IFN-γ diagnostic tests in the context of bovine mycobacterial infections in Belgium

In countries where cattle tuberculosis caused by Mycobacterium bovis (Mbov) and paratuberculosis caused by Mycobacterium avium subsp. paratuberculosis (Mptb) are present, testing strategies for the Mbov eradication have to discriminate between these two infections. Present indirect tests are based on the analysis of the specific cellular immune response (DTH, IFN-γ) against crude mycobacterial antigens (avian and bovine PPD). In this study, we compared the evolution of the IFN-γ responses of animals experimentally infected with Mbov, Mptb, or inoculated with Mycobacterium phlei. Mbov inoculation induced a strong IFN-γ response that allows rapid classification of the status of the animals following interpretation criteria set up by us. Experimental inoculation with M. phlei induced sensitisation to mycobacterial antigens as detected by the IFN-γ test but these reactions were of short duration, therefore, repeated testing allows us to define these animals as aspecific reactors. IFN-γ response induced after oral inoculation of calves with Mptb was of low intensity and ratio of responses measured against avian versus bovine PPD did not allow a clear diagnostic at least for the six first month of infection.