To identify and describe histological and immunohistochemical criteria that may differentiate between skin and lymph node lesions associated with Mycobacterium (M.) bovis and M. microti in a diagnostic pathology setting.
Archived skin and lymph node biopsies of tuberculous lesions were stained with haematoxylin and eosin, Ziehl-Neelsen and Masson’s Trichrome. Immunohistochemistry was performed to detect the expression of calprotectin, CD3 and Pax5. Samples were scored for histological parameters (i.e. granulomas with central necrosis versus small granulomas without central necrosis, percentage necrosis and/or multinucleated giant cells), number of acid-fast bacilli (bacterial index) and lesion percentage of fibrosis and positive immunohistochemical staining.
Twenty-two samples were examined (M. bovis n=11, M. microti n=11). When controlling for age, gender and tissue, feline M. bovis-associated lesions more often featured large multi-layered granulomas with central necrosis. Conversely, this presentation was infrequent in feline M. microti-associated lesions, where small granulomas without central necrosis predominated. The presence of an outer fibrous capsule was variable in both groups, as was the bacterial index. There were no differences in intralesional expression of immunohistochemical markers.
Differences in the histological appearance of skin and lymph node lesions may help to infer feline infection with either M. bovis or M. microti at an earlier stage when investigating these cases, informing clinicians of the potential zoonotic risk. Importantly, cases of tuberculosis can present with numerous acid-fast bacilli. This implies that a high bacterial index does not infer infection with non-zoonotic non-tuberculous mycobacteria.

© 2021 The Authors. Journal of Small Animal Practice published by John Wiley & Sons Ltd on behalf of British Small Animal Veterinary Association.