Histoplasmosis

Organism & Morphology

• Mold phase (room temperature, environment): produces characteristic spiked macroconidia (8–15 µm) and small microconidia (2–5 µm). The microconidia are the infectious particles — small enough to reach the terminal bronchioles.
• Yeast phase (37°C, in host): small (2–5 µm), oval yeasts with narrow-based budding, found intracellularly within macrophages.

Epidemiology

• Most prevalent endemic mycosis in North America, particularly the Ohio and Mississippi River valleys; also Mexico, Central/South America (especially Brazil), Africa, and Asia.
• Up to 75% of adults in endemic areas show serologic/radiographic evidence of prior infection without clinical illness.
• The organism thrives in humid, acidic soil enriched with bird or bat droppings (roosting sites, caves, chicken coops, old buildings).
• High-risk exposures: spelunking, excavation, demolition/renovation of old buildings, cleaning chicken coops, cutting dead trees.
• Outside endemic areas, most cases represent imported disease (travelers, immigrants).
• At-risk populations are growing due to immunosuppressive therapies (anti-TNF agents, transplant immunosuppression, etc.).

Pathogenesis

1. Inhalation of microconidia → reach alveolar spaces
2. Engulfed by alveolar macrophages → transform to yeast (calcium- and iron-dependent process)
3. Yeasts evade intracellular killing and can spread to regional lymph nodes and hematogenously
4. In immunocompetent hosts, cell-mediated immunity (IFN-γ, TNF-α pathways) eventually controls infection → granuloma formation → calcification
5. In immunocompromised hosts: uncontrolled intracellular proliferation → progressive disseminated histoplasmosis (PDH)

Clinical Syndromes

1. Asymptomatic / Subclinical Infection

2. Acute Pulmonary Histoplasmosis

• Arthritis/arthralgia + erythema nodosum: 5–10% of cases; resembles sarcoidosis; responds to anti-inflammatories
• Pericarditis: <10%; chest pain, pericardial rub; may cause tamponade; managed with anti-inflammatories ± drainage

3. Subacute Pulmonary Histoplasmosis

• Subacute onset: fever, cough, chest pain
• CXR: mediastinal lymphadenopathy + patchy infiltrates
• Most recover in weeks; some have prolonged fatigue

4. Chronic Cavitary Histoplasmosis

• Occurs in smokers with underlying structural lung disease (bullous emphysema)
• Productive cough, dyspnea, low-grade fever, night sweats, weight loss
• CXR/CT: upper-lobe infiltrates, cavitation, pleural thickening — closely resembles pulmonary tuberculosis
• Progressive without treatment; ~15% relapse rate even with adequate therapy

5. Progressive Disseminated Histoplasmosis (PDH)

• ~1 in 2,000 infections; ~70% in immunocompromised hosts
• Risk factors: AIDS (CD4 <200/µL), solid organ transplant, anti-TNF-α agents (infliximab, adalimumab, etanercept), methotrexate, corticosteroids, extremes of age
• Manifestations:
  • Fever, weight loss, cough, dyspnea (most common)
  • Hepatosplenomegaly (~50%), lymphadenopathy (~33%)
  • Sepsis-like syndrome in severe immunosuppression
  • Meningitis or focal brain lesions (5–10%)
  • Oral mucosal ulcerations, GI ulcerations/bleeding
  • Adrenal insufficiency (adrenal gland involvement)
  • Thrombocytopenia, pancytopenia
• CXR abnormal in 70%: diffuse interstitial or reticulonodular infiltrates, miliary pattern

6. Mediastinal Complications

• Mediastinal lymphadenopathy: hilar/mediastinal nodes undergo necrosis and coalesce → large masses compressing great vessels, airways, or esophagus; may create bronchoesophageal fistulae
• Fibrosing mediastinitis: uncommon but serious; progressive fibrosis encases mediastinal structures → superior vena cava syndrome, pulmonary vessel obstruction, airway obstruction; fatal in up to one-third of cases; no effective antifungal treatment at this stage (fibrosis is irreversible)
• Broncholithiasis: calcified lymph nodes erode into bronchi → hemoptysis, obstruction, tracheoesophageal fistulae

7. Pulmonary Nodules

• Histoplasma nodules must be differentiated from malignancy
• Calcification favors histoplasmosis but does not exclude cancer
• PET scan can be positive in histoplasmosis (mimics malignancy)
• Slow enlargement (up to 2 mm/year) is not progressive infection and does not require treatment

Diagnosis

Before initiating immunosuppressive therapy for presumed sarcoidosis, active histoplasmosis must be excluded — the two conditions share many clinical and radiographic features (mediastinal adenopathy, granulomas, elevated ACE, elevated ESR/CRP). — Fishman's Pulmonary Diseases and Disorders

Treatment (IDSA 2007 / ATS 2011 Guidelines)

Mild to Moderate Acute Pulmonary Histoplasmosis

• If symptomatic >3 weeks or moderate disease: Itraconazole 200 mg TID × 3 days (load), then 200 mg BID × 6–12 weeks
• Fluconazole is less effective; ketoconazole should not be used
• Echinocandins are intrinsically ineffective against Histoplasma

Moderately Severe to Severe Acute Pulmonary Histoplasmosis

• Liposomal amphotericin B 3–5 mg/kg/day × 1–2 weeks → then itraconazole 200 mg TID × 3 days, then 200 mg BID × 12 weeks
• Deoxycholate amphotericin 0.7–1 mg/kg/day if liposomal unavailable or low nephrotoxicity risk
• Corticosteroids (methylprednisolone 0.5–1 mg/kg IV, or prednisone 40–60 mg/day) for hypoxemia or respiratory distress, especially in HIV patients at risk for IRIS

Chronic Cavitary Histoplasmosis

• Itraconazole 200 mg TID × 3 days load, then 200 mg BID × minimum 1 year (extending to 18–24 months reduces ~15% relapse rate)

Progressive Disseminated Histoplasmosis (PDH)

• Liposomal amphotericin B × 1–2 weeks → itraconazole for minimum 1 year total
• Mild-moderate PDH in immunocompetent hosts: itraconazole alone may suffice

Immunocompromised / HIV

• Itraconazole 200 mg once or twice daily indefinitely if immunosuppression cannot be reversed
• In HIV: continue until CD4 >200/µL (with ART)
• Annual urine Histoplasma antigen monitoring on maintenance therapy

Pleural Effusion from Histoplasmosis

• Lymphocytic exudate; pleural biopsy may show noncaseating granulomas
• Usually self-limited — no antifungal treatment needed unless:
  • Effusion persists >3–4 weeks, OR
  • Patient is immunocompromised

Key Associations to Remember

Recent Evidence (2024)

• Histoplasmosis in HIV patients in Europe (2024) — Systematic review highlighting histoplasmosis as an underrecognized imported opportunistic infection in HIV patients in Europe (PMID 38993488)
• WHO Fungal Priority Pathogens List systematic review (2024) — Histoplasma included; review documents global burden and diagnostic gaps, particularly in low-income countries (PMID 38935903)