Histological and immunohistochemical features suggesting aetiological differences in lymph node and (muco)cutaneous feline tuberculosis lesions

To identify and describe histological and immunohistochemical criteria that may differentiate between 4 skin and lymph node lesions associated with Mycobacterium (M.) bovis and M. microti in a diagnostic 5 pathology setting. 6
Materials and Methods 7
Archived skin and lymph node biopsies of tuberculous lesions were stained with haematoxylin and 8 eosin, Ziehl-Neelsen and Masson’s Trichrome. Immunohistochemistry was performed to detect 9 expression of calprotectin, CD3 and Pax5. Samples were scored for histological parameters (i.e. 10 granulomas with central necrosis vs small granulomas without central necrosis, percentage necrosis, 11 and/or multinucleated giant cells), number of acid-fast bacilli (bacterial index) and lesion percentage of 12 fibrosis and positive immunohistochemical staining. 13
Results 14
Twenty-two samples were examined (M. bovis n = 11, M. microti n = 11). When controlling for age, 15 gender and tissue, feline M. bovis-associated lesions more often featured large multi-layered 16 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.
Clinical Significance
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.