SAN FRANCISCO — A waterborne mycobacterium that infects humans through breaks in the skin causes lesions that easily are mistaken for staphylococcal infection, Dr. Peggy Weintrub said at the annual meeting of the American Academy of Pediatrics.
Mycobacterium marinum infection causes what's commonly known as “fish tank granuloma,” said Dr. Weintrub, chief of pediatric infectious diseases at the University of California, San Francisco. Lesions typically appear 1–4 weeks after exposure and start out as little papules and nodules that can be misdiagnosed as a staph infection. Later, however, they become verrucous, plaquelike lesions that start spreading up the skin along lymphatic tracts.
She described a case in a 3-year-old boy who presented with mild eczema and some other longstanding crusting lesions on his hand and arm that were spreading up the arm lymphocutaneously. The lesions had not responded to previous treatment with cephalexin. A previous culture from the lesions did grow some staphylococcal organisms, but two additional courses of antibiotics did not affect the lesions.
The boy was afebrile and systemically well, and had an infant sibling at home with no lesions.
The patient's history helped point clinicians toward M. marinum. The boy liked to be helpful, assisting his mother with the baby and helping his father with anything and everything, including gardening and cleaning the family's fish tank.
“I was imagining trying to get a 3-year-old to clean a fish tank,” said Dr. Weintrub. “Apparently, this was his favorite job, so he had done it multiple times in the weeks before he broke out in these lesions,” giving M. marinum an inoculation pathway through the boy's eczematous lesions.
M. marinum infects fish and amphibians. “You can sometimes get a history of a fish tank, but you also can get this infection from swimming in pools and in natural bodies of water—any kind of significant water exposure,” she said.
In rare cases, immunocompetent patients can develop disseminated M. marinum infection, affecting bones or joint tendon sheaths. “Particularly on the hand, it's a very worrisome diagnosis,” Dr. Weintrub said.
If a lesion is oozing, the secretions can be cultured for M. marinum. A skin biopsy from the boy grew the organism. Histopathology will show granulomas.
Patient history also can help sort through the differential diagnoses. A history of gardening raises the possibility of Sporothrix schenckii infection, which produces lesions that look very similar to M. marinum lesions, with a lymphocutaneous spread. S. schenckii resides in decaying vegetation, moss, soil, wood, and hay. It is more a disease of adults (typically farmers, gardeners, and forestry workers) than of children, with a history of skin trauma in 10%–60% of cases. A history of travel may suggest Leishmania, a parasite.
Clinicians also should consider Staphylococcus aureus infection, which most commonly causes pustular, draining lesions but rarely can cause granulomatous disease that looks like M. marinum, Dr. Weintrub said.
Usually two or three drugs are used to treat M. marinum infection, although no controlled studies back these strategies. “There aren't really good guidelines” on which drugs to use or for how long, she noted. Regimens may include clarithromycin or azithromycin, ethambutol, rifampin, and/or minocycline or doxycycline (for older patients). In general, 1–2 months of treatment may suffice, but 3 months of therapy was needed for the boy's lesions to clear completely. Localized infection is more likely to clear up than is deep infection. In rare cases, surgery may be needed to remove infected tissue.
This Mycobacterium marinum culture shows a rough colony of granular growth. CDC/Dr. CHARLES C. SHEPARD