Tuberculosis in free-ranging and captive wild animals: Pathological and molecular diagnosis with histomorphological differentiation of granulomatous lesions

Tuberculosis (TB) is a serious zoonotic threat, impacting the human-livestock-wildlife interface globally. Here, we evaluated the status and histomorphological differentiation of TB lesions in 89 morbid cases of wild animals (bovids, cervids, carnivores, non-human primates, and pachyderms) in India. Histomorphological and molecular studies were done using Ziehl-Neelsen (ZN) staining, immunohistochemistry, and polymerase chain reaction (PCR), whereas cultural isolation was performed on selected samples. A total of 32 (35.95%) cases were confirmed as TB, comprising of 12 carnivores, 09 bovids, 06 cervids, 04 non-human primates, and a pachyderm. The TB lesions in the lungs, liver, and lymph nodes varied from the large-sized caseous nodules filled with dry cheesy material in bovids and cervids to variable-sized cavitations containing liquefied caseum in carnivores' lungs. The lungs, livers, and spleens of non-human primates exhibited small to medium-sized nodules. Histologically, lesions were divided into four categories (Types I, II, III, and IV) based on the extent of necrosis, the presence of mineralization, giant cells, and fibrous encapsulation. Extensive caseous necrosis with calcification, abundant giant cells, and thick fibroblastic encapsulation were consistent findings in the lungs, livers, and lymph nodes of bovids and cervids, whereas airway impaction with cellular exudate containing a teeming number of acid-fast bacilli and, at times, alveolar rupture leading to cavity formation was present in the lungs of carnivores. Absence of calcification and fibrous encapsulation was recorded in lungs of non-human primates. Immunohistochemical labelling with anti-early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) antibodies showed mild, moderate, and intense positive reactions in type II and III, type I, and type IV granulomatous lesions, respectively. Molecular detection by PCR revealed Mycobacterium tuberculosis (12 carnivores, 02 non-human primates and 01 pachyderm), M. bovis (02 cervids and 01 bovid) and M. orygis (02 cervids and 01 bovid). Cultural isolation confirmed M. tuberculosis in 03 carnivores and M. orygis in 02 cervids and 01 bovid. Our findings imply that TB is quite prevalent in the wildlife of India and there are considerable differences in the histomorphological lesions induced by distinct Mycobacterium species in different wild animals. The circulation of TB organisms in wild animals warrants a strict surveillance programme to identify the carrier status of these animals so that effective TB control strategies can be formulated to prevent spillover and spillback incidences at the human-livestock-wildlife interface.