Boy’s progressive extremity rash looks fishy

Figure

The case

A previously healthy 8-year-old boy presents to the dermatology clinic with a progressively worsening elbow rash over the course of the last week. The rash does not itch. He spent the previous weekend sailing on the Chesapeake Bay. His pediatrician prescribed a course of cephalexin as well as a trial of topical antiviral ointment, neither of which improved the rash. The patient denies any other new exposures. 

Diagnosis: Mycobacterium marinum infection

Discussion

Mycobacterium marinum is a slow-growing, nontuberculous photochromogen found in a variety of bodies of water.¹ Also known as “swimming pool granuloma” or “fish tank granuloma,” cutaneous M marinum infections are most often reported among fish fanciers and those who participate in waterborne activities.²

Mycobacterium marinum disease in humans is uncommon, with rates of up to 0.27 cases per 100,000 persons noted in the literature.² Although cutaneous M marinum on the limbs is the most common form of the infection, invasive disease (tenosynovitis, arthritis, osteomyelitis) also can occur. The incubation period is usually within 4 weeks following trauma to the skin but can be up to 9 months.

Lesions resulting from cutaneous M marinum infection can present as nodules, ulcers, abscesses, and/or pustules (Figure).¹

Data on M marinum susceptibility are limited.¹ Biopsy is the preferred diagnostic modality. Unlike rapid-growing mycobacteria, M marinum usually requires several weeks before sufficient culture growth occurs.³ In this patient, granulomas were seen upon microscopic review of the biopsy sample, and culture eventually grew M marinum.

Management

For moderate cutaneous M marinum disease in pediatric patients, the following antibiotics can be considered: rifampin, trimethoprim-sulfamethoxazole, clarithromycin, or doxycycline.³ Extensive lesions may require surgical debridement. Susceptibility testing is not routinely required.

In one case series from France, adults with cutaneous infections were treated with a median duration of 4 months of antibiotics.¹ Patients with cutaneous infection in this series mostly received monotherapy with clarithromycin or a cycline. Deeper structure infections were often treated with combination antibiotic therapy with regimens including clarithromycin, rifampin and/or ethambutol. Of the 63 patients studied, 30 with cutaneous and/or deeper structure infections underwent surgery for excision and/or debridement.

Another retrospective case series from Duke University noted that boating or fishing exposure seemed to be associated with invasive disease, whereas fish tank exposure was associated with cutaneous disease.² In this series, median time to diagnosis for invasive infection was 4 months. This delay in diagnosis indicates that the indolent nature of invasive M marinum infection requires a high index of suspicion in those with aquatic exposure.

Of note, patients with a nontuberculous mycobacterium infection can have positive subsequent tuberculin test results because of shared antigens between M tuberculin-derived proteins and proteins from other nontuberculous species.³ Interferon-gamma release assays also may give positive results attributed to antigen cross-reactivity.

Patient outcome

The patient was treated with a 3-month course of trimethoprim-sulfamethoxazole, based on culture susceptibility testing. His lesions healed fully within 1 to 2 years.

References:

1. Aubry A, Chosidow O, Caumes E, Robert J, Cambau E. Sixty-three cases of Mycobacterium marinum infection: clinical features, treatment, and antibiotic susceptibility of causative isolates. Arch Intern Med. 2002;162(15):1746-1752.

2. Johnson MG, Stout JE. Twenty-eight cases of Mycobacterium marinum infection: retrospective case series and literature review. Infection. 2015;43(6):655-662.

3. Diseases caused by nontuberculous Mycobacteria (environmental Mycobacteria, Mycobacteria other than Mycobacterium tuberculosis. In: Baker CJ, ed. Red Book Atlas of Pediatric Infectious Diseases. 3rd ed. Elk Grove Village, IL: American Academy of Pediatrics; 2016:686-688.