[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Dimorphic fungi](https://durgical.com/en/microbiology/groups/dimorphic-fungi/) › Histoplasma capsulatum [Microbiology](https://durgical.com/en/microbiology/) · [Dimorphic fungi](https://durgical.com/en/microbiology/groups/dimorphic-fungi/) # Histoplasma capsulatum - Fungus - Exam priority: high A fungus of soil with bird or bat droppings (caves); inhaled, it spreads in advanced HIV with fever, weight loss and pancytopenia. Updated 2026-11-01 Also called: Histoplasma ## Key points - Caves, bats and birds: think Histoplasma - Tiny yeasts inside macrophages - Urine antigen: the key rapid test in disseminated disease - Advanced HIV with fever, pancytopenia and hepatosplenomegaly: histoplasmosis or tuberculosis - Lipid amphotericin B, then itraconazole ## At a glance **Shape** Small oval yeasts inside macrophages **Family** Ajellomycetaceae (Onygenales) **Type** Thermally dimorphic fungus: mold in the environment, yeast in tissue **Stain** Giemsa or Wright on blood or marrow; silver (GMS) and PAS on biopsy **Size** Yeasts of about 2–4 µm ## Morphology - In the environment: mold with large rough-surfaced macroconidia and small microconidia (the ones inhaled) - In tissue: tiny yeasts clustered inside macrophages and monocytes - Slow culture, taking several weeks ## Transmission - Inhaling microconidia when soil with bat guano or bird droppings is disturbed - Risk in caves, chicken coops, demolition, construction and cleaning old buildings - Not spread from person to person **Reservoir:** Soil enriched with bat guano or bird droppings ### Isolation precautions What the care team adds to standard precautions. - Standard ## Life cycle - In soil it grows as a mold and produces macroconidia and microconidia - Microconidia become airborne when soil is disturbed and are inhaled - At 37 °C in the alveolus they convert to yeasts - Macrophages engulf them and carry them to lymph nodes, liver, spleen and marrow - With good cellular immunity granulomas form; if it fails, it disseminates or reactivates ## Virulence factors - Survives and multiplies inside the macrophage - Switch to yeast at body temperature, essential to cause disease - Captures iron and calcium inside the phagosome ## Diseases it causes - Histoplasmosis: from mild or silent lung infection to chronic cavitary and disseminated disease [Histoplasmosis →](https://durgical.com/en/conditions/histoplasmosis/) - In advanced HIV, disseminated histoplasmosis is AIDS-defining and a leading cause of death [HIV infection and AIDS →](https://durgical.com/en/conditions/hiv-infection-and-aids/) ## Clinical picture - Acute pulmonary histoplasmosis: fever, cough and chest pain after a cave visit - Disseminated: prolonged fever, weight loss, hepatosplenomegaly, pancytopenia, oral ulcers and skin lesions; mistaken for tuberculosis - Residual lung and spleen calcifications ## Diagnosis - Histoplasma antigen in urine (or serum): among the most sensitive and fastest tests, especially in disseminated disease - Blood or marrow smear with Giemsa: yeasts inside phagocytes - Culture of blood, marrow or tissue: confirms, but takes weeks - Biopsy with silver or PAS; antibodies useful in chronic forms; PCR where available - The antigen can cross-react with Blastomyces and Paracoccidioides ## Treatment Severe or disseminated disease: amphotericin B (lipid preferred) followed by itraconazole; mild to moderate disease: itraconazole. Many mild acute lung infections resolve without treatment. Drug class only; doses and regimens are set by the clinician from the current guideline. In Pharmacology: - [Amphotericin B](https://durgical.com/en/pharmacology/amphotericin-b/) - [Itraconazole](https://durgical.com/en/pharmacology/itraconazole/) ## Prevention - High-efficiency mask and wetting the soil before disturbing guano or droppings - People with advanced HIV: avoid caves and chicken coops - Early HIV diagnosis and antiretroviral therapy; antigen screening in advanced HIV ## In Colombia Histoplasmosis is common in Colombia and throughout Latin America; the disseminated form is one of the most important opportunistic infections in late-diagnosed HIV. Outbreaks have been described in people visiting caves and tunnels. PAHO pushes for antigen testing in advanced HIV. ## Memory aid Histo hides in the macrophage like a bat in a cave: look for tiny yeasts tucked inside the cells. ## Test yourself Think of the answer, then tap the question to see it. ## Which fast, non-invasive test is key to diagnosing disseminated histoplasmosis? Histoplasma antigen in urine (it can also be measured in serum). ## In which cells are Histoplasma yeasts seen on a bone marrow smear? Inside macrophages and other phagocytes, as small clustered yeasts. ## Which exposure history suggests histoplasmosis in a patient with pneumonia? Visiting caves or disturbing soil with bat guano or bird droppings. ## Which disease is disseminated histoplasmosis most often confused with in advanced HIV? Disseminated tuberculosis: both cause fever, weight loss and multi-organ involvement. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/histoplasma-capsulatum/) ## Related - [Blastomyces dermatitidis](https://durgical.com/en/microbiology/blastomyces-dermatitidis/) - [Paracoccidioides brasiliensis](https://durgical.com/en/microbiology/paracoccidioides-brasiliensis/) - [Cryptococcus neoformans](https://durgical.com/en/microbiology/cryptococcus-neoformans/) ## Related conditions - [Histoplasmosis](https://durgical.com/en/conditions/histoplasmosis/) - [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/) ## What the colours mean **Gram-positive** Thick peptidoglycan wall: stains purple. **Gram-negative** Thin wall with an outer membrane: stains pink. **Acid-fast** Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain. [← Back to the group: Dimorphic fungi](https://durgical.com/en/microbiology/groups/dimorphic-fungi/) Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given. Sources: [About Histoplasmosis](https://www.cdc.gov/histoplasmosis/about/index.html) (CDC, 2024); [Clinical Overview of Histoplasmosis](https://www.cdc.gov/histoplasmosis/hcp/clinical-overview/index.html) (CDC, 2024); [Early HIV diagnosis could reduce AIDS-related deaths in Latin America and the Caribbean: PAHO](https://www.paho.org/en/news/28-11-2025-early-hiv-diagnosis-could-reduce-aids-related-deaths-latin-america-and-caribbean) (PAHO/WHO, 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. 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