INFECTIOUS BURSAL DISEASE

Infectious bursal disease (IBD), caused by an avibirnavirus, has been an economically significant, widely distributed condition affecting im mature chickens since 1960. The classical type 1 conventional strain i s responsible for up to 5% mortality in susceptible flocks. As a resul t of immunosuppression, growth rate, liveability, and productivity may be adversely affected by subsequent exposure to a wide range of viral , bacterial, and protozoal agents. The emergence of highly pathogenic strains of IBD virus in 1988 caused heavy losses in broiler and replac ement breeder and commercial egg pullets in Europe, and subsequently i n Africa and Asia. The causal virus is extremely lymphocidal with an a ffinity for immature B cells resulting in bursal atrophy approximately four days after infection. IBD virus is resistant to environmental ex posure and is transmitted laterally by direct and indirect contact bet ween infected and susceptible flocks. Conventional IBD is controlled b y immunization of parent stock followed by vaccination of progeny afte r maternal antibody levels have waned. Parent level stock is immunized by one or two successive doses of live, mild or intermediate strain, attenuated vaccine to prime the immune system, followed by inactivated oil emulsion vaccines at maturity and at the mid-point of the laying cycle to boost immunity and ensure transfer of protective maternal ant ibody to progeny. Highly pathogenic IBD is controlled by a number of a lternative strategies, including administration of live vaccines of lo w attenuation or simultaneous doses of live intermediate strain attenu ated vaccine with inactivated oil emulsion during the first week. All successful programmes require diligent care regarding handling and adm inistration of vaccine. Serological monitoring using automated ELISA t echnology is an accepted method of confirming that flocks are adequate ly protected. In the future recombinant and subunit vaccines will be d eveloped to control existing strains and the variants which will emerg e in areas with a high density of poultry production.