The effect of different levels of iron intake on the multiplication of Mycobacterium paratuberculosis in C57 and C3H mice.

Two groups of C57 and C3H mice of 5 weeks of age were infected via the intraperitoneal route with 2.0 mg (wet weight) of Mycobacterium paratuberculosis. These were maintained with a similar number of segregated and non-inoculated mice of the same strains under specially controlled conditions of low, medium and high iron intake. Mice were killed and bled at 7 months post-infection and assessments of haematological parameters and the degree of mycobacterial granulomatous involvement of abdominal and other tissues were made. In addition, the total mycobacterial numbers visible in macrophages in standardized histological sections of liver, spleen and bone marrow in the presence or absence of stainable iron storage compounds were assessed using a double staining technique for iron and mycobacteria. Moderate to marked anaemia in both C57 and C3H mice on low iron intake, irrespective of infection, indicated that an effective low iron status was achieved in the animals by dietary manipulation. Medium and high iron intake groups exhibited normal haematological parameters. Iron storage compounds were readily visible in liver microgranulomas of mice on medium and high, but not on low iron intake. In liver, spleen and bone marrow samples, mycobacterial counts in iron-containing microgranulomas were significantly higher than in those without stainable iron. Increased frequencies of residual and progressive infection were associated with increased iron intake. The greater susceptibility of the C57 strain was evident from the significantly higher liver microgranuloma counts, higher mycobacterial numbers and greater progressive infection when compared with the C3H strain. These findings in mice strongly suggest that slow multiplication of M. paratuberculosis is enhanced in iron-replete compared with iron-deficient macrophages. This enhancement occurs despite the capacity of the less susceptible strain of mouse to limit the spread of the organism within the body.