Tuberculosis in Small Ruminants in Portugal: A Retrospective Laboratory-Based Study (2012-2023)

Animal tuberculosis caused by mycobacteria of the Mycobacterium tuberculosis complex (MTBC) represents a significant challenge to both public and animal health. Although eradication programs focus predominantly on cattle, small ruminants, especially goats, may play a relevant role as reservoirs. This study aimed to characterize tuberculosis cases in small ruminants in Portugal diagnosed at the national reference laboratory for Tuberculosis (Instituto Nacional de Investigação Agrária e Veterinária) over the period from April 2012 to July 2023. In this study, samples from 79 animals suspected of TB (64 goats and 15 sheep) were analyzed by integrating histopathology; bacteriology; and, when applicable, PCR results, thereby allowing the distribution by species to be described, MTBC agents to be identified, and diagnostic agreement to be assessed. Of these samples, 29 positive cases (36.7%) were identified, all of them in goats (45.3% of goats tested), whereas no cases were confirmed in sheep. Mycobacterium caprae was the most frequently identified species (89.7%), followed by Mycobacterium bovis (10.3%). Geographic distribution was concentrated in the Alentejo (48.3%) and Norte (44.8%) regions, where 93.1% of cases occurred. Topographical analysis of lesions revealed respiratory tract involvement (lung and/or thoracic lymph nodes) in 82.7% of positive cases, with gross patterns including caseous/caseocalcified lesions (n = 11), necrosis (n = 9), and nodular granulomatous lesions (n = 10). Agreement between histopathology and bacteriology was 81.5%. Parasitic coinfections were observed in 24.1% of positive cases, complicating histopathological interpretation. The results of this study indicate goats, as opposed to sheep, as non-negligible reservoirs of TB, suggesting species-level differences that warrant targeted investigation. The predominance of respiratory lesions and the detection of extensive cavitary forms indicate potential aerogenous transmission with implications for multi-host systems. Diagnostic discordance and parasitic interference reinforce the need for combined testing methodologies. It is therefore advisable to consider the integration of goats into tuberculosis surveillance and control programs, with greater relevance in Alentejo and Norte.