Nontuberculous mycobacterial infections

[Merna]

Mycobacteria other than the tubercle bacillus sometimes infect humans. These organisms (called nontuberculous mycobacteria) are commonly present in soil and water and are much less virulent in humans than is Mycobacterium 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 and immunocompromised people are at the highest risk. 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, M. fortuitum, M. abscessus, and M. chelonae. Person-to-person transmission has not been documented.

The lungs are the most common site of disease; most of these 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).

Diagnosis of nontuberculous mycobacterial infections is typically made via acid-fast stain and culture of samples.

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.