Mycobacterium marinum Infection and Aquatic Exposure: A Clinical Reasoning Pathway

<i>Mycobacterium marinum</i> is a freshwater nontuberculous mycobacterial infection that can lead to cutaneous soft tissue infections in humans, with diagnosis frequently delayed if key historical details remain overlooked. We describe the case of a 57-year-old diabetic man who first presented with a left middle finger skin lesion, as well as a nodular area of erythema and swelling on his left ulnar styloid process that failed a trial of oral clindamycin. After multiple weeks of discoloration and progression of the ulnar lesion, he presented to an orthopedic clinic with purulent discharge from both affected areas, resulting in incision and drainage of small abscesses near his ulnar styloid as well as his left third metacarpophalangeal joint. After a thorough history taken by the infectious diseases consultant, it was revealed to be antecedent aquatic exposure; a punch biopsy for acid-fast bacilli grew <i>Mycobacterium marinum</i> on culture. Clinical resolution ultimately occurred after a four-month course of clarithromycin and ethambutol, a common regimen used in <i>Mycobacterium marinum</i> species with wild-type antibiotic sensitivity. This case is representative of the clinical challenges involved in non-tuberculous mycobacteria diagnosis. Relevant epidemiologic risk factors included aquatic exposure, while medical risk factors included poorly controlled diabetes. The exam was characteristic of common disease presentations, as the patient presented with multiple cutaneous lesions in a sporotrichoid distribution. Differential diagnosis can include bacterial infections, as well as multiple endemic dimorphic fungal and parasitic infections. A clinical reasoning pathway is proposed to streamline the diagnosis of <i>Mycobacterium marinum and</i> ensure early and accurate identification.