[Diagnosis of paratuberculosis in the preclinical stage]

Those tests which are common practice in the diagnosis of Johne's disease in the Netherlands, namely the intradermal johnin hypersensitivity test, the complement fixation test and microscopic examination as well as examination of the faeces by culture, were carried out in twenty-one cattle from four herds infected with Johne's disease. In addition, the johnin lymphocytic stimulation test was performed. A number of animals of various ages, which were tested, showed positive johnin hypersensitivity tests prior to the beginning of the 1982 grazing period (Table 1). M. paratuberculosis was isolated from five of these twenty-one animals after death. Six animals showed a positive complement fixation test one or several times; M. paratuberculosis was isolated from four of these animals after death. On the other hand, M. paratuberculosis was isolated from an animal in which this hypersensitivity test had been constantly negative. The other animals from which M. paratuberculosis was isolated, showed positive johnin hypersensitivity tests in some and negative tests in other cases. Not a single shedder of M. paratuberculosis was identified using microscopic examination of the faeces. When the faeces were examined by culture, one of the three animals found to be positive after death was identified. The animals from the suspected herds showed a higher average johnin stimulation index (SI) than did those from herds free from Johne's disease. However, when the LST was used, all carriers of M. paratuberculosis also were not detectable. In conclusion, it has to be stated that none of the tests studied in these cases removed all doubt as to whether the animals were or were not subclinically infected with M. paratuberculosis.