Atypical Mycobacterial Infections
Saturday, January 01, 2011
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 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 systemic infection with involvement of
- M. marinum infection is suspected if the patient
presents with localized or sporotrichoid keratotic papules and
nodules that may suppurate. Risk exposure is most commonly through
an aquarium or a penetrating injury obtained while cleaning
- Once the diagnosis is established, minocycline 100 mg twice
daily for several months is usually curative.
- For patients who experience unacceptable side effects from
minocycline, doxycycline 100 mg twice daily is an alternative.
- Trimethoprim-sulfamethoxazole 1 tablet twice daily (equivalent
to 160 mg trimethoprim / 800 mg sulfamethoxazole) is also
- Clarithromycin 500 mg twice daily is a more expensive
- Rifampin 600 mg daily plus ethambutol 15 mg/kg daily is used in
patients where the above therapies have been ineffective or not
- For all regimens, therapy is continued for 1-2 months beyond
complete clinical resolution of the lesion.
Rapid Growers (Group IV Mycobacteria)
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.
- Clarithromycin 500 mg twice daily for an average of 6 months
(depending on the clinical response) is effective in
- 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.
- Mycobacterial infections can appear sporotrichoid; hence,
biopsies from patients with this pattern should be examined and
cultured for both mycobacteria and deep fungi.
- 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.
- Rarely, M. marinum infections produce concurrent
synovitis, arthritis, and/or osteomyelitis.