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Mycobacterium marinum is an uncommon but well documented infection in man. It has the ability to hide behind many guises, which can make diagnosis difficult.
A woman aged 74 was referred because of stiffness and triggering of her right thumb (dominant hand). On examination there was no evidence of inflammation; triggering was evident in the absence of palpable Notta's nodule. Radiographs of the thumb and hand showed nothing of note. Treatment options were discussed, including injection of steroids into the affected thumb to relieve triggering, but the patient declined at that stage and made an appointment to return two weeks later. By the time of her second visit she had developed a well circumscribed, nodular lesion over the dorsum of the first web space of her right hand and similar lesions tracking up her arm (Figure 1a). On detailed inquiry she then revealed that four weeks before her initial presentation she had sustained a minor laceration to the thumb after striking a metal object; moreover, she kept tropical fish and had continued to handle the aquarium without gloves.
Blood tests including white cell count and C-reactive protein were within normal limits. A single lesion was excised and sent for histological and microbiological examination. Culture grew Mycobacterium marinum and, on the recommendation of the microbiologist, the patient began treatment with clarithromycin, rifampicin and ethambutol. Four weeks into treatment the triggering of her thumb resolved.
M. marinum is a Runyon group I photochromogenic mycobacterium that causes tuberculosis in fish.1 It grows at 30-32°C, about five degrees less than typical mycobacteria, and this may explain why it is frequently confined to the superficial structures in the limbs.2 Infection in man typically begins with a single, papulonodular granulomatous lesion (Figure 1b) but can progress to a sporotrichoid form in a single limb (as in our patient).3 The main differential diagnosis is indeed sporotrichosis, which tends to affect gardeners. The cutaneous expression of M. marinum infection is known as fish tank or swimming pool granuloma. In the initial stages of infection, the resemblance of symptoms to inflammatory joint disease has prompted administration of intralesional steroids, with consequent exacerbation.1,2,4,7
Contrary to the opinion expressed in certain reports,1,5M. marinum does not restrict itself to superficial structures. It can present with tenosynovitis, bursitis, gout, rheumatoid arthritis, and even destructive polyarthritis.3,4,6,7 The unwary may treat these with local steroids and make matters worse. When the diagnosis has been made, the usual treatment is excision and chemotherapy. Occasionally, as in our case, antimicrobial treatment alone has sufficed. This case adds to the two previously reported cases of trigger finger caused by M. marinum.
Competing interests None declared.