Mycobacterium avium complex (MAC)

Microbiology

A slow-growing [non-tuberculous mycobacterium](Non-tuberculous mycobactera (NTM).md)
Comprises M. avium (subspecies hominissuis in humans) and M. intracellulaire
- 28 serovars between the two
Can colonize airways and GI tract as well as causing disease

Common in the environment, likely acquired by inhalation or ingestion (not person-to-person)
- Water, soil, and animals
- Natural water, indoor water, pools, hot tubs
Risk factors
- Pulmonary disease: COPD, preior pneumonia, prior TB, steroid use, silicosis, scoliosis, pectus excavatum
- Disseminated disease: HIV, interferon-gamma defects
- Cervical lymphadenitis: children 1-5 years old, HIV with IRIS

In pulmonary disease, likely inhalation ± microaspiration
In disseminated disease in AIDS, likely GI colonization then replication and dissemination

Incubation months to years
Typically presents with non-specific subacute to chronic respiratory syndrome
- Productive cough, weight loss, fevers, night sweats
CXR often shows upper-lobe infiltrates ± cavitations ± pleural thickening
- Bronchiectasis on CT in 94%
Mortality is 15% within 2 to 10 years
Specific populations/presentations:
- Smokers/COPD/alcohol: typical presentation as above
- Lady Windermere syndrome: slower progression
- Hot tub lung disease: a hypersensitivity pneumonitis associated with inhalation

Almost exclusive to HIV patients with CD4 <100
- Median CD4 was 13
Typically presents with high fevers, weight loss, night sweats, anemia
Also abdo pain, diarrhea, intraabdo lymphadenopathy, hepatosplenomegaly, and elevated ALP
Organ-specific involvement: spleen, lymph nodes, liver, adrenals, stomach, CNS
- Often not lung involvement
Mortality 50% at 4 months

(1) chest radiograph or, in the absence of cavitation, chest high-resolution computed tomography (HRCT) scan
(2) three or more sputum specimens for acid-fast bacilli (AFB) analysis
(3) exclusion of other disorders, such as tuberculosis

Diagnosis based on presence of both clinical and microbiological evidence
Clinical:
- Pulmonary symptoms, nodular or cavitary opacities on chest radiograph, or an HRCT scan that shows multifocal bronchiectasis with multiple small nodules; and
- Appropriate exclusion of other diagnoses.
Microbiological:
- Positive cultures from at least two separate expectorated sputum samples; or
- Positive culture from at least one bronchial wash or lavage; or
- Transbronchial or other lung biopsy with mycobacterial histopathologic features, and either positive culture for NTM or one or more sputum or BALs positive for NTM

Nodular/bronchiectatic disease:
- Clarithromycin (1,000 mg) or azithromycin (500 mg)
- Rifampin (600 mg)
- Ethambutol (25 mg/kg)
- Three times weekly (because daily never tolerated)
If fibrocavitary MAC lung disease or severe nodular/bronchiectatic disease:
- Clarithromycin (500–1,000 mg) or azithromycin (250 mg) daily
- Rifampin (600 mg) or rifabutin (150–300 mg) daily
- Ethambutol (15 mg/kg) daily
- And consider adding three-times-weekly amikacin or streptomycin early in therapy
Duration is until culture negative on therapy for at least 1 year
Refer to Ophthalmology for monitoring while on ethambutol

Antibiotics
- Clarithromycin (1,000 mg/d) or azithromycin (250 mg/d)
- Ethambutol (15 mg/kg/d)
- ± rifabutin (150–350 mg/d)
Duration is until resolution of symptoms and reconstitution of cell-mediated immune function (e.g. CD4 >100)

Indicated for adults with (AIDS) with CD4􏰀 <50; but not done and no longer in the HIV guidelines
Antibiotics: azithromycin 1,200 mg/week or clarithromycin 1,000 mg/day
Second-line: rifabutin 300 mg/day (less well tolerated)