Numerous species widely present in environment; worldwide distribution
– Transmission typically by inhalation or ingestion
Mycobacterium avium and Mycobacterium intracellulare, known together as Mycobacterium avium-intracellulare complex (MAC), most commonly cause hepatic disease
Clinical Issues
• Common opportunistic infection in AIDS patients, particularly those that have CD4 counts < 50 cells/μL
Up to 35% of all AIDS patients develop disseminated MAC eventually
• Helpful laboratory tests include mycobacterial blood culture and PCR
• Treatment is multidrug regimen for many months
Drug resistance is problematic
Prognosis generally poor
Microscopic
• Wide range of histologic features depending on immune status of patient with numerous acid-fast organisms on acid-fast bacterial stain
Poorly formed granulomas, loose aggregates of histiocytes, or of foamy histiocytes
Fibrin ring granulomas
Necrosis/abscess
Spindle cell pseudotumor
• AFB stains often show large numbers of organisms in immunocompromised patients
• In patients with preserved T-cell function, granulomas can be well formed, similar to tuberculosis
Diagnostic Checklist
• Aggregates of foamy histiocytes in liver biopsy from patient with AIDS warrants stains for acid-fast bacteria
Aggregates of Foamy Histiocytes This liver from an AIDS patient who died of a Mycobacterium avium-intracellulare complex (MAC) infection shows loose aggregates of foamy histiocytes and poorly formed granulomas with minimal inflammation in the lobule or associated with the granulomas.
Numerous Organisms on AFB Stain Ziehl-Neelsen stain shows innumerable acid-fast bacteria within clustered histiocytes in a patient with MAC infection of the liver.
Poorly Formed Granuloma This high-power view of a poorly formed granuloma in a MAC infection shows a rounded, loose collection of histiocytes.
Scattered Organisms on AFB Stain Scattered acid-fast rods are seen on this AFB stain. Immunocompetent patients often have fewer lesional organisms than immunocompromised patients.
• Infection by any one of many species of nontubercular mycobacteria
Classified according to growth rate, presence, or absence and type of pigment
Includes M avium, M. intracellulare, M. kansasii, M. marinum, M. gordonae, M. chelonae, M. scrofulaceum, M. szulgai, M. malmoense, M. xenopi, M. abscessus, and M. fortuitum
ETIOLOGY/PATHOGENESIS
Infectious Agents
• Numerous species widely present in environment; worldwide distribution
Transmission typically by inhalation or ingestion
Mycobacterium avium and Mycobacterium intracellulare, known together as MAC, most commonly cause hepatic disease
Next to MAC, Mycobacterium kansasii is most common cause of nontuberculous mycobacterial infection in HIV patients
CLINICAL ISSUES
Epidemiology
• Incidence
Up to 35% of all AIDS patients develop disseminated MAC eventually
– 1-year incidence is 3% among patients with CD4 counts between 100-199 cells/μL and 39% for patients with CD4 counts < 10 cells/μL
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Mycobacterium avium and Mycobacterium intracellulare, known together as Mycobacterium avium-intracellulare complex (MAC), most commonly cause hepatic disease
with minimal inflammation in the lobule or associated with the granulomas.
are seen on this AFB stain. Immunocompetent patients often have fewer lesional organisms than immunocompromised patients.
