Mortalities and economic loss result from secondary infections due to the immunosuppression rather the primary viral infection in the subclinical form of the disease. Clinical signs consist of diarrhea, anorexia, depression, trembling, prostration, dehydration, and death. Common gross lesions include hemorrhages within the thigh and pectoral muscles. The spleen may also be enlarged. Histologic lesions are seen primarily within lymphoid organs including the cloacal bursa, spleen, thymus, harderian gland, and cecal tonsil.
Lesions consist primarily of lymphoid degeneration and necrosis.
In the cloacal bursa, there is lymphoid necrosis and infiltration by heterophils and macrophages. As the inflammation and necrosis decline, cystic cavities filled with necrotic debris are left in the medullary region of the follicles. These cystic spaces are delineated by the reticuloepithelial cells of the bursa which are apparently unaffected by the virus. In chickens less than 2 weeks old, infection is clinically inapparent but destruction of the cloacal bursa results in immunosuppression, primarily affecting the humoral immune system. The cellular immune system is affected to a lesser degree and in a transient fashion.