Pain and swelling along the nail fold of a 51-year-old man: Mycobacterium marinum (fish tank granuloma)
Kelly J. Warren, M.D. and Janet A. Fairley, M.D
Dermatology Online Journal 4(1): 1

Discussion

Figure 4
Figure 4: Right third finger following four months of ethambutol and rifampin. 

The diagnosis of M. marinum was confirmed by mycobacterial culture of lesional tissue. Skin sections stained by the Ziehl-Neelson method for acid-fast bacilli revealed no organisms. Treatment with a combination of oral ethambutol (2400 mg q.d.) and rifampin (300 mg q.d.) for 4 months was successful in clearing the skin lesions. 

The source of M. marinum infection is freshwater or saltwater from pools or aquaria, hence the terms "swimming pool granuloma" and "fish tank granuloma." Clinically, the typical presentation is of a single inflammatory nodule, usually on the elbows, knees, or feet of swimmers and on the hands of fish-keepers. Because the nodules on the hands spread along the lines of lymphatic vessels, lesions caused by M. marinum may be confused with those of sporotrichosis. Disseminated cutaneous infections can rarely occur in both immunocompetent and immunocompromised hosts [1]. 

Definitive diagnosis can only be made by culturing M. marinum from lesional tissue. Because the organisms grow best at 30 C to 33 C, samples incubated at 37 C, the usual temperature used for culturing Mycobacterium tuberculosis, may yield minimal or no growth. On histologic exam it is difficult to find bacilli, however, the presence of tuberculoid granuloma and an appropriate clinical history (e.g., handling of fish, use of swimming pools) suggests the diagnosis. Skin testing with PPD is of little value. 

The treatment of choice for M. marinum is unknown. Like many atypical mycobacteria, M. marinum is only poorly susceptible to antituberculous drugs. Various antibiotics including minocycline, tetracycline, trimethoprim-sulfamethoxazole, doxycycline, amikacin, and ethambutol plus rifampin have been reported to be effective, although treatment failures with all of these agents have been reported [2]. Treatment with levofloxacin, a broad-spectrum quinolone, has recently been reported to effective in the treatment of M. marinum skin infections [3]. Lesions may also heal spontaneously, although this may require several years. 



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References

[1] King AJ, Fairley JA, Rasmussen JE. Disseminated cutaneous Mycobacterium marinum infection. Arch Dermatol 1983;119:268-270. 

[2] Edelstein H. Mycobacterium marinum skin infections. Arch Intern Med 1994;154:1359-1364. 

[3] Iijima S, Saito J, Otsuka F. Mycobacterium marinum successfully treated with levofloxacin. Arch Dermatol 1997;133:947-949.


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