Acute infectious bursal disease in poultry: a review.

This review is focused on the acute form of infectious bursal disease (IBD) caused by very virulent IBD virus (vvIBDV). First described in Europe about 10 years ago, this new form of the disease has rapidly spread all over the world, causing dramatic losses; after a decade, it still represents a considerable threat to the poultry industry. Emergence of the acute forms of the disease has drastically changed the epidemiology of IBD. Although their origin is still under investigation, vvIBDVs have spread all over the world in a very explosive but conserved manner. This raises the question of the origin of vvIBDVs, of the possible existence of reservoirs and of the possible emergence of new, distinct lineages in the future. While it has become clear that amino acids within the variable region of virus protein VP2 account for the molecular basis of antigenic variation, no definite hot spot that determines pathogenicity has been identified. Fingerprints of VP2 on vvIBDVs have to be considered more as common evolutionary markers than as virulence markers. The search for such markers is in progress. Pathogenesis of the disease is still poorly understood, and cytokines might play a crucial role in the onset of the disease and in the development of immunosuppression. Mechanisms such as apoptosis and necrosis have been described in lymphoid organs and are involved in the severity of the disease. Macrophages, especially, could play a specific role in the acute phase. Classical serotype 1 vaccines still induce good protection, but the actual problem for control of the disease has became the interference of maternally derived antibody in the establishment of the vaccination schedule. The development of safe vaccines that could either transmit a high passive immunity which could protect broilers during the whole growing period or prime an immune response before or at hatching in the presence of passive immunity might be established in the near future. In this context, recombinant vaccines and virus-neutralizing factor technology might have an advantage over other approaches.