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

J L Mitchell,J Del Pozo,J C Hope,R Dheendsa,C S C Woolley,D A Gunn‐Moore

doi:10.1111/jsap.13386

Abstract

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.

Highlights

Mycobacterial infections are increasingly recognised as a substantial cause of morbidity in the domestic cat population, especially in Great Britain (Broughan et al 2013), where approximately 1% of all feline biopsy submissions show changes suggestive of mycobacterial disease (Gunn-Moore et al 2013)
Control FFPE tissues for IHC were a submandibular lymph node from a cat euthanased for reasons not related to mycobacterial infection, and a cutaneous lesion from a cat diagnosed with a non-tuberculous mycobacteria (NTM) infection on interferongamma (IFNγ) release assay (IGRA) with abundant acid-fast bacilli (AFB)
Case 1 was an indooronly cat with no reported contact with potentially infected wildlife; it was fed the commercial raw food diet associated with the outbreak of M. bovis in cats in England and Scotland in 2018 to 2019 (O’Halloran et al 2019)

Summary

Introduction

Mycobacterial infections are increasingly recognised as a substantial cause of morbidity in the domestic cat population, especially in Great Britain (Broughan et al 2013), where approximately 1% of all feline biopsy submissions show changes suggestive of mycobacterial disease (Gunn-Moore et al 2013). Neous lesion (Gunn-Moore et al 2011a) This clinical presentation is recognised in cases of infection with members of the Mycobacterium (M.) tuberculosis-complex (MTBC) as well as non-tuberculous mycobacteria (NTM) such as members of the M. avium-complex (Gunn-Moore 2014). Mycobacterial lesions have been reported in the eyes (Stavinohova et al 2019), joints (Lalor et al 2017) and other organs and tissues, resulting in a range of clinical signs (GunnMoore 2014)

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