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Abstract
Infectious bursal disease (IBD), caused by an avibirnavirus, has been an economically significant, widely distributed condition affecting immature chickens since 1960. The classical type 1 conventional strain is responsible for up to 5% mortality in susceptible flocks. As a result 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 replacement breeder and commercial egg pullets in Europe, and subsequently in Africa and Asia. The causal virus is extremely lymphocidal with an affinity for immature B cells resulting in bursal atrophy approximately four days after infection. IBD virus is resistant to environmental exposure and is transmitted laterally by direct and indirect contact between infected and susceptible flocks. Conventional IBD is controlled by immunization of parent stock followed by vaccination of progeny after 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 antibody to progeny. Highly pathogenic IBD is controlled by a number of alternative strategies, including administration of live vaccines of low attenuation or simultaneous doses of live intermediate strain attenuated vaccine with inactivated oil emulsion during the first week. All successful programmes require diligent care regarding handling and administration of vaccine. Serological monitoring using automated ELISA technology is an accepted method of confirming that flocks are adequately protected. In the future recombinant and subunit vaccines will be developed to control existing strains and the variants which will emerge in areas with a high density of poultry production.