The silent period of infection lasts for variable periods until granulomatous inflammatory foci arise that may become generalized, with shedding of MAP. There is a systemic cellular immune response that can be detected by the intradermal skin test, lymphocyte proliferation assay or whole blood IFN-gamma tests, indicating exposure to MAP and offering potential early detection of exposed individuals and farms, although they are still unable to distinguish between active infection and exposure. This evidence concerns the gene IFNG and infection.