Lepromatous leprosy presenting as adult-onset Still's disease: a 5-year diagnostic challenge

Leprosy, known as the “great imitator”, can present with systemic inflammatory manifestations that closely resemble autoimmune diseases, particularly during type 2 leprosy reactions (erythema nodosum leprosum, ENL). We report a 23-year-old Chinese male with a 5-year history of recurrent fever, arthralgia, papular and nodular skin lesions, and generalised lymphadenopathy. He met Yamaguchi criteria for adult-onset Still’s disease (AOSD) and received prolonged immunosuppressive therapy with glucocorticoids, methotrexate, hydroxychloroquine, and adalimumab but experienced recurrent flares upon steroid tapering. Skin biopsy from a right costal margin lesion revealed a dense dermal infiltrate predominantly composed of foamy histiocytes, with collagen degeneration and periadnexal lymphohistiocytic inflammation. Well-formed granulomas were absent. Acid-fast staining (Ziehl-Neelsen) demonstrated abundant acid-fast bacilli within the foamy histiocytes forming characteristic globi, and Wade-Fite stain confirmed Mycobacterium leprae with a high bacterial index. The diagnosis was revised to lepromatous leprosy with type 2 reaction (ENL). After referral for multidrug therapy (MDT) and gradual prednisolone tapering, the patient achieved complete clinical remission with normalisation of inflammatory markers. This case underscores a critical diagnostic pitfall for dermatopathologists and clinicians: in non-endemic regions where clinical suspicion for leprosy is low, the histopathologic finding of a foamy histiocytic infiltrate without granuloma formation should prompt acid-fast staining to exclude lepromatous leprosy, particularly in patients with unexplained systemic inflammation and epidemiologic risk factors.