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Abstract

A 76-year-old patient presented with persistent ulcerative dermatitis on the left thigh. Initial excisional biopsy demonstrated granulomatous dermatitis with mixed caseation necrosis, extensive dermal fibrosis, and pseudoepitheliomatous hyperplasia. The accompanying inflammatory infiltrate was comprised of lymphocytes, histiocytes, plasma cells, neutrophils, and occasional giant cells with a diffuse perivascular and interstitial distribution. Special stains, including PAS-D, GMS, and acid-fast bacilli (AFB) stains, were negative for microorganisms. A subsequent punch biopsy again revealed a granulomatous dermatitis with dermal fibrosis, perivascular and interstitial infiltration by lymphocytes, histiocytes, giant cells, and numerous hemosiderin-laden macrophages. Special stains were again negative for microorganisms (PAS-D, GMS, and AFB). Given the recurrent granulomatous inflammation and negative routine stains, molecular testing was pursued. PCR targeting the hsp65 gene, encoding a 65 kDa heat shock protein widely validated for specific identification of nontuberculous mycobacteria, detected Mycobacterium marinum DNA from the first biopsy tissue. This highly sensitive and specific assay confirmed the diagnosis despite negative histochemical stains and cultures. This case highlights the diagnostic difficulty of paucibacillary atypical mycobacterial infections presenting with persistent granulomatous dermatitis, especially in an elderly patient. It underscores the critical role of hsp65 gene PCR as a valuable molecular diagnostic tool in confirming an atypical mycobacterial infection when conventional methods fail, guiding timely and appropriate management.