Characteristics of very virulent infectious bursal disease viruses isolated from Chinese broiler chickens (2012-2013)

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.

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.

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.

Phylogenetic analyses and pathogenicity of a variant infectious bursal disease virus strain isolated in China

Infectious bursal disease is an acute, highly contagious, immunosuppressive disease of young chickens caused by infectious bursal disease virus (IBDV). In recent years, there has been a notable increase in the isolation rates of variant IBDV strains in China; however, the pathogenicity of these variants is unclear. In the current study, we characterized variant IBDV strain ZD-2018-1 and assessed its pathogenicity in specific-pathogen-free chickens. Phylogenetic analysis revealed that ZD-2018-1 belonged to the variant IBDV strain, which showed several unique amino acid mutations compared with other previously-isolated variant IBDV strains. Pathogenicity assays showed that ZD-2018-1 was less virulent than very virulent IBDV strain SD-2013-1, and did not cause an obvious symptoms or death. In comparison, strain SD-2013-1 had a high mortality rate and caused severe lesions in various tissues. However, both of the strains induced obvious pathological lesions on the bursa of Fabricius, resulting in severe immunosuppression in the infected chickens. The results of this study present a systematic evaluation of the genetic characteristics, pathogenicity, and immunosuppressive properties of a new variant IBDV strain, and may help in the development of strategies for the prevention and control of IBDV in poultry.

Further evidence for very virulent infectious bursal disease virus in vaccinated chickens in Nigeria

Reverse transcription polymerase chain reaction (RT-PCR) and partial sequencing of the VP2 hypervariable region was performed on clinical samples from two infectious bursal disease (IBD) outbreaks in Plateau state, Nigeria. IBD virus RNA was detected in all four bursa of Fabricius samples. Nucleotide sequencing and analysis of the four samples revealed high similarity to previous IBDV sequences from northern and southern Nigeria. The deduced amino acid sequences were compared to reference IBDV strains retrieved from the GenBank; virulence markers A222, I256, and I294 were conserved in both outbreak and reference sequences. Amino acid residue S254 was conserved in the outbreak viruses and previous viruses from northern Nigeria. Phylogenetic analysis revealed that all four viruses were very virulent IBDVs. These viruses clustered with vv2-1 variant viruses from Oyo and Ogun states and less closely with vv2-2 isolates from Tanzania. The nucleotide identity of the sequences in this study ranged from 99.6 to 100 % with each other. These findings are further evidence of IBD outbreaks in vaccinated chicken flocks in Nigeria.