Question
Mycobacterium avium – intracellular complex
Answer
Mycobacterium avium-intracellulare complex (MAC/MAI) comprises a group of slow-growing, non-tuberculous (atypical) mycobacteria, ubiquitous in the environment (soil, water), that cause opportunistic disease mainly in immunocompromised hosts, especially patients with advanced HIV/AIDS (typically at CD4 counts below 50 cells/μL).
Clinical significance:
- In AIDS patients — causes disseminated MAC infection, with prolonged fever, night sweats, weight loss, diarrhoea, hepatosplenomegaly, and lymphadenopathy, reflecting widespread haematogenous dissemination; a classic AIDS-defining opportunistic infection at very low CD4 counts.
- In immunocompetent individuals — can cause chronic pulmonary disease resembling tuberculosis, particularly in patients with pre-existing structural lung disease (bronchiectasis, COPD), and cervical lymphadenitis in children.
Laboratory diagnosis: acid-fast bacilli seen on ZN staining (morphologically indistinguishable from M. tuberculosis on smear alone); culture on Lowenstein-Jensen medium or liquid culture systems, with species identification by biochemical tests, high-performance liquid chromatography, DNA probes, or molecular sequencing to distinguish it from M. tuberculosis; blood culture (using lysis-centrifugation technique) is useful for disseminated disease in AIDS patients.
Treatment: multidrug regimen combining a macrolide (clarithromycin or azithromycin), ethambutol, and rifabutin, given for a prolonged course; unlike M. tuberculosis, MAC is often resistant to first-line antitubercular drugs, requiring this different regimen. Prophylaxis with azithromycin is given to AIDS patients with very low CD4 counts to prevent disseminated disease, and antiretroviral therapy to restore immune function is central to management.