Atypical Mycobacterial Infections

Although several mycobacterial organisms have been associated with skin lesions, only two, M. fortuitum / chelonei/ abscessus (Group IV, rapid growers) and M. marinum, are common pathogens. Diagnosis requires a culture to identify the causative agent and sensitivity testing to clarify therapeutic options. Since cultures may be negative or take months to be completed, empiric strategies based on the likely pathogen are outlined. Sensitivity testing will confirm the efficacy of the agent(s) selected in each case. Immunosuppressed hosts may require additional therapy (multiple agents simultaneously) and are more likely to have a systemic infection with the involvement of extracutaneous sites.

M. marinum

First Steps

1. M. marinum infection is suspected if the patient presents with localized or sporotrichoid keratotic papules and nodules that may be suppurate. Risk exposure is most commonly through an aquarium or a penetrating injury obtained while cleaning fish.
2. Once the diagnosis is established, minocycline 100 mg twice daily for several months is usually curative.

Alternative Steps

1. For patients who experience unacceptable side effects from minocycline, doxycycline 100 mg twice daily is an alternative.
2. Trimethoprim-sulfamethoxazole 1 tablet twice daily (equivalent to 160 mg trimethoprim / 800 mg sulfamethoxazole) is also effective.
3. Clarithromycin 500 mg twice daily is a more expensive alternative.
4. Rifampin 600 mg daily plus ethambutol 15 mg/kg daily is used in patients where the above therapies have been ineffective or not tolerated.
5. For all regimens, therapy is continued for 1-2 months beyond the complete clinical resolution of the lesion.

Rapid Growers (Group IV Mycobacteria)

First Steps

Infections by the Group IV rapidly growing mycobacteria usually produce subcutaneous abscesses or cellulitis of the lower extremity. Sporotrichoid spread may occur. Risk exposure is to water (foot baths in nail salons) or contaminated injections. Immunosuppression enhances the risk of infection.

1. Clarithromycin 500 mg twice daily for an average of 6 months (depending on the clinical response) is effective in nonimmunosuppressed hosts.
2. Combination therapy determined by sensitivities may be required in immunosuppressed hosts. Initial treatment with systemic amikacin plus cefoxitin or a quinolone plus clarithromycin may be effective in more severe cases.

Pitfalls

1. Mycobacterial infections can appear sporotrichoid; hence, biopsies from patients with this pattern should be examined and cultured for both mycobacteria and deep fungi.
2. Side effects of treatment may occur from the antibiotics recommended. Patients should be queried about these adverse reactions, as they may reduce compliance. Since treatment durations are months long, and most patients cannot "tough out" adverse events for the duration of treatment, so they may either reduce the dose or miss doses, increasing the risk of drug resistance.
3. Rarely, M. marinum infections produce concurrent synovitis, arthritis, and/or osteomyelitis.