Studies on apoptosis caused by vaccine and field strain of infectious bursal disease virus

Infectious bursal disease is an immunosuppressive disease caused by a double stranded, bisegmented RNA virus of the Birnaviridae family. It is known to cause apoptosis in lymphoid cells. The present study was undertaken to determine extent of apoptosis in bursa of Fabricius when exposed to IBD virus. 45 birds of one week of age and 45 birds of six weeks of age were divided into three groups each. Both age groups along with one group were infected with field isolate and an intermediate invasive strain of vaccine (IV-95). Qualitative estimation of apoptosis was done by DNA fragmentation assay and electron microscopy. Quantitative estimation by flow-cytometry revealed percentage of apoptotic cells to be continuously higher in birds below three weeks of age, while it was greater on second and fifth day in birds above three weeks of age within the period of study ie. 5 days. There was no significant difference among field virus infected and vaccinated birds of same age group. Annexin-V staining of bursal impression smear showed higher scoring on second and fifth day in all groups. DNA gel electrophoresis of infected birds of both age groups indicated ladder pattern consistent to apoptotic processes and TEM showed apoptotic changes such as condensation of chromatin, its margination towards nuclear envelop, shrinkage of cytoplasm, fragmentation of nucleus and formation of apoptotic bodies. The investigation indicated greater apoptotic indices in birds below three weeks of age suggesting greater possibility of immunosuppression in birds of this age. Results indicated that vaccine was causing almost equal apoptosis as field virus in terms of DNA fragmentation and apoptotic histo-morphology.