Sequential bacteriological observations in relation to cell-mediated and humoral antibody responses of cattle infected with Mycobacterium paratuberculosis and maintained on normal or high iron intake.

Twenty calves were orally infected with Mycobacterium paratuberculosis before weaning. Ten of these plus 4 non-infected controls were maintained on elevated dietary iron intake from 6 to 33 months of age. During this time, in which the majority of animals were bred, the influence of increased dietary iron upon tests of cellular and humoral immune responsiveness to antigens of the organism were monitored. Results were examined in relation to the organism's capacity to multiply and infect up to 7 portions of the intestinal tract. No significant differences were detected in the degree of intestinal disease or pattern of faecal excretion of M. paratuberculosis in iron supplemented and non-supplemented cattle. Cutaneous delayed-type hypersensitivity (DTH) to johnin PPD developed at 1 month and in-vitro lymphocyte and immunostimulatory activity (LS) to this antigen at 2 months after infection. LS indices were significantly reduced in magnitude in iron-supplemented cattle (p less than 0.01). Most ELISA antibody responses were positive 10 to 17 months after infection and preceded the fewer number of CF responses by several months. Neither of the antibody tests was affected by elevated iron intake. Generally, complete or partial resistance to paratuberculosis was associated with sustained positive monthly LS tests (index greater than or equal to 2.0), whereas antibody levels tended to be sustained only in the more severely affected cattle. Although neither test system was affected by pregnancy the ELISA failed to detect a significant proportion of cattle chronically shedding M. paratuberculosis in faeces.