Histopathological appearance of bursal sections (hematoxylin and eosin) derived from groups of chickens infected with the modified viruses based on the backbone of virulent strain rGx-F9VP2 (A) and attenuated strain rGt (B) at different days post-infection (X 200).

<p>(A) The virulent strain rGx-F9VP2 induced moderate and persistent histopathological bursal lesions including middling lymphocytic deletion and necrosis, regional atrophy, and fibrosis of the follicle. In the rGxHT-256 group, no pathological lesions were observed at 2 d p.i.; slight lymphocytic deletion and necrosis was observed at 3 and 5 d p.i.; moderate lesions including atrophy and mild vacuolation of the follicle were observed at 7 d p.i.; and then the bursa began to recover from 10 d p.i. and returned to normal at 14 d p.i. The rGxHT-249 group presented no pathological lesions other than very slight lymphocytic deletions at 5 d p.i. There were no microscopic lesions of the bursa in the control chickens infected with DMEM. (B) The atrophy of the follicle, lymphocytic deletion, and connective tissue hyperplasia appeared in the bursa of rGt-249 group at 7 and 10 d p.i. No bursal lesions were observed in rGt-256, rGt, and DMEM group.