33-35: Nontuberculous Mycobacterial Diseases

About 10% of mycobacterial infections are caused by nontuberculous mycobacteria. Nontuberculous mycobacterial infections are among the most common opportunistic infections in advanced HIV disease. These organisms have distinctive laboratory characteristics, occur ubiquitously in the environment, are not communicable from person to person, and are often resistant to standard antituberculous drugs.

1. PULMONARY INFECTIONS

Mycobacterium avium complex (MAC) causes a chronic, slowly progressive pulmonary infection resembling tuberculosis in immunocompetent patients who typically have underlying pulmonary disease. Susceptibility testing for macrolide-resistance should be performed on clinical isolates. Treatment of nodular or bronchiectatic pulmonary disease is clarithromycin (1000 mg orally three times per week) or azithromycin (500 mg orally three times per week) plus rifampin (600 mg orally three times per week) or rifabutin (300 mg orally three times per week) plus ethambutol (25 mg/kg orally three times per week). Patients with fibrocavitary lung disease or severe nodular or bronchiectatic disease should receive three-drug therapy with clarithromycin (500–1000 mg orally daily) or azithromycin (500 mg orally daily) plus rifampin (600 mg orally daily) or rifabutin (300 mg orally daily) plus ethambutol (15 mg/kg orally daily). Therapy is continued for at least 12 months after sterilization of cultures.

M kansasii can produce clinical disease resembling tuberculosis, but the illness progresses more slowly. Most such infections occur in patients with preexisting lung disease, though 40% of patients have no known pulmonary disease. Microbiologically, M kansasii is similar to M tuberculosis and is sensitive to the same drugs except pyrazinamide, to which it is resistant. Therapy with isoniazid, ethambutol, and rifampin for 2 years (or 1 year after sputum conversion) has been successful.

Less common causes of pulmonary disease include M xenopi, M szulgai, and M gordonae. These organisms have variable sensitivities, and treatment is based on results of sensitivity tests. The rapid growing mycobacteria, M abscessus, M chelonae, and M fortuitum, also can cause pneumonia in the occasional patient.