Infectious Disease · Mycobacterial Infections
The facts most likely to be tested
Mycobacterium avium complex (MAC) is the most common atypical mycobacterial infection in patients with advanced HIV/AIDS when the CD4 count is <50 cells/µL.
Mycobacterium marinum presents as a nodular, ulcerative skin lesion on the extremities following exposure to aquarium water or saltwater fish.
Mycobacterium kansasii causes a chronic pulmonary infection that is clinically and radiographically indistinguishable from tuberculosis.
Mycobacterium scrofulaceum is a classic cause of unilateral cervical lymphadenitis in young children.
Prophylaxis for MAC in HIV patients with CD4 <50 cells/µL is achieved with azithromycin or clarithromycin.
Disseminated MAC infection manifests with fever, night sweats, weight loss, diarrhea, and hepatosplenomegaly.
Treatment for MAC involves a combination of clarithromycin or azithromycin plus ethambutol.
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A 34-year-old male with a history of untreated HIV presents to the clinic with a 3-week history of fever, night sweats, and a 10-lb weight loss. Physical examination reveals hepatosplenomegaly and generalized lymphadenopathy. Laboratory studies show a CD4 count of 32 cells/µL. A blood culture is obtained and later grows acid-fast bacilli.
What is the most appropriate prophylactic regimen that should have been initiated for this patient?
Azithromycin
The patient has disseminated MAC infection, which is prevented in patients with CD4 counts <50 cells/µL using macrolide prophylaxis (Bet 1 and Bet 5).
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Etiology / Epidemiology
Non-tuberculous mycobacteria (NTM) are ubiquitous in soil and water. Risk factors include structural lung disease (COPD, bronchiectasis) and immunocompromise (HIV/AIDS).
Clinical Manifestations
Presents as chronic cough, hemoptysis, and weight loss. Lady Windermere syndrome is a classic presentation in elderly women with bronchiectasis.
Diagnosis
Sputum culture is the gold standard. Diagnosis requires two positive cultures from separate samples.
Treatment
Treatment is complex; Clarithromycin plus Ethambutol is the backbone for *M. avium complex* (MAC). Ototoxicity is a major concern.
Prognosis
Treatment is prolonged, often 12 months post-culture conversion. Relapse is common if therapy is prematurely discontinued.
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Epidemiology & Etiology
NTM are environmental organisms, not transmitted person-to-person. M. avium complex (MAC) is the most common pathogen in the US. Patients with cystic fibrosis or prior TB are at significantly increased risk.
Pertinent Anatomy
Pulmonary involvement typically targets the upper lobes or areas of pre-existing bronchiectasis. Disseminated disease involves the reticuloendothelial system, including the liver, spleen, and bone marrow.
Pathophysiology
NTM exploit impaired mucociliary clearance or localized structural damage to establish infection. In HIV, CD4 count < 50 cells/µL allows for systemic hematogenous spread. The inflammatory response is typically granulomatous, similar to *M. tuberculosis*.
Clinical Manifestations
Symptoms mimic TB: fever, night sweats, and fatigue. Lady Windermere syndrome involves chronic cough in thin, elderly women who suppress the cough reflex. Hemoptysis is a red flag for cavitary disease. In children, cervical lymphadenitis is the most common manifestation.
Diagnosis
Diagnosis requires clinical, radiographic, and microbiological evidence. Sputum culture is the gold standard. Radiographic findings often show nodular bronchiectasis or thin-walled cavities on high-resolution CT.
Treatment
MAC treatment requires a multi-drug regimen: Clarithromycin (or Azithromycin), Ethambutol, and Rifampin. Visual disturbances are a classic side effect of Ethambutol. Ototoxicity and QT prolongation are risks with macrolides.
Prognosis
Treatment is long-term, requiring 12 months of negative cultures. Drug resistance is a major barrier to cure. Patients require serial sputum monitoring to ensure clearance.
Differential Diagnosis
Tuberculosis: Person-to-person transmission
Histoplasmosis: Exposure to bird/bat droppings
Sarcoidosis: Non-caseating granulomas without infection
Lung Cancer: Weight loss without positive cultures
Bronchiectasis: Chronic sputum production without mycobacteria