شناسایی و ارزیابی بیماری زایی ویروس بیماری بورس عفونی
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGMENTS
REFERENCES
ABSTRACT
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.
INTRODUCTION
Infectious bursal disease virus (IBDV) is the primary etiological agent of a highly contagious disease among young chickens. The virus damages the immune organs, particularly the bursa of Fabricius, resulting in severe immune suppression and secondary infections with high morbidity and mortality (Rosenberger and Gelb, 1978; Sharma et al., 2000; Ye et al., 2017). As a member of the Birnaviridae family, IBDV contains a doublestranded RNA genome with segments A and B. The 2.8 kb segment B encodes the VP1 protein (879 amino acids (aa)), which is responsible for viral genome replication and RNA synthesis as a RNA-dependent RNA polymerase. The 3.2 kb segment A contains two partially overlapping open reading frames (ORFs), 1 and 2. The smaller ORF1 precedes and overlaps the larger ORF2 and encodes the non-structural protein VP5. The ORF2 encodes a precursor polyprotein (approximately 1012 aa), which is self-processed into three mature proteins: VP2, VP4, and VP3. VP2 is a major structural protein containing the antigen epitope that induces the production of neutralizing antibodies (Alfonso-Morales et al., 2013). Among different strains, the aa sequences of the highly variable region of VP2 are different and commonly affect the virulence of different virus strains (Hoque et al., 2001; Letzel et al., 2007; Lai et al., 2014; Yilmaz et al., 2019). The highly variable region is usually located between the aa residues 206 and 350 and is widely used for molecular epidemiology and phylogenetic studies (Ren et al., 2009; Drissi Touzani et al., 2019).