Makindo Medical Notes

Related Subjects: |Adrenal Physiology |Addison's Disease

🌍 About Histoplasmosis

• Caused by the dimorphic fungus Histoplasma capsulatum (mould in the environment, yeast in tissue).
• Endemic in soil enriched with bat or bird droppings – classic exposure sites: caves, chicken coops, old barns.
• Most common in the Ohio and Mississippi river valleys (USA), parts of Latin America, and sporadically in Eastern Europe. Rare in the UK – usually imported cases.
• Often described as “the fungal TB mimic” due to its granulomatous and disseminated forms.

🧬 Aetiology & Pathology

• Inhaled spores transform into yeast in alveoli and are taken up by macrophages.
• Immune response → calcified granulomas with caseous necrosis, resembling TB on imaging.
• Potential to disseminate in immunocompromised hosts.

⚠️ Risk of Disseminated Infection

• AIDS patients (esp. CD4 <150 cells/µL).
• Patients on corticosteroids or TNF-α antagonists (e.g., infliximab, etanercept).
• Haematological malignancies, recent solid organ transplants.

🩺 Clinical Features

• Most exposures: asymptomatic.
• Acute pulmonary infection (2–3 weeks post-exposure): Fever, malaise, cough, chest pain; may present with erythema nodosum.
• Chronic pulmonary histoplasmosis: Seen in male smokers with COPD; apical cavitation and fibrosis resembling reactivation TB.
• Complications: Fibrosing mediastinitis → SVC obstruction; adrenal involvement → Addison’s disease.
• Disseminated disease: Fever, pancytopenia, hepatosplenomegaly, lymphadenopathy. High mortality if untreated.

🔎 Investigations

• Imaging: Bihilar lymphadenopathy, pulmonary nodules, calcified granulomas.
• Adrenal tests: Cortisol and Short Synacthen if adrenal involvement suspected.
• Antigen detection: Serum & urine antigen – sensitive in disseminated disease.
• Serology: Anti-M and Anti-G antibodies (false positives in TB, lymphoma, sarcoidosis).
• Blood cultures: May grow fungus in disseminated disease.

💊 Management

• Mild pulmonary histoplasmosis: Often self-limiting; supportive care.
• Moderate-severe acute or chronic disease: Itraconazole PO for 6–12 weeks.
• Severe disseminated disease: IV Amphotericin B initially, then oral itraconazole for months (sometimes ≥1 year).
• Monitor treatment response with repeat urine/serum antigen levels.

📚 Exam Pearls

• Think histoplasmosis if: “traveller + cave exposure + pulmonary disease mimicking TB.”
• Adrenal failure in disseminated cases is a high-yield clue.
• Chronic pulmonary form is linked with COPD and male smokers.