In later phases of the immune response, bacteria escape macrophages into the intestinal lumen and animals begin to shed Map in feces. Eventually the bacteria also escape into the vascular system which stimulates antibody. This humoral immunity however does not play a significant role in controlling Map because of its intracellular localization. Figure 1 . A graph showing hypothetical concentrations of mycobacteria and antibodies through stages of infection.
Horizontal line suggests test detection level.
Ultimately the body s ineffective immune response to Map yields a combination of factors which contribute to the development of clinical disease in an individual. The factors are poorly understood but involve the inflammation and impaired nutrient absorption from the intestine, interaction of systemic and gut immune components, and the eventual collapse of the cellular immune response allowing the unhindered proliferation of the infection. However, some animals may be infected, appear normal and be culled before they show any clinical signs. So, some owners may never realize their herd is infected. A complaint in these herds could be that herd production not as high as it should be.