Paratuberculosis (Johne’s disease) is a chronic enteritis of ruminants caused by Mycobacterium
avium subsp. paratuberculosis (M. paratuberculosis) (49).
Identification of the agent: The diagnosis of paratuberculosis is divided into two parts: the
diagnosis of clinical disease and the detection of subclinical infection. The latter is essential for
control of the disease at the farm, national or international level.
Diagnosis of paratuberculosis is made on clinical grounds confirmed by the demonstration of
M. paratuberculosis in the faeces by microscopy, culture, or by the use of DNA probes and the
polymerase chain reaction. Diagnosis is made at necropsy by the finding of the pathognomonic
lesions of the disease in the intestines, either grossly with the demonstration of typical acid-fast
organisms in impression smears of the lesions or histologically, and by isolation of
M. paratuberculosis in culture.
The detection of subclinical infection depends on the detection of specific antibodies by serology, or
culture of M. paratuberculosis from faeces or tissues collected at necropsy, or the demonstration of
cell-mediated responses. The choice of test depends on the circumstances and the degree of
sensitivity required at individual animal or herd level.
Cultures of M. paratuberculosis may be obtained from faeces or tissues, after treatment to eliminate
contaminants, by inoculation into artificial media with and without the specific growth factor –
mycobactin – that is essential for the growth of M. paratuberculosis.