Infectious Bursal Disease (IBD) affects primarily young chickens. The condition is considered economically significant due its ability to induce profound immunosuppression in chickens often resulting in susceptibility to secondary bacterial and viral infections. The symptoms include depression, anorexia, ruffled feathers, and diarrhea. Mortality may be as high as 50%. Antibodies to avian viruses in serum can be detected by the Enzyme Linked Immunosorbant Assay (ELISA). The levels of Variant and Classic-type IBD antibodies in serum, as well as the immune status of entire flocks can be monitored using ELISA. 

Inactivated Variant-type Bursal antigen is bound to a microwell. Diluted sera is added to the microwell and incubated. Any anti-IBD antibodies present in the serum will bind to the IBD on the microwell forming an IBD antigen-antibody complex. After washing away any unbound material, anti-Chicken IgG Alkaline Phosphatase conjugate is added to the microwell. The conjugate will bind to the IBD antigen-antibody complex. After incubation any unbound conjugate is removed by washing. Enzyme substrate is then added to the microwell. The substrate reacts with any Alkaline Phosphatase present and forms a yellow product. In the final step the reaction is stopped, fixing the color. The intensity of the color is measured photometrically at 405 - 410 nm.