This is a crusted, violaceous or hyperkeratotic papulonodule that develops on the dorsal surface of the finger or hand, often a knuckle in this disease. This condition is known as a swimming pool or fish tank granuloma. The granuloma is caused by an atypical mycobacteria, generally Mycobacterium marinum.

If this condition is acquired from an aquarium then often the dominant hand is affected. If this condition is acquired from a swimming pool then any site of trauma may be affected. Often the patient is treated with several courses of antibiotics until correct diagnosis is made. The granuloma (s) - one or more dusky red nodules - occur along the site of lymphatic drainage from the initial point of entry.

Diagnosis is via biopsy for histopathology, specialist staining and culture.

Treatment with antibiotics e.g. doxycycline.