Subcutaneous exposure of calves to Myobacterium paratuberculosis compared with intravenous and oral exposures

Three groups of calves, 21 days of age, were exposed to Mycobacterium paratuberculosis by oral, intravenous, and subcutaneous routes. Fecal cultural examination and intradermal tests were made, and at 150 days after exposure, the calves were euthanatized and tissues were collected for histopathologic and bacteriologic cultural examination. Calves in the 3 groups were infected. Those exposed intravenously had significantly higher colony counts than did either of the other groups. There was no significant difference between colony counts of cultures from tissues of calves exposed orally and those of calves exposed subcutaneously.