Deep Mycobacterium marinum Infection of the Digit Presenting Without Cutaneous Lesions: A Case Report

Mycobacterium marinum is a slow-growing nontuberculous mycobacterium acquired through breaches of the skin during contact with fish or aquatic environments. It classically produces a cutaneous papulonodular lesion of the hand, but deeper involvement of tendon sheaths, joints, and bone occurs in a minority of patients and carries greater morbidity and a higher risk of failure of medical therapy alone. We report a rare case in which establishing the diagnosis was challenging: M. marinum infection presenting as an isolated deep swelling of a digit without any cutaneous lesion in an immunocompetent fisherman. A 53-year-old man presented with a four-month history of a painful fusiform swelling over the proximal interphalangeal joint (PIPJ) of the right ring finger following a penetrating injury from a fish fin. Plain radiographs showed pressure erosion of the proximal phalanx. Because deep structure involvement is associated with a poorer response to antibiotics alone, the patient underwent excision biopsy and debridement; histopathology demonstrated caseating granulomatous inflammation, and polymerase chain reaction confirmed M. marinum. Antimycobacterial therapy with rifampicin and clarithromycin was given for six months. The patient regained PIPJ flexion up to 110° and returned to work pain-free. This case highlights that M. marinum should be considered in chronic monoarticular digital swelling with relevant aquatic exposure, even when the characteristic skin lesion is absent, and illustrates the value of tissue biopsy with molecular confirmation and combined surgical and medical management for deep disease.