This study was aimed to investigate changes in the number and distribution of lymphocytes in the mucosa associated lymphoid tissues (MALT) of digestive tract (proventriculus, duodenum, jejunum, ileum, cecum and cecal tonsils) and respiratory system (lungs) of chicken infected by Infectious Bursal Disease Virus (IBDV). Samples were divided into two groups; IBDV infected group (21, 24 and 30 days old) and control group (non infected birds; 21 days old). Haematoxylin and eosin stained slides were prepared for microscopic studies to observe the distribution and the number of lymphocytes in the mucosa of the digestive tract and respiratory system. Lymphocytes were significantly (P<0.05) lower in proventriculus, duodenum, jejunum, ileum, cecum, cecal tonsils and lungs of IBDV infected chickens than the control. Moreover, the reduction in lymphocytes number was maximum in duodenum and cecal tonsils, while minimal in lungs. Depletion of lymphocyte was mainly in the lamina propria and the core of the villi and depletion increased with the advance of age of IBDV infected chicken. These results demonstrate that IBDV destroys the lymphocytes of the MALT and suppresses the immunity.

Key words: Chicken, Distribution, IBDV, Lymphocytes, MALT