P041 Acral cutaneous Mycobacterium marinum infection in an adolescent receiving adalimumab

Abstract

An 18-year-old man presented with a 2-month history of a painful, progressive rash affecting the left fifth toe, with extension into the midfoot and ankle, resulting in difficulty with ambulation. He had a background of enthesitis-related arthritis, well controlled on adalimumab monotherapy since childhood. Adalimumab had been temporarily withheld perioperatively for anterior cruciate ligament repair and was not recommenced following the onset of the rash. Clinical examination revealed erythematous, indurated nodules and plaques with secondary erosions and impetiginization affecting the dorsal and plantar aspects of the toe. The patient reported fatigue but no systemic symptoms. Despite multiple courses of oral antibiotics and antifungals, alongside topical therapies, there was no clinical improvement. There was no history of trauma, travel, aquarium exposure or contact with natural water sources; however, he reported regular use of chlorinated swimming pools. Initial investigations, including viral swabs and bacterial and fungal cultures, were negative. Repeat QuantiFERON-TB testing was positive, having been negative on prebiologic screening. Skin biopsy demonstrated a dense inflammatory infiltrate with epidermal acanthosis, parakeratosis and intracorneal micropustules. While routine tissue cultures were negative, mycobacterial culture was positive, with species identification confirming Mycobacterium marinum via MALDI-TOF mass spectrometry at a national reference laboratory. The patient was commenced on combination therapy with rifampicin, ethambutol and clarithromycin following multidisciplinary discussion with infectious diseases. Marked clinical improvement was observed by 3 months, with complete resolution at 6 months. Cutaneous infection with M. marinum is an uncommon presentation that may pose significant diagnostic challenges, particularly in immunosuppressed patients where exposure history may be subtle or atypical. Early recognition is essential to avoid prolonged morbidity. Early biopsy with mycobacterial culture is essential to avoid diagnostic delay in this setting.