Histological and immunohistochemical characterization of granulomas in alpacas (Vicugna pacos) naturally infected with tuberculosis

Irene Agullo-Ros¹INÉS RUEDAS TORRES²Laura Hunter³Alison Bird²Claire Whitehead⁴Francisco Javier Salguero^3*

• ¹Universidad de Cordoba Facultad de Veterinaria, Córdoba, Spain
• ²UK Health Security Agency Porton, Salisbury, United Kingdom
• ³School of Veterinary Medicine. Department of Pathology and Infectious Diseases, UK Health Security Agency (UKHSA), London, United Kingdom
• ⁴Camelid Veterinary Services Ltd, Reading, United Kingdom

Tuberculosis (TB), caused by the Mycobacterium tuberculosis complex (MTBC), is a chronic zoonotic disease of increasing concern in alpacas (Vicugna pacos), a species highly susceptible to the disease. Given the growing alpaca population in Europe and documented zoonotic infection in humanszoonotic potential, understanding TB pathology in alpacas is crucial. This study provides the first comprehensive histopathological and immunohistochemical characterization of TB lesions in naturally infected alpacas. Granulomas from the lungs (n=175), liver (n=241),, and lymph nodes (n=55), were classified into four developmental stages (I, II, III and IV) based on their morphology, necrosis, fibrosis, cellular composition, and presence of acid-fast bacilli (AFBs). Advanced granulomas (stages III and IV) predominated in all tissues, indicating chronic infection. High numbers of AFBs were observed in lung and lymph node granulomas across all stages, with very rare presence of multinucleated giant cells (MNGCs)., with numbers increasing as lesions progressed in the lungs. This pattern in the lung, with extensive necrosis and lack of fibrous encapsulation, together with the presence of abundant AFBs, suggests deficient immune control and significant transmission risk. In contrast, liver granulomas, particularly encapsulated stage IV lesions, showed fewer detectable AFBs, implying better mycobacterial control in this organ. Immunohistochemistry in selected granulomas revealed ionized calcium-dinding adaptor protecin macrophages (IBA1+) immunopositive macrophages were most prevalent cells in early stages (stage I and II) , while T (CD3+) and B lymphocytes (B-cell SAPspecific activator protecin+) increased in advanced (stage III and IV) granulomas, forming peripheral lymphoid follicle-like structures. Neutrophils (MPOimmunopositve to myeloperoxidase+) were less abundant, but more prominent in advanced lesions showing extensive necrosis. The high incidence of liver lesions suggests the oral route as a notable infection pathwayhigh dissemination of pathogenic mycobacteria and generalised tuberculosis in this species. This research fills knowledge gaps about TB tuberculous granulomas in camelids and highlights alpacas as potential sources of mycobacterial excretion, posing a transmission risk to domestic animals, wildlife, and humans.