Ocular mycobacterial lesions in cats

Ocular mycobacterial infections are an under-recognised cause of morbidity in the domestic cat. This study aimed to explore the distribution, histopathological appearance, and severity of feline ocular mycobacterial lesions, and to characterise the immune cell population with immunohistochemistry. Routine histological staining with haematoxylin and eosin, and Masson’s trichrome, was performed to identify ocular lesions and assign an inflammation score based on the number of cells present. Acid-fast bacilli were detected with Ziehl-Neelsen, and immunohistochemistry for Iba1, calprotectin, CD3 and Pax5 was undertaken on formalin-fixed paraffin-embedded tissue samples from 24 cases of ocular mycobacteriosis. Posterior or panuveitis with concurrent retinitis was identified in 20/24 cases (83%), with retinal detachment in 16/20 (80%) of these cases. Choroidal lesions had the highest median inflammation score. Ziehl-Neelsen-positive organisms were detected in 20/24 cases (83%), with the highest prevalence of acid-fast bacilli detected in choroidal lesions (16/20, 80%). Lesions were typically granulomatous to pyogranulomatous, characterised by abundant numbers of Iba1-positive macrophages, followed by calprotectin-positive granulocytes and monocytes, fewer T cells and rarer B cells. However, where iritis was identified, inflammation was typically lymphoplasmacytic (11/16 cases, 69%). Where diagnostic testing was performed, tuberculosis (ie, infection with Mycobacterium bovis, M. microti or a non-speciated M. tuberculosis-complex pathogen), was diagnosed in 20/22 cats (91%), with M. lepraemurium infection identified in the other 2/22 cats (9%). These results suggest the choroid is the primary site of lesion development in most cases of feline ocular mycobacteriosis, and inflammatory changes are associated with the presence of mycobacteria localised to ocular tissues.