Paratuberculosis

Mycobacterium paratuberculosis is the etiologic agent of bovine paratuberculosis. The disease is characterized by diarrhoeas, a decrease of the production level (milk, reproduction) and a loss of weight leading to death. The most important mode of transmission of paratuberculosis is the faecal-oral route, although transmission can occur in utero, via infected semen, colustrum and milk. In the infected animal organisms, the bacteria can then spread through the macrophages.

In 1898, Johne and Frothingham detect acid-fast bacillus in intestinal mucosa of affected animals. These bacteria, similar to tuberculosis bacillus, are responsible for a thickening of intestinal mucosa corresponding to an enteropathy called paratuberculosis or Johne’s disease (Thorel et al., 1990). The incubation of illness is slow (between 2 and 5 years) so the majority of affected animals presents clinical symptoms between 2 and 7 years.

Affected animals can shed varying numbers of M. paratuberculosis organisms in their faeces (from some bacteria/g of faecal material to 10 4- 10 10germs/g at the clinical stage) (Collins et al., 1993).

Feacal culture for the causative organims is the definitive method of diagnosis but it is very slow, requiring 6-8 weeks. Immunologically-based tests for Johne's disease are rapid but lack of specificity and sentitivity.

A shift of biologists of St George's Hospital of London conducted by the Dr J. Hermon-Taylor identified in 1985 a repetitive genomic fragment called IS900, specific of M. paratuberculosis strains (Greens et al. 1989). Since this sequence has been used as probe in molecular diagnostic test in particular for PCR test (Guillou et al. 1993).