Pathogenesis of infectious bursal disease in chickens infected with virus at various ages.

Chickens free from infectious bursal disease (IBD) maternal antibody were inoculated with a virulent strain of IBD virus at 1, 5, or 11 weeks of age. Chickens inoculated at 5 weeks developed severe clinical signs and had reduced levels of serum complement within 2-4 days postinoculation, but those inoculated at 1 or 11 weeks did not. However, at 1, 2, 4, and 8 days postinoculation, the rate of virus recovery from different tissues, severity of microscopic lesions, and frequency of detection of viral antigens in lymphoid organs of chickens inoculated at 5 weeks were comparable to those of chickens inoculated at 1 or 11 weeks of age. These findings suggest that age resistance to clinical manifestations of IBD is probably independent of the ability of virus to replicate and induce lesions in the host.