Paratuberculosis: Diagnostic Methods and their Constraints

Paratuberculosis (Johne’s disease) is a chronic intestinal disease of ruminants and the disease is caused by Mycobacterium avium subspecies paratuberculosis (MAP). The disease has a prolonged pre-patent period and the clinical symptoms of the disease are apparent only during the later stages of the disease. However, the infected animal can shed the organism much before the clinical disease with the risk of spreading the disease to other susceptible animals. The disease manifestation is in four stages with each of these stages requiring a different diagnostic method. During the early stages of the disease Cell mediated immune response (CMI) is higher, and shedding of the organism and serum antibody response is absent or minimized. Delayed type hypersensitivity (DTH) response upon injection of Johnin purified protein derivative (Johnin PPD) in the skin of the animal and interferon gamma release assay (IGRA) is the common CMI based tests for the diagnosis of paratuberculosis. As the disease progress, CMI response reduces and humoral response predominates with progressively increased shedding of the organism. ELISA based tests are commercially available to detect the MAP specific antibodies in serum or milk, and nucleic acid detection methods are used to identify the organism. However, all these CMI and humoral antibody based tests have advantages and disadvantages. It is essential to apply a combination of these tests to detect or rule out the disease in farms of endemic countries.