This project was designed to study the pathological changes associated with experimental Infectious bursal disease in broiler chickens. Two hundred and forty broiler chicks on 21 days of age were randomnly divided into six groups, A through F, each containing 40 birds. The inoculum of Infectious bursal disease virus was inoculated intraocularly in the chicks of group A at the age of 21 days, in group C at the age of 28 days, and in group E at the age of 35 days. Groups B, D and F were kept as control for groups A, C and E respectively. Blood samples and lymphoid organs of seven randomnly selected chickens from each group were collected at day 4 post inoculation (P1), 8 P1 and 12 P1, and haematology (TLC, DLC), serology (AGPT) and histopathology of lymphoid organs were conducted.

Typical clinical signs of IBD were noted. The chief and consistent gross lesion produced in the chickens infected at different ages was an initial increase in the size of bursa. At day 4 P1 bursa was oedematous and double in size but at days 8, 12, P1 it was smaller in size than in control chickens. The spleen and thymus appeared almost normal grossly except a few gray foci on spleen.

Histopathological study of bursa of Fabricius (regardless of the age at infection) revealed an initial macrophage and heterophil infiltration of the lymphoid follicle and inter-follicular spaces followed by increased inter-follicular connective tissue, plasma cells, macrophage and thickening of bursal epithelium. Spleen showed lymphocytic necrosis and depletion in the initial stages followed by repopulation. Statistical analysis of lymphoid organ: body weight indices of spleen, bursa and thymus showed a significant difference in the inoculated and control chickens but there was no difference in the chickens inoculated at different ages. All the inoculated birds were found positive for the Infectious bursal disease virus antibodies. The results of hematological study indicated that there was a decrease in the total number of leukocyte and percentage of hetrophils and lymphocytes in all the inoculated chickens regardless of the age at the time of infection. Thus it can be concluded from this study that age susceptibility, from 21 to 35 days, among broiler chickens for Infectious bursal disease was nearly the same.