Department: Applied Microbiology And Brewing
Infectious bursal disease (IBD) is an economically important, acute, highly contagious and immunosuppressive viral disease of poultry that affects young birds between the ages of 3 to 6 weeks. The disease targets lymphoid tissues with a special predilection for the Bursa of Fabricius. Infectious bursal disease virus (IBDV) can cause high mortality and morbidity in chickens aged 3 weeks upwards depending on the virulence of the infecting strain. In the field, despite the massive vaccination program designed to curb IBD virus, outbreaks are still being reported in vaccinated flocks. This study investigated the underlying factor(s) responsible for the constant and persistent post vaccinal outbreaks of IBD with high mortalities and morbidities recorded in poultry flocks in Nigeria, by studying the genes of IBD viruses obtained from outbreaks in the field. Bursae samples obtained from chickens suspected to have died of IBDV were first screened using Agar gel immunodiffusion (AGID) test followed by isolation attempts of field IBD viruses using embryonated eggs and chicken embryo fibroblast (CEF) cell culture. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to detect the genome of IBDV. Reverse transcriptase-polymerase chain reaction/Restriction fragment length polymorphism (RT-PCR/RFLP) was used to study the number and size of the restriction fragments obtained following digestion of RT-PCR products with restriction endonucleases. The hypervariable (hv) VP2 (viral protein 2) and VP1 (viral protein 1) regions of segments A and B genomes of the field IBDVs were sequenced, respectively. Nucleotide, amino acid and phylogenetic analysis were carried out on the VP2 region for accurate strain identification/characterization and on the VP1 (viral protein 1) region to identify reassortant strains of IBDV. Out of the139 bursae initially screened, 93 (66.9%) were positive by AGID. At second passage of the field viruses, IBDV antigens were not detected except from one sample identified with the result of the sequence analysis confirming the need for the use of specific pathogen free (SPF) hosts for the isolation of IBDV and the difficulty in isolation of field IBDV in embryonated eggs and tissue culture. A 743bp fragment of the hvVP2 region was amplified from 143 (66.2%) out of 216 of the samples tested while a 722bp fragment of the VP1 gene was amplified from 86 (60.14%) of these by RT-PCR. The sequence results of the hvVP2 region revealed that the very virulent Infectious bursal disease virus (vvIBDV) and classic strain were present in the flocks sampled with vvIBDV being more prevalent. The amino acid sequences also showed mutations that were previously not present. The phylogenetic analysis revealed that the Nigerian IBDVs were closely related to vvIBDV from Europe and Asia but formed distinct clusters of their own. Results obtained from the sequence analysis of the VP1 region showed that reassorted IBDV strains were present in Nigerian poultry flocks. Sequence results also revealed motifs at the triplet amino acid positions especially at amino acid position 145 and showed that the Nigerian IBDV had vvIBDV-like and non-vvIBDV-like segment B with a segment A derived from vvIBDV. Phylogenetic analysis of the Nigerian IBDV revealed a novel lineage of reassorted IBDV segment B. These findings clearly show that the vvIBDV is the prevalent strain affecting the poultry population in Nigeria breaking through any protection conferred by the vaccine strains used in the field thereby causing post vaccination outbreaks. The presence of reassortant strains reported in this study has further compounded the problem of effective control of IBDV in Nigeria. Therefore, there is a need for continuous molecular and epidemiological study of infectious bursal disease virus in Nigeria to keep track of the changes (mutations) observed to avoid sudden emergence of new strains. There is also a need to review the current infectious bursal disease virus vaccines used in the field to prevent the high mortalities observed during outbreaks especially by the very virulent strain of infectious bursal disease virus.