Comparison of the Histopathological Observation in Pulmonary Tuberculosis between Elk (Cervus canadensis) Naturally Infected with Mycobacterium bovis and Human with Mycobacterium tuberculosis

Introduction: Bovine tuberculosis (TB) is an infectious disease caused by Mycobacterium bovis (M. bovis) and human TB by Mycobacterium tuberculosis (M. tuberculosis). The difference in lung pathology between bovine and human TB is not fully understood. In this study, the histopathological lung observations of elk naturally infected with M. bovis were examined and compared with those of humans with pulmonary TB infected with M. tuberculosis. Materials and Methods: Tuberculous lung lesions from elks and human patients were processed for the paraffin sections. The prepared sections were stained with hematoxylin and eosin (H & E) or Ziehl-Neelsen or Masson’s trichrome staining. The histopathological findings were compared between elk and human. Results and Discussion: Upon gross examination, the lesions in the elk lung demonstrated chronic granulomatous caseous necrotizing tuberculous pneumonia with numerous caseous granulomas and some cavities, similar to those observed in the human TB lung. Microscopic examination revealed a general granulomatous inflammatory process characterized by the accumulation of alveolar macrophages and bronchial obstruction by granulomatous tissues, observed in both elk and humans. However, two differences were noted between M. bovis and M. tuberculosis infections in the morphological patterns of granulomas and cavities. First, while elk lung lesions manifested only primary-type granulomas, human pulmonary lesions contained both primary-type and post-primary granulomas, showing traces of alveolar structures in the central caseous region. Second, nearly all cavities observed in elk lung lesions showed an eroded connection to adjacent bronchioles, whereas no erosion into bronchioles was found in any human cavity. Conclusion: The reasons for the differences in lung pathology between M. bovis and M. tuberculosis infections are discussed in detail.