Pathogenicity of infectious bursal disease virus variant AL2 in young chickens.

Limited information is available on the effects of the recently emerged infectious bursal disease virus (IBDV) variant AL2. In this study, the effects of inoculation of 4-day-old chickens with increasing doses of IBDV AL2 were characterized. IBDV AL2 induced neither overt clinical signs nor mortality. Infected chickens showed reduced bursa indices (BI) and bursa lymphocytic depletion, as determined by histomorphometry. However, histomorphometry and BI values differed during the early stages of the infection. Because data from bursa histomorphometry were consistent with viral RNA detection, such values seem to be more appropriate for the assessment of AL2 viral infectivity in chickens. Both the histomorphometry and BI data indicated a dose-effect pattern. However, with time, even low doses of the virus ultimately resulted in significant damage to the bursa. Samples of spleen were used to assess B- (IgM+) and T- (CD4+ and CD8+) cell populations by flow cytometry. Infected chickens showed a significant increase of splenic IgM+ cells at 5 and 8 days postinoculation (PI). On day 8 PI, the number of total IgM+ cells in the spleen was inversely related to the virus concentration. Others have shown that cell-mediated immunity is essential for protection against IBDV. Our results indicate a significant increase (P < 0.05) of total spleen CD4+ cell counts on day 8 PI in birds that received higher virus concentrations, indicating a role for these cells in protective immunity, while CD8 cell counts remained unchanged. We speculate that the changes in splenic CD4+ and IgM+ cell populations are associated with protective immune responses against IBDV in the host.