SHM Converge Scientific Abstract ePoster Gallery-Fish Tank Granuloma: When Environmental History Makes the Diagnosis in Atypical Mycobacterial Infection

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This case report describes a 53-year-old man with type 1 diabetes who developed a chronic, worsening skin lesion on his right index finger after cleaning a fish tank and sustaining minor abrasions. He was repeatedly treated for presumed bacterial cellulitis with amoxicillin-clavulanate, TMP-SMX, and doxycycline, but only had temporary or no improvement. Imaging showed soft tissue swelling without bone involvement, and multiple cultures were negative.<br /><br />The diagnosis became clear only after an infectious disease consultation revealed the key environmental history of aquatic exposure. Biopsy showed suppurative granulomatous dermatitis with pseudoepitheliomatous hyperplasia, findings strongly suggestive of Mycobacterium marinum infection. Although AFB cultures were negative, this did not rule out the diagnosis because M. marinum requires incubation at 30°C, and false-negative cultures are common.<br /><br />He was treated with clarithromycin plus rifampin for six months and had complete symptom resolution.<br /><br />Main lessons from the case:<br />- Always ask about aquatic exposure, fish tanks, swimming pools, and water-related injuries when evaluating chronic skin lesions.<br />- Negative AFB cultures do not exclude M. marinum infection.<br />- Granulomatous inflammation on biopsy is a major clue when paired with the right history.<br />- Immunocompromised patients may be at higher risk for invasive disease.<br />- Early collaboration between infectious disease and microbiology can prevent delayed diagnosis and guide appropriate therapy.<br /><br />Overall, the report emphasizes that environmental history was the decisive diagnostic clue in a case that initially masqueraded as routine cellulitis.

Asset Subtitle

Christiane Martell

Meta Tag

Author List Christiane Martell, Christina Chen-Milhone, Sarika Chauhan

Category Clinical Vignettes

Concept Mycobacterium marinum

Concept Aquatic exposure

Concept Acid-Fast Bacillus Culture

Concept Nontuberculous mycobacteria

Concept Skin injury

Distinguished Non-Finalist

Presenter Organization Lakeland Regional Health

Presenting Author Christiane Martell

Track Adult

Keywords

Mycobacterium marinum

fish tank exposure

aquatic injury

chronic skin lesion

granulomatous dermatitis

pseudoepitheliomatous hyperplasia

type 1 diabetes

negative AFB culture

clarithromycin rifampin

cellulitis misdiagnosis

Mycobacterium marinum

Aquatic exposure

Acid-Fast Bacillus Culture

Nontuberculous mycobacteria

Skin injury