Experimental infection model for Johne's disease using a lyophilised, pure culture, seedstock of Mycobacterium avium subspecies paratuberculosis

Johne's disease is a severe chronic enteritis of ruminants caused by Mycobacterium avium subspecies paratuberculosis (Map). Repeatable infections of known duration are required for validation of new diagnostic tests, evaluation of pathogenesis and development of improved vaccines. In the first study of its type, a standardised experimental model for Johne's disease was developed based on a lyophilised, low passage, pure culture, seedstock of Map. Experimental inoculations of sheep with accurately enumerated doses of Map resulted in infection outcomes across multiple trials that were modulated by the interval between inoculation and examination. Compared to an inoculum consisting of an intestinal mucosal homogenate from a naturally affected sheep, clinical signs from the pure culture of Map were manifested later, but other measures of infection were similar. Immunological assays showed that most of the inoculated animals were IFN-γ positive in the early stages of the infection. Over time, an increasing number of sheep became Map-specific antibody positive, developed typical histopathological lesions and shed Map in their faeces. The repeatability and utility of this experimental infection model will enable study of many aspects of Johne's disease. It is the first study to show that models for Johne's disease can be standardised in relevant species using traditional microbiological approaches to production and storage of seedstock. It is recommended that an international bank of master seedstock be established, containing low passage isolates that are representative of the major strains of Map, S and C.