Effects of challenge with very virulent infectious bursal disease virus reassortants in commercial chickens.

Pathogenicity and immune responses were characterized in commercial broilers and layers challenged with very virulent infectious bursal disease virus (vvIBDV) reassortants (vvIBDV segment A + serotype 2 segment B and vvIBDV segment A + classic virulent segment B) at 7 days of age. In addition, functional immunosuppression was evaluated after challenge with infectious bronchitis virus (IBV) at 15 days of age. Layers showed higher levels and increased persistence of IBDV- and IBV-specific maternal antibodies than broilers at 1, 13, and 28 days of age. Cytokine gene expression was evaluated, after IBDV challenge, as an indicator of the innate immune function. Similar results were detected between the groups inoculated with vvIBDV reassortants. Interleukin-1β (IL-1β) in the bursa of layers demonstrated down-regulation at 1 day postinfection (DPI; 8 days of age), and no changes at 4 DPI (11 days of age) compared with controls. In broilers, IL-6 expression in the bursa was down-regulated 1 DPI (8 days of age) and up-regulated at 4 DPI (11 days of age). A significant lymphoid depletion was detected at 21 DPI (28 days of age) in broilers exposed to a reassortant of vvIBDV segment A and classic virulent IBDV segment B. Finally, reduced specific antibodies against IBV measured 13 days after challenge were detected in layer and broiler chickens inoculated with a reassortant serotype 2 IBDV in segment B, suggesting functional immunosuppression. These results provide evidence indicating that current IBDV vaccination of breeders does not completely protect progeny chickens from challenge with reassortant vvIBDV.