Mycobacterium avium Complex (MAC): Risk Groups and Transmission

Mycobacterium avium complex (MAC)—M. avium and M. intracellulare—are environmental nontuberculous mycobacteria (NTM) causing pulmonary and disseminated disease in susceptible hosts.

At-Risk Populations

• Immunocompromised: HIV/CD4 <50 (disseminated); post-transplant (especially lung); biologic therapy (anti-TNF, JAK inhibitors); high-dose steroids.
• Structural Lung Disease: COPD, bronchiectasis, cystic fibrosis, prior TB, pneumoconiosis — "Lady Windermere" phenotype (older, thin, nonsmoking women with CF/bronchiectasis).
• Other: Diabetes, malnutrition, achalasia, GERD (microaspiration), silicosis, alcohol use disorder.

Transmission

• Environmental: Aerosolized water/soil (showers, hot tubs, pools, potting soil) — primary route for pulmonary MAC.
• Foodborne: Unpasteurized milk/dairy (rare; M. bovis more common).
• Person-to-person: Not documented.

Clinical Presentation

• Pulmonary (Nodular-Bronchiectatic): Chronic cough, fatigue, weight loss, hemoptysis, low-grade fever. CT: tree-in-bud, bronchiectasis, nodules (RML/lingula).
• Fibrocavitary: Upper lobe cavities; resembles TB; more common in smokers/COPD.
• Disseminated: Fever, night sweats, anemia, hepatosplenomegaly, diarrhea (HIV/CD4 <50).

Diagnosis (ATS/IDSA Criteria)

• Clinical + radiographic + microbiologic (≥2 positive sputum or 1 BAL/biopsy)
• Exclude TB and other mimics

Treatment

• Pulmonary: 3-drug macrolide-based (azithromycin/clarithromycin + ethambutol + rifampin) × 12 months post-culture conversion.
• Disseminated (HIV): Clarithromycin/azithromycin + ethambutol ± rifabutin; ART essential.
• Macrolide resistance = treatment failure risk.