Mycobacterium avium complex infection

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Mycobacterium avium complex (MAC) infection is caused by the Mycobacterium avium complex (M. avium and M. intracellulare). It more commonly occurs in patients with advanced immunosuppression and is considered an AIDS-defining condition. Diagnosis is based on isolation of the organism (acid-fast bacilli) on culture and treatment consists of at least two anti-mycobacterial drugs (e.g., a macrolide with ethambutol).

• Caused by the Mycobacterium avium complex, which consists of the ubiquitous pathogens M. avium and M. intracellulare.
• AIDS-defining condition; frequent opportunistic bacterial infection in patients with AIDS
• Infection occurs via the gastrointestinal or respiratory epithelium through inhalation, ingestion, or inoculation.
• Bacteria may remain confined to the area of entry (localized) or spread via the lymphatic system and the blood (disseminated).

References: ^[2]

MAC infection often manifests as disseminated disease and typically occurs in patients with advanced immunosuppression (CD4 counts < 50/mm³). ^[2]

• Night sweats, fever, weight loss
• Focal lymphadenitis
• Abdominal pains, diarrhea
• Hepatomegaly, splenomegaly
• Localized symptoms depend on the organ system involved and include pericarditis, cough, joint pains, or signs of CNS infection.

Diagnosis of disseminated MAC is based on isolation of the organism in a patient with characteristic clinical features. ^[2]

• Laboratory studies
  • CBC: anemia, leukopenia
  • CMP: ↑ alkaline phosphatase, ↑ gamma-glutamyl transpeptidase, ↑ LDH, ↓ albumin ^[3]
  • Isolation of the organism (confirmatory test): acid-fast bacilli staining and culture from blood, bone marrow, lymph nodes, or other sterile tissue or body fluids
• Imaging: depends on the presenting symptoms; examples include
  • Chest x-ray/CT chest: variable findings including diffuse pulmonary opacities, lymphadenopathy; may appear normal
  • CT abdomen: retroperitoneal and mesenteric lymphadenopathy, hepatosplenomegaly, thickening of the small bowel wall

Disseminated MAC may mimic disseminated TB. Findings that favor MAC include elevated alkaline phosphatase and gamma-glutamyl transpeptidase, hepatosplenomegaly, and leukopenia. ^[3]

Manage all patients in consultation with an infectious disease specialist. In patients with HIV, initiate ART (if not already started) concurrently with MAC treatment.

• Initial regimen ^[2]
  • At least two-antimycobacterial drugs; preferred agents include:
    • Macrolide: clarithromycin; (first-line) OR azithromycin
    • AND ethambutol
• Optional third and fourth drugs ^[2]
  • Consider in patients with any of the following:
    • A high risk of mortality (e.g., CD4 count < 50/mm³)
    • Drug resistance
    • High mycobacterial load
    • Inadequate response to ART
  • Agents (any of the following):
    • Rifabutin
    • Fluoroquinolone: levofloxacin or moxifloxacin
    • Injectable agent: amikacin or streptomycin

See “Primary prevention of opportunistic infections in HIV.”