Paratuberculosis (Johne disease) a chronic infectious disease of ruminants caused by Mycobacterium paratuberculosis, is potentially an important problem in deer farming. Primarily a disease of cattle, sheep, and goats, paratuberculosis is characterized by progressive wasting and diarrhea and granulomatous inflammation of the intestines and mesenteric lymph nodes. Infection usually occurs early in life, but the disease manifests itself after a long period of incubation. In deer, paratuberculosis affects a range of animals from yearlings to mature breeding stock. The clinical disease is characterized by a sudden onset, rapid loss of condition and weight and in many but not all cases, diarrhea; and it runs a more acute course than in other ruminants. The typical gross pathological pathological changes of M. paratuberculosis infection in deer are often variable and mild, but the microscopic changes are often quite extensive. In severe cases, lymph node changes are sometimes associated with necrosis, mineralization, and reactive fibrosis. Characteristics of isolates of M. paratuberculosis from deer by culture, pathogenicity tests, SDS-PAGE, and Western blots did not show significant differences from the organism isolated from other ruminants. From experimental infection, the pathogenesis of paratuberculosis in deer was found to be basically similar to that of the disease in other ruminants.