Interference of anti - M. bovis antibodies in serological tests for paratuberculosis

ELISA assays are broadly used as a diagnostic tool of paratuberculosis. Nevertheless, probl ems of specificity are due to the great number of shared antigens between MAP and other Mycobacteria , especially the closely related M. avium. An M. phlei preabsortion step in the ELISA eliminates much of the cross - reacting antibody due to environmental ba cteria. Despite this, other infections such as bovine tuberculosis, a frequent occurring disease in Brazil and in many other developing countries, may interfere with the specificity of the test. In this study, 97 sera from M. bovis infected cows that were skin - test reactive and confirmed as infected by histopathology. Sera were tested in a PPA (paratuberculosis protoplasmatic antigen – Allied Monitor, USA) ELISA that is routinely used for the diagnosis of paratuberculosis. From the 97 sera 25 (26.59%) pre sented an OD >0.60; the cut - off point for this test was standardized at OD 0.35. From these, twenty sera were randomly chosen for immunoblot analysis. Eighteen sera (90%) strongly reacted with four PPA bands: one 32 - 36KDa, one 42KDa and two others between 77 and 100KDa. An ELISA - positive control serum sample from a confirmed case of Johne’s disease from a region free of tuberculosis also recognized these same four bands. The findings indicate that the proteins are shared between M. bovis and MAP . In conclu sion, although ELISA tests may be used as valuable tools for the detection of paratuberculosis, interpretation of results in herds known to be infected with M. bovis should be done carefully.