed mutagenesis, three rescued recombinant viruses with a single amino acid substitution in the C-terminus of VP3 of the Gt strain (L981P, A990V and T1005A) were characterized in a similar manner. Amino acid substitution at position 990 reduced viral replication of Gt and reduced its efficacy of protection against vvIBDV Gx challenge in vivo. This study provides important information for the design and development of more effective IBDV vaccines using reverse genetics.
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ABSTRACT: Rapid and reliable detection and classification of infectious bursal disease viruses (IBDVs) is of crucial importance for disease surveillance and control. This study presents the development and validation of a real-time RT-PCR assay to detect and discriminate very virulent (vv) from non-vv (classic and variant) IBDV strains. The assay uses two fluorogenic, minor groove-binding (MGB) TaqMan probes targeted to a single nucleotide polymorphism (SNP) embedded in a highly conserved genomic region. The analytical sensitivity of the assay was determined using serial dilutions of in vitro-transcribed RNA. The assay demonstrated a wide dynamic range between 10(2) and 10(8) standard RNA copies per reaction. Good reproducibility was also detected, with intra- and inter-assay coefficients of variation ranging from 0.13% to 2.23% and 0.26% to 1.92%, respectively. The assay detected successfully all the assessed vv, classical, and variant field and vaccine strains and correctly discriminated all vvIBDV strains from non-vvIBDV strains. Other common avian RNA viruses tested negative, indicating high specificity of the assay. The high sensitivity, rapidity, reproducibility, and specificity of the real-time RT-PCR assay make this method suitable for general and genotype-specific detection and quantitation.
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ABSTRACT: Infectious bursal disease virus (IBDV) causes an economically significant disease of chickens worldwide. Very virulent (vv) IBDV strains have emerged and induce up to 60% mortality. The molecular basis for vvIBDV pathogenicity is not understood and the relative contribution of the two genome segments A and B in this phenomenon is not known. Isolate 94432 was previously shown as being genetically related to vvIBDVs but exhibits an atypical antigenicity and does not cause mortality. Here, the full-length genome of 94432 was determined and a reverse genetics system was established. The molecular clone was rescued and exhibited the same antigenicity and reduced pathogenicity as 94432 isolate. Genetically modified viruses derived from 94432, whose vvIBDV consensus nucleotide sequence was restored in segment A and/or B, were produced and their pathogenicity assessed in specific pathogen free chickens. We found that a valine (position 321) that modifies the most exposed part of the capsid protein VP2 critically modified the antigenicity and partially reduced the pathogenicity of 94432. However, a threonine (position 276) located in the finger domain of the virus polymerase (VP1) contributed even more significantly to attenuation. This threonine is partially exposed in an hydrophobic groove on VP1 surface, which suggests possible interactions between VP1 and another, as yet unidentified, molecule at that this amino acid position. The restored vvIBDV-like pathogenicity was associated with increased replication and lesions in the thymus and spleen. These results demonstrate that both genome segments influence vvIBDV pathogenicity and may provide new targets for the attenuation of vvIBDVs.