Fungos filamentosos

Histoplasma capsulatum

Dimorphic fungus: intracellular yeasts in tissue, hyphae in the environment

Identification & Growth

Identification

Filamentous phase at 25 °C with characteristic tuberculate macroconidia
Yeast phase at 37 °C: small yeasts inside macrophages (Giemsa/Grocott)

Growth Conditions

Sabouraud and BHI, 25 °C and 37 °C, for up to 4–6 weeks

Clinical Significance

Pulmonary histoplasmosis after exposure to caves, chicken coops and bat guano
Disseminated form in advanced HIV, with pancytopenia and mucocutaneous lesions

Intrinsic Resistance

Echinocandins have no activity; itraconazole and amphotericin B are the mainstay of treatment

Bench Alerts

Biosafety level 3 pathogen: handle cultures only inside a cabinet, never open the plate on the open bench
No routine susceptibility testing; diagnosis relies on antigen detection and histopathology

Clinical Notes

Clinical presentations

Self-limited acute pulmonary histoplasmosis after exposure to a high inoculum (caves, chicken coops), ranging from asymptomatic to a flu-like illness with pulmonary infiltrates.
Chronic cavitary pulmonary histoplasmosis, mimicking tuberculosis, predominantly in patients with pre-existing structural lung disease.
Progressive disseminated histoplasmosis in patients with advanced HIV infection, transplant recipients and other severely immunocompromised hosts, involving bone marrow, liver, spleen and mucosal surfaces.

Specimens and collection

Histoplasma antigen testing in urine and serum is the most sensitive test for disseminated disease and should be requested early in at-risk patients.
Blood culture using lysis-centrifugation systems and bone marrow culture increase yield in disseminated disease, but growth is slow (weeks).
Serology (double immunodiffusion and complement fixation) and tissue histopathology (silver stain) complement the diagnosis, especially in chronic forms.

Epidemiology and at-risk populations

Dimorphic fungus endemic to the Ohio and Mississippi River valleys in the US, and widely distributed across Latin America, including endemic areas in Brazil.
Associated with soil contaminated by bat and bird droppings, with typical exposure in caves, demolition sites, construction and cleaning of chicken coops.
Patients with HIV and low CD4 counts, transplant recipients and users of anti-TNF therapies have substantially higher risk of disseminated disease.

Resistance and therapeutic implications

Routine antifungal susceptibility testing is not recommended for individual clinical management, as MIC-outcome correlation is weak for dimorphic fungi.
Itraconazole is the azole of choice for mild to moderate disease, with lipid amphotericin B reserved for severe, disseminated or CNS disease, per IDSA guidance.
Monitoring of itraconazole serum levels is recommended to ensure adequate therapeutic exposure, given variable absorption and clinically relevant drug interactions.

Bench and reporting notes

Thermally dimorphic fungus: filamentous form with characteristic tuberculate microconidia at 25–30 °C and small intracellular yeast form at 35–37 °C.
Culture on Sabouraud medium may take 2 to 6 weeks to grow; molecular probes or exoantigen testing accelerate confirmation of identification.
Handling of the filamentous phase should occur in a biosafety cabinet, as conidia are infectious via inhalation.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Histoplasma capsulatum in the lab?

Histoplasma capsulatum is identified through filamentous phase at 25 °c with characteristic tuberculate macroconidia, yeast phase at 37 °c: small yeasts inside macrophages (giemsa/grocott).

What are the intrinsic resistances of Histoplasma capsulatum?

This organism is naturally resistant to echinocandins have no activity; itraconazole and amphotericin b are the mainstay of treatment. These drugs should not be reported as susceptible.

Where is Histoplasma capsulatum commonly found?

It is typically associated with pulmonary histoplasmosis after exposure to caves, chicken coops and bat guano, disseminated form in advanced hiv, with pancytopenia and mucocutaneous lesions.

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