The current invention relates to the diagnosis and treatment of diseases resulting from infections by Mycobacterium avium subsp. paratuberculosis. In particular the invention relates to the use of an antigen selected among (a) a synthetic peptide 5P having the following formula: DPhe-NMeVal-Ile-Phe-Ala-OMe (SEQ ID NO: 1)； (b) a lipopeptide L5P consisting of the synthetic peptide a) wherein the N-terminal phenylalanine residue is N-acylated with an eicosanoic acid acyl chain； (c) a variant of peptide a) or lipopeptide b) able to react with anti-Mycobacterium paratuberculosis antibodies； for in vitro detection or quantification of specific anti-Mycobacterium paratuberculosis antibodies in a biological sample.
