Tissue localisation of Mycobacterium avium subspecies paratuberculosis following artificially induced intracellular and naked bacteraemia.

Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of Johne's disease or paratuberculosis, a chronic enteritis of ruminants. While Johne's disease is primarily expressed in the gastrointestinal tract, isolation of MAP from extra-intestinal tissues indicates that microbial dissemination via the haematogenous route may occur during the infection. This study examined the movement of peripheral blood mononuclear cells (PBMCs) infected with MAP and the dissemination of MAP following mycobacteraemia induced by IV inoculation over a time frame of 3 days. Viable MAP were detected in the peripheral blood by culture at 24, 48 and 72 h post IV inoculation in 3 of 4 sheep while a fourth sheep was blood culture positive at 24 and 48 h. Seventy-two hours following IV inoculation, MAP in tissues was unevenly distributed. MAP was predominantly distributed to the filtering organs, i.e. the lung, liver and spleen, however MAP was also isolated from lymph nodes, especially those draining the neck. MAP was not isolated from gastrointestinal tissues or lymph nodes, heart or kidney at 72 h following IV inoculation. The rapidity of localisation of MAP to tissues after onset of induced bacteraemia suggests that intermittent seeding into blood in natural infection would be sufficient to account for dissemination.