Answer to last month's Clinical Quiz

Question:

A 22 years old aquarium hobbyist had noticed a non-tender nodule on the base between his fourth and fifth fingers of his right hand for one year. A similar lesion emerged on the dorsum of his right hand half a year later followed by a nodule on his right elbow. He took no notice of them until a month before consultation the nodules on the dorsum of his right hand ruptured with some purulent discharge. Having been treated with a course of ampicillin and cloxacillin with no response, he was referred to a dermatologist for opinion.

Answer:
A. Fish tank granuloma

The history of an aquarium hobby, long indolent course of disease, nodular lesions, and sporotrichoid spread in a local patient are highly suggestive of fish tank granuloma caused by Mycobacterium marinum, an atypical mycobacteria cutaneous infection, although our patient could not recall any history of trauma while aquarium or fish handling. An urgent skin biopsy was done and specimen sent for histopathology, mycobacteria smear and culture, fungal stain and culture. Histology showed a tuberculoid inflammation in dermis with abscess formation and stain for acid fast bacilli was positive. Culture yielded Mycobacterium marinum. M. marinum is resistant to antituberculosis treatment. He was given minocycline 100mg BD with good initial response and the treatment was carried on for four months. Nodules shrunk after treatment and post-treatment biopsy confirmed cure with scar formation, mycobacteria culture negative. Differential diagnoses include sporotrichosis, other mycobacterial infections and blastomycosis. Mycobacterium marinum is found both in fresh and sea water. The typical lesion is a violaceous papule on site of inoculation which is usually the hands, feet, elbows or knees. Lesion may spread centripedally. Spontaneous healing with scarring may result but occasionally deep penetration in bursae or joints may happen. The treatment of choice is minocycline. Alternatives are trimethoprim-sulfamethoxazole, tetracycline, rifampicin with ethambutol. Skin biopsy for histology with mycobacteria and fungal culture is mandatory in management of this presentation.