Pathogenicity of embryo-adapted serotype 2 OH strain of infectious bursal disease virus in chickens and turkeys

Isolation and genomic characteristics of the novel variant infectious bursal disease virus in China

The infectious bursal disease virus (IBDV) is a member of the viruses that can induce immunosuppression in chickens. In recent years, more and more IBDV-infected cases by the novel variant IBDV were reported in China, and it has been demonstrated that currently used vaccines could not provide complete protection against these new IBDV variants. However, a lack of comprehensive analysis of the genomic characteristics of the novel variant strain IBDV has hampered its vaccine development. In this study, a strain of IBDV, designated HB202201, was phylogenetically analyzed, and it was found that the hypervariable region (HVR) of VP2 belonged to the novel variant strain. Furthermore, the 5'- and 3'-ends of segments A and B were analyzed using the rapid amplification of cDNA end (RACE) method. After the full-length of segment A and segment B were determined, the phylogenetic analysis of the segment A and segment B showed that the isolated HB202201 belonged to A2dB1 genotype, which demonstrated the HB202201 belonged to the novel variant strain. In addition, the specific mutations in VP1-VP5 amino acids were analyzed, which showed that there were multiple typical mutations in novel variant IBDV proteins, including VP1 (G24, I141, V163, and E240), VP2 (K221, and I252), VP3 (Q167 and L196), and VP5 (R7, P44, R92, G104, and E147), whereas there was no typical mutation in VP4. This study provides insights into the genomic and antigenic characteristics of the novel variant IBDV, which will promote the development of novel vaccine against the novel variant IBDV.

Adaptation and Molecular Characterization of Two Malaysian Very Virulent Infectious Bursal Disease Virus Isolates Adapted in BGM-70 Cell Line

Two Malaysian very virulent infectious bursal disease virus (vvIBDV) strains UPM0081 and UPM190 (also known as UPMB00/81 and UPM04/190, respectively) isolated from local IBD outbreaks were serially passaged 12 times (EP12) in specific pathogen free (SPF) chicken embryonated eggs (CEE) by chorioallantoic membrane (CAM) route. The EP12 isolate was further adapted and serially propagated in BGM-70 cell line up to 20 passages (P20). Characteristic cytopathic effects (CPEs) were subtly observed at P1 in both isolates 72 hours postinoculation (pi). The CPE became prominent at P5 with cell rounding, cytoplasmic vacuoles, granulation, and detachment from flask starting from day 3 pi, up to 7 days pi with titers of 10^9.50 TCID₅₀/mL and log10^9.80 TCID₅₀/mL for UPM0081 and UPM190, respectively. The CPE became subtle at P17 and disappeared by P18 and P19 for UPM0081 and UPM190, respectively. However, the presence of IBDV was confirmed by immunoperoxidase, immunofluorescence, and RT-PCR techniques. Phylogenetic analysis showed that these two isolates were of the vvIBDV. It appears that a single mutation of UPM190 and UPM0081 IBDV isolates at D279N could facilitate vvIBDV strain adaptability in CEE and BGM-70 cultures.

Pathogenicity of Genome Reassortant Infectious Bursal Disease Viruses in Chickens and Turkeys

Infectious bursal disease virus (IBDV) contains two genome segments (segment A/segment B) that can reassort among the viruses. Reassortant IBDVs have been identified in several countries including the United States. These reassortant viruses usually include at least one genome segment from a very virulent (vv)IBDV strain. In vivo virulence of six reassortant IBDV from the United States was assessed relative to the virulence of three frequently described IBDV pathotypes: vvIBDV (rB strain), classic virulent (cv)IBDV (STC strain), and subclinical (sc)IBDV (Del-E strain). Morbidity and mortality in 4-wk-old specific-pathogen-free (SPF) leghorns indicated that reassortant IBDV with a vv genome segment A and non-vv segment B were less pathogenic than the vv/vv rB strain but more pathogenic than the cv/cv STC strain. The sc/vv IBDV strain D6337 (sc/vv) was comparable to the STC strain in pathogenicity. Viruses with a serotype 2 (ser2) genome segment A, regardless of the type of genome segment B, did not cause clinical disease in SPF chickens or turkeys. None of the reassorted viruses caused morbidity, mortality, or gross lesions in SPF turkeys. Histopathologic lesions in the bursa of turkeys were not observed in any group except those challenged with the serotype 2 OH strain, which had a mild lymphocytic depletion. No mortality was observed in maternally immune broilers inoculated with any of the IBDV pathotypes at 1, 2, 3, and 4 wk of age. No bursal lesions were observed in any of the broiler chicken groups at 1 wk of age except for the D2712 (ser2/cv)-inoculated birds that had mild lymphocyte depletion. Based on evaluation of bursal lesion scores and IBDV reverse transcriptase-PCR on broilers challenged at 2 wk of age, the K669 (vv/ser2) virus broke through the maternal immunity while the STC, Del-E, rB, D2712 (ser2/cv), and 7741 (vv/cv) viruses did not. All viruses broke through maternal immunity in the broilers at 3 wk of age except the Del-E scIBDV and D2712 (ser2/cv) reassortant IBDVs. At 4 wk of age, maternal antibodies were very low and bursal lesions were observed in all broilers challenged with the viruses. The data indicate that genome reassortant IBDVs are less pathogenic than is the rB (vv/vv) IBDV. However, the reassortant viruses with a vv genome segment A can still cause morbidity and mortality in SPF chickens, and they were able to break through maternal immunity produced via use of commercial classic and variant vaccines at an early age. This suggests that current breeder vaccination programs may not adequately protect against the reassortant vv/ser2 and vv/cv IBDV strains.

