Histological and immunohistochemical features suggesting aetiological differences in lymph node and (muco)cutaneous feline tuberculosis lesions by Danielle Gunn-Moore and colleagues at the University of Edinburgh

Mycobacterium bovis and M. microti are two species of mycobacteria that cause tuberculosis (TB) in a wide range of animals. Infections of both produce identical macroscopic lesions, but they differ with respect to their zoonotic potential. M. bovis causes disease across multiple species of veterinary interest, and commonly infects domestic cats in TB endemic areas. M. microti infection is rarely seen in species other than the domestic cat and its maintenance host the field vole, although it is more common in parts of the UK with little or no prevalence of bovine TB such as Scotland.

The ability to distinguish between the two infections is essential to determine potential zoonotic risks, and may influence the owner decision to treat or euthanise. Histology is often used as the initial diagnostic test but many feline mycobacterial lesions have few acid-fast bacilli (AFB) on staining, so a negative result does not rule out mycobacteriosis. Other tests such as culture, PCR or interferon γ release assays have low sensitivity, are expensive or require expert handling. Immunohistochemistry (IHC) can characterise the cellular composition of granulomas, so this study examined the histological and IHC patterns of feline TB granulomas to help differentiate between the two causes of TB in cats.

Twenty-two archived formalin-fixed paraffin-embedded (muco)cutaneous and lymph node biopsies of TB lesions (11 infected with M. bovis and 11 with M. microti) from cats throughout England and Scotland were studied. Histopathological analysis found that ‘organised’ lesions were significantly more common in infections with M. bovis. These resembled typical TB granulomas, with a central necrotic core, a macrophage-dominant cellular layer and an outer rim of lymphocytes. They also often had abundant neutrophils within zones of granulomatous inflammation, lacked multi-nucleated giant cells and showed no mineralisation. By contrast, M. microti lesions were usually composed of tightly packed, smaller macrophage clusters with or without neutrophils. Most lacked a necrotic core, and were surrounded by intermittent, thin capsules.

Ziehl-Neelsen staining revealed AFB of M. microti to be S-shaped, although infections from both species were associated with numerous AFB. Therefore, a high bacterial index does not infer infection with non-zoonotic non-TB mycobacteria. There was no difference in the intralesional expression of IHC markers.

M. bovis-associated lesions more often contained large multi-layered granulomas with central necrosis, while M. microti-associated lesions mainly had small granulomas without central necrosis. Distinguishing between the histological appearance of skin and lymph node lesions from feline TB infections could help identify the cause at an early stage before further testing.

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