Viral diseases of the immune system and strategies to control infectious bursal disease by vaccination.

Viral infections which are immunosuppressive can affect the economics of poultry production, often as a result of the chicken's increased susceptibility to secondary infections and sub-optimal response to vaccinations. The mechanism of this immunosuppression has been studied in detail for certain chicken viruses. The replicating virus can have both direct and indirect effects on the cells of the immune system. The special role of the bursa of Fabricius, as a lympho-epithelial organ, will be mentioned. The effects of oncogenic viruses (MDV, REV and ALV) on the immune system will be discussed as will the present status of our knowledge on the immunosuppressive effects of certain respiratory viruses such as ILT, NDV and reovirus. Two major immunosuppressive agents are CAV and IBDV. The effects of IBDV will be described in more detail because of its economic importance. Advances made in the molecular biology of both the virus and the immune system give new opportunities to control the disease by vaccination. Successful vaccination strategies applied in the past and options for the future will be discussed.