Newcastle Disease (ND) has worldwide distribution and Newcastle Disease Virus is by far one of the most important pathogens of all birds. The symptoms of ND include respiratory distress, diarrhea, cessation of egg production, depression, edema of the head, face, and wattles, nervous signs, and death. Birds may display all, some, or none of these symptoms, Newcastle Disease can vary from bird to bird and cause sudden death with 100% mortality to subclinical disease. Antibodies to avian viruses in serum can be detected by the Enzyme Linked Immunosorbant Assay (ELISA). The levels of NDV antibodies in serum, as well as the immune status of entire flocks can be monitored using ELISA. 

Inactivated antigen is bound to a microwell. Diluted sera is added to the microwell and incubated. Any anti-NDV antibodies present in the serum will bind to the NDV on the microwell forming an NDV 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 ND 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.