Running Head: Bursal Follicles after Ibdv Infection

27 Infectious bursal disease (IBD) is characterized by immunosuppression due to 28 the depletion of lymphocytes in the atrophied bursa of Fabricius (BF). We have 29 sometimes encountered contradictory findings: chickens infected with the 30 vaccine IBD virus (IBDV) strain have sometimes exhibited a highly atrophied 31 BF but not immunosuppression. In this study, chickens administered vaccine or 32 wild-type strains of IBDV were later vaccinated with the B1 strain of the 33 Newcastle disease virus (NDV). Bursal changes were examined histologically 34 with a focus on the bursal follicle. The immunoreactivity to NDV was also 35 evaluated with the hemagglutination inhibition test. In gross examination, we 36 observed a few chickens with a severely atrophied BF in vaccine 37 strain-administered groups (vaccine groups), and the level of severity was the 38 same as that in the wild-type strain-administered group (wild-type group). 39 However, these chickens retained humoral antibody responses to NDV and 40 were revealed to possess a higher number of bursal follicles than those of the 41 wild-type group. These results indicated that macroscopic evaluation dose not 42 accurately reflect the immunoreactivity and degree of bursal damage in 43 IBDV-administered chickens. We also found non-immunosuppressed chickens 44 in the wild-type group. These non-immunosuppressed chickens retained a 45 significantly higher number of normal follicles and total follicles according to 46 our statistical analysis. Furthermore, a high correlation coefficient between the 47 NDV-HI titer and the number of normal follicles was found in the wild-type 48 group. These results implied that the retained number of normal follicles is 49 important for the immunoreactivity of chickens infected with IBDV. 50