Antemortem and postmortem examinations of the cattle calf naturally infected with Mycobacterium avium subsp. paratuberculosis

摘要

A male cattle calf was detected as subclinically and naturally infected with Mycobacterium avium subspecies paratuberculosis (MAP) by a series of antemortem and postmortem tests. The MAP infection was identified by strong antibody and cell-mediated immune (CMI) response by a commercial ELISA kit and an intradermal Johnin test, respectively, in the initial antemortem examination. The antemortem status of the calf was further confirmed by MAP-specific interferon gamma (IFN-γ) response. For detection of IFN-γ response, MAP-specific IFN-γ release assays (IGRAs): (a) immuno capture ELISA (IC-ELISA) and (b) ELISPOT was employed. In addition, the presence of intracellular cytokine IFN-γ was detected by flow cytometry. For all cytokine assays, MAP-specific recombinant antigens HSP65 and 35 kDa were employed to overcome the poor sensitivity and specificity resulting from the use of Johnin, the crude protein purified derivative of MAP. Postmortem examination of the MAP-infected/suspected cattle calf did not reveal any pathognomonic gross lesions in the gastro-intestinal tract. Histopathological examination of multiple organs showed the presence of epithelioid cells/macrophages and edematous lesions in the mesenteric lymphnodes suggestive of MAP; however, no granulomas were observed in the intestinaltract. The necropsy samples of rectum and mesenteric lymph nodes were positivefor isolation of MAP by culture in the BACTEC™ MGIT™ 960 system, and acid fastbacilli were demonstrated by fluorescence microscopy confirming the infection.Due to differential and complex expression patterns of MAP antigens reported inliterature, a combination of assays such as those based on IGRAs and antibodydetection is essential. Therefore, the current experimental evidence confirmsthe efficacy of the approach adopted. However, further studies will be needed tounderstand the optimal combination MAP-specific antigens for use in IGRAs orantibody assays that can be used for detecting MAP infection in every stage ofthe disease.