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Microbiology · 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

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

  1. In soil it grows as a mold and produces macroconidia and microconidia
  2. Microconidia become airborne when soil is disturbed and are inhaled
  3. At 37 °C in the alveolus they convert to yeasts
  4. Macrophages engulf them and carry them to lymph nodes, liver, spleen and marrow
  5. 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 →
  • In advanced HIV, disseminated histoplasmosis is AIDS-defining and a leading cause of death 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:

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

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.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: About Histoplasmosis (CDC, 2024); Clinical Overview of Histoplasmosis (CDC, 2024); Early HIV diagnosis could reduce AIDS-related deaths in Latin America and the Caribbean: PAHO (PAHO/WHO, 2025).

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Representative model, for educational purposes only.