Characterization and pathogenicity of infectious bursal disease virus in southern China

Rapid identification, pathotyping and quantification of infectious bursal disease virus by high-resolution melting curve quantitative reverse transcription PCR analysis: An innovative technology well-suited for real-time large-scale epidemiological surveillance

With the virus continuing to evolve, very virulent IBDV (vvIBDV) and novel variant IBDV (nvIBDV) have become the predominant epidemic strains in China, exacerbated by the widespread use of attenuated vaccine strains (attIBDV), making a complex infection situation of IBDV in the field. Therefore, developing a rapid and accurate high-resolution melting curve quantitative reverse transcription PCR (HRM-qRT-PCR) for the identification and pathotyping of IBDV is crucial for clinical monitoring and disease control. Extensive data analysis and genome-screening of the three dominant IBDV pathotypes identified a specific region (nucleotides 2450-2603 in segment A) with distinct GC content as the detection target. Experimental testing of HRM-qRT-PCR revealed distinct melting curves and high sensitivity, with the detection limits of 61.2 copies/μL, 61.1 copies/μL and 67.5 copies/μL for vvIBDV, nvIBDV and attIBDV, respectively. The method exhibited excellent specificity, with no inter-genotypes cross-reactivity among the three pathotypes and no reactivity to other common avian pathogens. Applied to samples with double and triple co-infections of different IBDV pathotypes, the method displayed specific melting peaks corresponding to the viruses present in the samples, with an accuracy rate of 100 %. This method precisely identifies and differentiates all the single or co-infected samples, generating distinct peaks corresponding to the Tm values of each virus pathotype in traditional melting curve plots. Furthermore, the method overcomes the limitations of traditional pathotyping methods, requiring only one reaction to achieve rapid viral pathotyping and facilitating quantitative analysis of viruses within the samples. This study introduces an innovative HRM-qRT-PCR method, offering new technology to rapid and accurate identification, pathotyping and quantification of vvIBDV, nvIBDV, and attIBDV. With strong discriminatory power, user-friendliness and a short processing time, this method is highly attractive for the rapid IBDV pathotyping in real-time large-scale epidemiological surveillance during outbreaks.

The complete protections induced by the oil emulsion vaccines of the novel variant infectious bursal disease viruses against the homologous challenges indicating the important roles of both VP2 and VP1 in the antigenicity and pathogenicity of the virus

Novel variant infectious bursal disease virus (nvIBDV) is an emerging genotype (A2dB1b) that can cause severe and prolonged immunosuppression in young chickens. Despite current commercial vaccines being proven to lack complete protection against nvIBDV, it remains unclear whether the oil emulsion inactivated vaccines (OEVs) of the homologous and heterologous virus or booster immunization can provide effective protection. In this study, OEVs with two types of nvIBDV isolates QZ191002 (A-nv/B-nv) and YL160304 (A-nv/B-HLJ0504-like) were prepared and evaluated the protective effects of OEVs plus the booster immunizations with different current commercial vaccines against the challenge of nvIBDVs. The results from vaccination-challenge experiments showed that nvIBDV could break through the protection provided by only one immunization dose of the commercial vaccines, with the protection rates ranging from 40% to 60%. Interestingly, even with booster immunization with different commercial vaccines, the protection rates could only be increased to 60%-80%. As expected, only the OEVs of the homologous virus could provide 100% protection against the homologous nvIBDV, which could induce high-level specific antibodies, ameliorate target organ damage, and significantly reduce the viral load of the bursal in the challenged chickens. Notably, YL160304-OEV performed better than QZ191002-OEV, providing 100% protection not only against the challenge of homologous strain but also against that of heterologous QZ191002 strain. Antibody levels of the immunized chickens gradually increased after a short decline and reached the highest level on the age of 28 days. Similarly, the percentages of lymphocytes CD4+, CD8+ T, and B in peripheral blood lymphocytes (PBLs) were significantly increased on 21 d and 28 d. Notably, despite the nvIBDV, OEVs initially induced a delayed responses in the early stages but ultimately reach higher levels of CD4+ and CD8+ T lymphocytes. The results of study suggest that even booster immunization with different commercial vaccines cannot provide complete protection against nvIBDV, while the OEVs made by the nvIBDVs can provide full protection. Moreover, YL160304-OEV exhibits a broader protective spectrum against different nvIBDV strains, making it a potential candidate for the development of new vaccine.

