Evaluation of techniques for post-mortem diagnosis of Mycobacterium tuberculosis complex infection in goats

Goats are highly susceptible to Mycobacterium tuberculosis complex (MTBC) infection and can play an important role in the epidemiology of animal tuberculosis (TB) in certain epidemiological scenarios. Historically, culture has been considered the only gold standard technique for post-mortem confirmation of MTBC infection. However, it is constrained by its low sensitivity, the slow growth of MTBC, and stringent biosecurity requirements. Thus, alternative post-mortem techniques are essential for effective TB control and eradication. This study aimed to compare the use of different diagnosis techniques for post-mortem confirmation of MTBC infection in goats positive for official tuberculin skin tests (TST). The techniques evaluated were direct tissue real-time polymerase chain reaction (RT-PCR), culture, observation of gross and histopathological tuberculosis-like lesions (TBL), and Ziehl-Neelsen staining. Lung and retropharyngeal and mediastinal lymph node (LN) samples were taken from 205 TST-positive goats. Direct tissue RT-PCR showed the highest rate of positive animals for post-mortem confirmation of TB infection (86.3 %) after the official in vivo diagnostic test, followed by histopathological (49.3 %), and gross (47.8 %) observation of TBL, MTBC culture (42.4 %) and ZN staining (8.5 %). Spoligotyping was obtained for 86 of the mycobacteria cultures, revealing SB0157 in 86.0 % (74/86) and SB1081 in 15.1 % (13/86) of them, both belonging to Mycobacterium caprae . Histopathological study showed a predominance of necrotic TBL in 85.2 % (52/61) and 83.5 % (66/79) of the granulomas observed in lung and LN samples, respectively. Our results indicate that direct RT-PCR on tissue is a sensitive and reliable technique for post-mortem confirmation of MTBC infection in the caprine TB control and eradication programs, even in animals without TBL. • A multi-method diagnostic strategy is crucial for the diagnosis of caprine TB. • Direct RT-PCR on tissue is a sensitive method for post-mortem diagnosis of caprine TB. • Direct RT-PCR on tissue is highly reliable even in animals without visible TBL. • Direct RT-PCR on tissue could be used as screening or alternative to culture. • MTBC culture is important for studying the origins and patterns of TB outbreaks.