Abstract
The present study was designed with the aim at developing live attenuated vaccines for the effective control of infectious bursal disease (IBD) in chickens in Bangladesh. Infectious bursal disease viruses (IBDVs) from seven outbreaks of acute IBD of the year 2007 were isolated using embryonated chicken eggs and identified by agar gel immunodiffusion test, histopathology, immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) assay. Of the 28 isolates, five were subjected to pathogenicity test using 9-day-old chicken embryos and 3-week-old layer chickens, and to RT-PCR coupled with restriction enzyme (RE) analysis. The isolates were similar in their pathogenicity which caused lower bursa/body weight ratios and higher bursal lesion scores. The results of RT-PCR-RE assay lead to speculate that four isolates were very virulent type and one was classical virulent type. Two isolates of the four those were very virulent by RT-PCR-RE assay were further characterized by nucleotide sequencing and found that these two isolates were of mild virulent type and were completely different from previously isolated very virulent IBDV (isolated in 1999) in Bangladesh. The classical strain of IBDV was adapted and attenuated through successive passage into 9-day-old embryonated chicken eggs followed by chicken embryo fibroblast cell (CEF) culture for the preparation of live attenuated tissue culture IBDV vaccines. Two types of live attenuated IBDV vaccines were prepared based on number of passage, namely, IBDV1 (low passaged vaccine) and IBDV2 (high passaged vaccine). Both the vaccines at two dilutions (lower dilution - \(10^{5.0}\text{ TCID}_{50}\text{/ml}\), higher dilution - \(10^{3.0}\text{ TCID}_{50}\text{/ml}\)) were tested for their pathogenicity, immune response and protective efficacy with comparison with two commercial IBDV vaccines (intermediate type and intermediate plus type) in commercial broiler chickens. The vaccine IBDV2 at both dilutions was found to be more immunogenic and efficacious in terms of protection and production of protective antibody titers which could be considered to immunize the commercial chickens against IBD after successful field trials.