Summary
Johne's disease is a chronic granulomatous enteritis of ruminants and camelids that is caused by Mycobacterium avium subspecies paratuberculosis (Mptb). In cattle, the disease typically presents as intractable diarrhoea and severe wasting in mature animals. The observation that cattle do not become clinically affected by Johne's disease when grazing with infected sheep for extended periods and the fact that culture media required for laboratory growth of isolates from sheep or cattle strains are different, support the hypothesis that there are distinct strains of Mptb; a cattle (C) strain that predominantly affects cattle but is also capable of infecting goats, deer and alpaca, and a sheep (S) strain that predominantly affects sheep.
Recent Australian studies 2,3,4 using polymorphism in the IS1311 element and other DNA techniques have confirmed this general proposition. However, it is now clear that this species-strain adaptation or preference is not absolute. S strain Mptb was retrospectively diagnosed from archived histological sections collected in the late 1990s from three individual cattle from three NSW properties3 and investigation of clinical Johne's disease cases in Victorian cattle has identified further cases of cross-infection. Detection of S strain Mptb in cattle raises the possibility that control of Johne's disease using current strategies which assume there is no cross-infection between cattle and sheep could, on occasions, be compromised. This project was undertaken to determine the within herd distribution of S strain Mptb infection in cattle in a single beef cattle herd in the Ballarat region of Victoria, to provide a better understanding of the epidemiology of S strain Mptb infection in cattle and assist in the development of control strategies for other similarly affected herds.
