Mycobacterium marinum cellulitis: A fishy etiology

Skin and soft tissue infections account for 2% of emergency department visits annually, though more unusual causative bacteria associated with saltwater exposure may result in morbidity. Mycobacterium marinum represents a rare but important cause of cellulitis, which if untreated or improperly managed, can progress to dactylitis or osteomyelitis. This unusual diagnosis is made more challenging due to the prolonged incubation period of approximately 21 days, temporally separating the inoculation from the disease. Patients will present with a nodular rash in a sporotrichoid pattern. While doxycycline is one antibiotic providing saltwater coverage, M. marinum has variable sensitivities to anti-mycobacterial antibiotics, and thus biopsy helps confirm the diagnosis as well as provide sensitivities for treatment. Emergency clinicians should inquire about environmental risk factors when caring for patients with cellulitis, especially with atypical skin presentations, and consider M. marinum as a rare but important cellulitis etiology.