Cell-Mediated Tests

There are two tests available for detecting a cell- mediated immune (CMI) response to MAP in­fection: the intradermal tuberculin test (or skin test), using johnin or avian PPD, and the IFN-γ test.

Se and Sp estimates for IFN-γ tests have not been reported for the detection of MAP-infected animals. It has been suggested that some ani­mals with CMI reactions may be able to control and eradicate MAP (Mikkelsen et al., 2009). If this is the case, a test-positive reaction may be an indicator of a past infection, and the test can be expected to have a specificity of less than 100%, regardless of the antigens used. For animals deemed to be MAP infectious, Se estimates for IFN-γ tests have been reported between 0.13 and 0.85, with Sp estimates between 0.88 and 0.94 (Nielsen and Toft, 2008). There are limited data available to support the use of CMI diagnostics for purposes other than determining whether animals have been exposed to MAP. If IFN-γ tests are to be used, animals should be >15 months of age to avoid false-positive reactions caused by in­nate IFN-γ production by natural killer (NK) cells (Jungersen et al., 2002).

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