Abstract

In the United States all species of Cervidae are included in the USDA’s uniform rules and methods for the eradication of bovine tuberculosis and, therefore, are subject to regulations regarding intradermal tuberculin testing. In reindeer, infection with M. bovis is exceedingly rare. The objectives of the present study were to describe the pathologic changes associated with M. bovis infection in reindeer and evaluate the effectiveness of intradermal tuberculin testing and an in vitro blood based assay for interferon-γ (IFN-γ) as means of diagnosis of tuberculosis in reindeer. Eleven reindeer were inoculated intratonsilarly with 2×10⁴ CFU of M. bovis while four non-inoculated reindeer served as negative controls. The comparative cervical test (CCT) was done on all reindeer 3 and 8 months after inoculation. Blood was collected monthly for IFN-γ analysis. Thirteen months after inoculation, all reindeer were euthanatized and examined. Various tissues were collected for bacteriologic culture and microscopic examination. All experimentally inoculated reindeer developed lesions in the medial retropharyngeal lymph nodes. Tracheobronchial, mediastinal and mesenteric lymph nodes, tonsils and lungs were less frequently affected. The CCT accurately identified M. bovis inoculated reindeer, but false positive results were common among negative control reindeer. Modifications in the USDA’s method for interpretation of the CCT decreased false positive results without increasing false negative results. An in vitro blood-based assay to measure IFN-γ production showed that mycobacteria-specific IFN-γ responses from M. bovis-infected reindeer exceeded those of negative control reindeer. However, positive IFN-γ responses to M. bovis purified protein derivative (PPDb) were also detected in negative control reindeer. ESAT-6 and CFP-10 are antigens unique to Mycobacteria spp. within the tuberculosis complex. While use of these antigens did not diminish detection of M. bovis-infected reindeer, it did decrease false positive results in negative control reindeer. Reindeer are susceptible to infection with M. bovis; however, lesions are fewer in number, less severe in nature and less widely disseminated than those seen in white-tailed deer similarly inoculated. Comparative cervical skin testing of reindeer can be highly sensitive but has low specificity. Specificity can be improved by modification of criteria for interpretation of the CCT. A blood-based IFN-γ assay may prove useful for tuberculosis diagnosis when recombinant CFP-10 or ESAT-6/CFP-10 antigens are used to enhance the specificity of the IFN-γ assay.