Histoplasmosis
Related Subjects:
|Adrenal Physiology
|Addison's Disease
๐ 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 โ 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 โ usually imported cases.
- Often described as โthe fungal TB mimicโ due to its granulomatous and disseminated forms.
๐งฌ Aetiology & Pathology
- Inhaled spores transform into yeast in alveoli and are taken up by macrophages.
- Immune response โ calcified granulomas with caseous necrosis, resembling TB on imaging.
- Potential to disseminate in immunocompromised hosts.
โ ๏ธ Risk of Disseminated Infection
- AIDS patients (esp. CD4 <150 cells/ยตL).
- Patients on corticosteroids or TNF-ฮฑ antagonists (e.g., infliximab, etanercept).
- Haematological malignancies, recent solid organ transplants.
๐ฉบ Clinical Features
- Most exposures: asymptomatic.
- Acute pulmonary infection (2โ3 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 โ SVC obstruction; adrenal involvement โ Addisonโs disease.
- Disseminated disease: Fever, pancytopenia, hepatosplenomegaly, lymphadenopathy. High mortality if untreated.
๐ Investigations
- Imaging: Bihilar lymphadenopathy, pulmonary nodules, calcified granulomas.
- Adrenal tests: Cortisol and Short Synacthen if adrenal involvement suspected.
- Antigen detection: Serum & urine antigen โ 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.
๐ Management
- Mild pulmonary histoplasmosis: Often self-limiting; supportive care.
- Moderate-severe acute or chronic disease: Itraconazole PO for 6โ12 weeks.
- Severe disseminated disease: IV Amphotericin B initially, then oral itraconazole for months (sometimes โฅ1 year).
- Monitor treatment response with repeat urine/serum antigen levels.
๐ Exam Pearls
- Think histoplasmosis if: โtraveller + cave exposure + pulmonary disease mimicking TB.โ
- Adrenal failure in disseminated cases is a high-yield clue.
- Chronic pulmonary form is linked with COPD and male smokers.
