Thymic lesions in chicken infected with infectious bursal disease virus.

Thymic lesions were investigated in specific-pathogen-free chickens infected with the highly virulent HPS-2 strain or virulent reference GBF-1 strain of infectious bursal disease virus (IBDV). Both viruses induced follicular lymphoid necrosis and depletion in the bursa, as well as thymic lesions. However, cortical lymphocyte condensation and depletion in the thymus were more prominent in the HPS-2 group than in the GBF-1 group. Electronmicroscopically, alterations of cortical lymphocytes were characterized by nuclear disintegration resembling apoptosis. Immunohistologically, the virus-antigen-positive epithelial reticular cells in the medulla were more extensive in the HPS-2 group. These results suggest that HPS-2 IBDV persists in the thymic medulla and that cortical lymphocyte depletion is caused by virus-induced apoptosis.