Vaccination of lambs with live-attenuated vaccine 2 weeks after oral inoculation with M a paratuberculosis stimulated the host response against the organism and led to a reduced mycobacterial burden. The diminished IFN-gamma responses in experimentally infected sheep with acid-fast organisms suggest a positive relationship between the magnitude of the systemic cell-mediated immune response and an animal's ability to control infection.

The lower respiratory tract (LRT) of 6 adult sheep was fixed in either isotonic formalin-acetic acid or neutral buffered formalin in order to study the heterogeneity, morphology and density of mast cells (MCs). Two subtypes of MCs were found, one histochemically similar to connective tissue MCs (formalin resistant) and the other similar to mucosal MCs as found in the intestine of the rat (formalin sensitive). Although both subtypes were present at all levels of the tract, formalin-sensitive MCs were significantly more abundant (p < 0.01) at all levels, and their density increased distally from the trachea to the peripheral lung. The formalin-sensitive MCs were predominantly located in the alveolar septa and in the superficial lamina propria of airways and less frequently within the airway epithelium. The MCs in the ovine LRT were found to be morphologically heterogeneous at both the light-microscopic and electron-microscopic levels. These findings indicate that ovine respiratory tract MCs have similarity to human lung MCs, and therefore potential for use as a model for the study of human allergic disease of the respiratory system.

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