Infection with Some Infectious Bursal Disease Virus Pathotypes Produces Virus in Chicken Muscle Tissue and the Role of Humoral Immunity as a Mitigation Strategy

Infectious bursal disease virus (IBDV) causes important economic losses and negatively affects global trade in poultry and poultry products. This study determined the presence of IBDV in primary lymphoid tissues and muscle tissue of infected broilers and the role of vaccination as a mitigation strategy. In the first study, specific-pathogen-free (SPF) broiler chickens were challenged with STC (classical [cIBDV]), Indiana (variant [varIBDV]), rA (very virulent [vvIBDV]), or Ohio (serotype 2, avirulent) IBDV. Infection was confirmed in all groups, but only the cIBDV group experienced morbidity or mortality. Virus was only isolated in low titers from a few breast and/or thigh muscle tissue samples from cIBDV and vvIBDV-infected chickens. For the second study, SPF broilers from three different treatment groups were challenged with IBDV viruses that currently circulate in the United States, varIBDV or vvIBDV: 1) maternal antibody-positive (MAb+), vaccinated with recombinant HVT-IBDV vaccine (Vaxxitek^®, Merial; MAb+/Vax); 2) MAb+, not-vaccinated (MAb+/Unvax); and 3) maternal antibody-negative, not-vaccinated chickens (MAb-/Unvax). MAb+/Vax and MAb+/Unvax chickens had significantly lower virus titers in primary lymphoid tissues compared to MAb-/Unvax chickens. No virus was detected in muscle tissues from any of the groups challenged with varIBDV, confirming the results of the first experiment. Only 1 of 36 (MAb+/Vax) and 2 of 36 (MAb+/Unvax) muscle samples were positive at minimal amounts (10^1.97 EID₅₀/ml) in vvIBDV challenge, compared to the 9 of 36 muscle samples that were positive in the MAb-/Unvax group. This study indicates that only cIBDV and vvIBDV strains can be found in muscle at low titers of SPF meat chickens and that the breeder vaccination with MAb transfer to progeny with or without accompanying progeny vaccination, as practiced in the United States, was an effective mitigation strategy for vvIBDV-challenged birds.

Detection of antibodies against serotypes 1 and 2 infectious bursal disease virus by commercial ELISA kits

Two distinct serotypes of infectious bursal disease virus (IBDV) are recognized in chicken and turkey flocks in the United States. Serologic testing of chicken flocks for serotype 1 viruses is routinely performed to monitor disease status and vaccination. Earlier studies indicated that enzyme-linked immunosorbent assay (ELISA) test detects antibodies to both serotypes of the virus, while the virus neutralization (VN) test is serotype specific. It is useful to evaluate currently available commercial ELISA kits for their ability to differentiate between antibodies elicited by the two serotypes. Three trials were performed in which chickens were orally inoculated with either a high or a low dose of serotype 1 STC or serotype 2 OH strains of IBDV. Sera collected at 0, 7, 14, and 21 days from these chickens and antisera procured from naturally infected broiler (n=20) and layer (n=30) flocks were tested with five different commercial ELISA kits and by VN. All ELISA kits detected different levels of antibodies elicited against serotype 1 of the virus and moderate and high levels of antibodies against serotype 2 virus. A correlation existed between the ELISA and the VN titers of experimentally infected chickens. All serum samples tested from the commercial layer flocks and 65% of the broiler flocks had antibodies against the OH strain. However, no correlation between the VN titers and ELISA titers was observed for the commercial broilers and layers sera by the majority of the kits. The results indicated that currently available commercial ELISA kits detect antibodies elicited by the two serotypes of IBDV. Hence, the prevalence of serotype 2 antibodies in the flocks should be considered while determining antibody profiles of the flocks against serotype 1 viruses.