Effects of DHAV-3 infection on innate immunity, antioxidant capacity, and lipid metabolism in ducks with different DHAV-3 susceptibilities

The aim of the experiment was to evaluate the status of innate immunity, oxidative status and lipid accumulation in ducklings exhibiting varying susceptibilities to DHAV-3 infection. In the experiment, ducklings with different DHAV-3 susceptibilities were used. Samples were collected at 6, 12, 15, and 24 h post infection (hpi), with 5 samples per time point. Plasma biochemistry, antioxidant enzyme activities, lipid content of liver and kidney were detected in the experiment. Elevated plasma level of total bilirubin, direct bilirubin, and creatinine indicated the injury of liver and kidney in susceptible ducklings (P < 0.05). The histopathological sections showed the injury in kidney. During the infection time, there was an increase in the concentrations of reactive oxygen species and oxidative damage markers (malondialdehyde and nitric oxide) in plasma of susceptible ducklings, particularly at 24 hpi (P < 0.05). Compared with the resistant ducklings, DHAV-3 infection resulted in a significant increase in the plasma total triglyceride (TG) level and a decrease in glucose level in susceptible ducklings. Gene expression of the innate immune response was both investigated in liver and kidney. In resistant ducklings, the expressions levels of pattern recognition receptors RIG-I, MDA5 remained constant. In contrast, the gene expressions peaked at 24 hpi in the susceptible ducklings. DHAV-3 infection promoted the expression of IFN, IL6, IL12β, caspase-8 or caspase-9 in both liver and kidney of susceptible ducklings. In conclusion, DHAV-3 infection led to the mobilization of antioxidant defenses, alterations in lipid metabolism, and oxidative stress in susceptible ducklings during DHAV-3 infection.

Phylogenotyping of infectious bursal disease virus in Vietnam according to the newly unified genotypic classification scheme

Since 1987, infectious bursal disease virus (IBDV) has circulated and evolved in Vietnam, but little is known about the genotypes present. IBDV samples were collected in 1987, 2001-2006, 2008, 2011, 2015-2019, and 2021 in 18 provinces. We conducted phylogenotyping analysis based on an alignment of 143 VP2-HVR (hypervariable region) sequences from 64 Vietnamese isolates (26 previous and 38 additional sequences and two vaccines, and alignment of 82 VP1 B-marker sequences, including one vaccine and four Vietnamese field strains. The analysis identified three A-genotypes, A1, A3, and A7, and two B-genotypes, B1 and B3, among the Vietnamese IBDV isolates. The lowest average evolutionary distance (8.6%) was seen between the A1 and A3 genotypes, and the highest (21.7%) was between A5 and A7, while there was a distance of 14% between B1 and B3 and 17% between B3 and B2. Unique signature residues were observed for genotypes A2, A3, A5, A6, and A8, which could be used for genotypic discrimination. A timeline statistical summary revealed that the A3-genotype predominated (79.8% presence) in Vietnam from 1987 to 2021 and that it remained the dominant IBDV genotype over the last five years (2016-2021). The current study contributes to a better understanding of the circulating genotypes and evolution of IBDV in Vietnam and worldwide.

gga-miR-20b-5p inhibits infectious bursal disease virus replication via targeting Netrin 4

MicroRNAs (miRNAs) involved host-virus interaction, affecting the replication or pathogenesis of several viruses. Frontier evidences suggested that miRNAs play essential roles in infectious bursal disease virus (IBDV) replication. However, the biological function of miRNAs and the underlying molecular mechanisms are still unclear. Here, we reported that gga-miR-20b-5p acted as a negative factor affecting IBDV infection. We found that gga-miR-20b-5p was significantly up-regulated during IBDV infection in host cells, and that gga-miR-20b-5p effectively inhibited IBDV replication via targeting the expression of host protein netrin 4 (NTN4). In contrast, inhibition of endogenous miR-20b-5p markedly facilitated viral replication associated with enhancing NTN4 expression. Collectively, these findings highlight a crucial role of gga-miR-20b-5p in IBDV replication.

Characterization and pathogenicity of a novel variant infectious bursal disease virus in China

Infectious bursal disease (IBD) is a highly epidemic and immunosuppressive disease of 3- to 6-week-old chicks caused by infectious bursal disease virus (IBDV). Since 2017, there has been a notable increase in the isolation rates of novel variant IBDV strains in China, of which characteristic amino acid residues were different from those of early antigen variants. In this study, one IBDV strain was isolated from a farm with suspected IBD outbreak in Shandong Province, China, which was designated LY21/2. The strain LY21/2 could replicate in MC38 cells with previous culture adaption in SPF chick embryos. Phylogenetic analysis revealed that LY21/2 formed one branch with novel variant IBDVs and shared 96.8–98.6% nucleotide sequence identity with them. Moreover, LY21/2 serving as the major parent underwent the recombination event of a variant strain (19D69), while the minor parent was a very virulent strain (Harbin-1). SPF chicks inoculated with LY21/2 showed no gross clinic symptom, whereas bursal atrophy was exhibited and apoptosis was occurred in 55.21% of bursal cells. The results of histopathology and immunohistochemical staining showed that lymphocyte depletion and connective tissue hyperplasia and IBDV antigen-positive cells were observed in the bursa of LY21/2-infected chicks. Besides, DNA fragmentation was detected in the LY21/2-infected bursal tissue section by TUNEL assay. Collectivtely, these data presented analysis and evaluation of the genetic characteristics and pathogenicity of a novel variant IBDV strain. This study may help in the development of biosafety strategies for the prevention and control of IBDV in poultry.