Effects of altered dietary iron intake in Mycobacterium paratuberculosis-infected dairy cattle: sequential observations on growth, iron and copper metabolism and development of paratuberculosis.

Twenty calves were orally inoculated with Mycobacterium paratuberculosis at six weeks old. At six months old, 10 of these, plus four uninfected controls were maintained on limited dietary copper and supplemented iron intake for a further 27 months. During this time all these animals, together with a further four untreated controls, were bred before being killed and examined for evidence of paratuberculosis. Despite significant reduction in weight gain, attributable to both iron supplementation and infection, no significant difference was found in the numbers of iron-supplemented and unsupplemented animals that developed clinical signs nor in the extent and severity of intestinal lesions between groups. Accumulation of iron in paratuberculosis lesions was not affected by iron supplementation but was positively correlated with the frequency of shedding of M paratuberculosis in faeces (P less than 0.05). Dietary iron supplementation alone resulted in serum hyperferraemia, hepatic siderosis and slight hypocuprosis, whereas, in infected animals, this resulted in marked hypocuprosis and anaemia within groups (P less than 0.05). Infection alone resulted in serum hypoferraemia and intestinal and hepatic siderosis which was positively correlated with the severity of infection within groups (P less than 0.05). Susceptibility to paratuberculosis may result from failure ultimately to limit monokine-mediated iron sequestration in intestinal tissue.