Mycobacteria other than the tubercle bacillus sometimes infect humans. These organisms are commonly present in soil and water and are much less virulent in humans than is M. tuberculosis. Infections with these organisms have been called atypical, environmental, and nontuberculous mycobacterial infections. Most exposures and infections by these organisms do not cause disease, which usually requires a defect in local or systemic host defenses; the frail elderly are sometimes infected. M. avium complex (MAC)—the closely related species of M. avium and M. intracellulare—accounts for most diseases. Other causative species are M. kansasii, M. xenopi, M. marinum, M. ulcerans, and the M. fortuitum complex (M. fortuitum, M. abscessus, and M. chelonae). Person-to-person transmission has not been documented.
The lungs are the most common site; most lung infections involve MAC but may be due to M. kansasii, M. xenopi, or M. abscessus. Occasional cases involve lymph nodes, bones and joints, the skin, and wounds. However, incidence of disseminated MAC disease is increasing in HIV-infected patients, and resistance to anti-TB drugs is the rule (except for M. kansasii and M. xenopi).
Nontuberculous mycobacterial infections are best managed by a specialist with particular expertise in that area. The American Thoracic Society publishes updated diagnostic and therapeutic guidelines on the diagnosis and management of these challenging infections.
The typical patient is a middle-aged or older white man with previous lung problems such as chronic bronchitis, emphysema, healed TB, bronchiectasis, or silicosis. MAC also commonly causes pulmonary disease in middle-aged and elderly women with bronchiectasis, scoliosis, pectus excavatum, or mitral valve prolapse but without known underlying lung abnormalities. This syndrome appears to be increasing in frequency for unknown reasons.
Cough and expectoration are common, often associated with fatigue, weight loss, and low-grade fever. The course may be slowly progressive or stable for long periods. Respiratory insufficiency and persistent hemoptysis may develop. Fibronodular infiltrates on chest x-ray resemble those of pulmonary TB, but cavitation tends to be thin-walled, and pleural effusion is rare. So-called tree-and-bud infiltrates are also characteristic of MAC disease.
Determination of drug susceptibility may be helpful for certain organism/drug combinations but can be done only in highly specialized laboratories.
For moderately symptomatic disease due to MAC with positive sputum smears and cultures, clarithromycin 500 mg po bid or azithromycin 600 mg once/day, rifampin (RIF) 600 mg po once/day, and ethambutol (EMB) 15 to 25 mg/kg po once/day should be used for 12 to 18 mo or until cultures are negative for 12 mo. For progressive cases unresponsive to standard drugs, combinations of 4 to 6 drugs that include clarithromycin 500 mg po bid or azithromycin 600 mg once/day, rifabutin 300 mg po once/day, ciprofloxacin 250 to 500 mg po or IV bid, clofazimine 100 to 200 mg po once/day, and amikacin 10 to 15 mg/kg IV once/day may be tried. Resection surgery is recommended in exceptional cases involving well-localized disease in young, otherwise healthy patients. M. kansasii and M. xenopi infections respond to isoniazid, rifabutin, and EMB, with or without streptomycin or clarithromycin, given for 18 to 24 mo. All nontuberculous mycobacteria are resistant to pyrazinamide.
In children 1 to 5 yr, chronic submaxillary and submandibular cervical lymphadenitis is commonly due to MAC or M. scrofulaceum. It is presumably acquired by oral ingestion of soil organisms.
Diagnosis is usually by excisional biopsy. Usually, excision is adequate treatment and chemotherapy is not required.
Swimming pool granuloma is a protracted but self-limited superficial granulomatous ulcerating disease usually caused by M. marinum contracted from swimming in contaminated pools or from cleaning a home aquarium. M. ulcerans and M. kansasii are occasionally involved. Lesions, reddish bumps, enlarging and turning purple, most frequently occur on the upper extremities or knees. Healing may occur spontaneously, but minocycline or doxycycline 100 to 200 mg po once/day, clarithromycin 500 mg po bid, or RIF plus EMB for 3 to 6 mo have been effective against M. marinum.
Wounds and foreign body infections:
M. fortuitum complex has caused serious infections of penetrating wounds in the eyes and skin (especially feet) and in patients receiving contaminated materials (eg, porcine heart valves, breast implants, bone wax).
Treatment usually requires extensive debridement and removal of the foreign material. Useful drugs include imipenem 1 g IV q 6 h, levofloxacin 500 mg IV or po once/day, clarithromycin 500 mg po bid, trimethoprim/sulfamethoxazole 1 double-strength tablet po bid, doxycycline 100 to 200 mg po once/day, cefoxitin 2 g IV q 6 to 8 h, and amikacin 10 to 15 mg/kg IV once/day, for 3 to 6 mo. Combination therapy is recommended with at least 2 drugs that have in vitro activity. Infections caused by M. abscessus and M. chelonae are resistant to most antibiotics, have proved extremely difficult or impossible to cure and should be referred to an experienced specialist.
MAC causes disseminated disease commonly in patients with advanced AIDS and occasionally in those with other immunocompromised states, including organ transplantation and hairy cell leukemia. In AIDS patients, disseminated MAC usually develops late (unlike TB, which develops early), occurring simultaneously with other opportunistic infections.
Disseminated MAC disease causes fever, anemia, thrombocytopenia, diarrhea, and abdominal pain (features similar to Whipple's disease). Diagnosis can be confirmed by cultures of blood or bone marrow or by biopsy (eg, percutaneous fine-needle biopsy of liver or necrotic lymph nodes). Organisms may be identified in stool and respiratory specimens, but organisms from these specimens may represent colonization rather than true disease.
Combination therapy to clear bacteremia and alleviate symptoms usually requires 2 to 3 drugs; one is clarithromycin 500 mg po bid or azithromycin 600 mg po once/day, plus EMB 15 to 25 mg/kg once/day. Sometimes rifabutin 300 mg once/day is also given. After successful treatment, chronic suppression with clarithromycin or azithromycin plus EMB is necessary to prevent relapse.
HIV-infected patients with a CD4 count < 100 cells/µL require prophylaxis for disseminated MAC with azithromycin 1.2 g po once/week or clarithromycin 500 mg po bid.
Last full review/revision September 2009 by Edward A. Nardell, MD
Content last modified November 2012