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School of Molecular & Biomedical Science
The University of Adelaide
AUSTRALIA 5005

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Dr David Ellis
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Histoplasma capsulatum

H. capsulatum exhibits thermal dimorphism by growing in living tissue or in culture at 37C as a budding yeast-like fungus or in soil or culture at temperatures below 30C as a mould.

On Sabouraud's dextrose agar at 25C, colonies are slow growing, white or buff-brown, suede-like to cottony with a pale yellow-brown reverse. Other colony types are glabrous or verrucose, and a red pigmented strain has been noted. Microscopic morphology shows the presence of characteristic large (8-14 um in diameter), rounded, single-celled, tuberculate macroconidia formed on short, hyaline, undifferentiated conidiophores. Microconidia, if present, are small (2-4 um in diameter), round to pyriform and borne on short branches or directly on the sides of the hyphae.

On brain heart infusion (BHI) blood agar incubated at 37C, colonies are smooth, moist, white and yeast-like. Microscopically, numerous small round to oval budding yeast-like cells, 3-4 x 2-3 um in size are observed.

Three varieties of H. capsulatum are recognized, depending on the clinical disease: var. capsulatum is the common histoplasmosis, var. duboisii is the African type and var. farciminosum causes lymphangitis in horses. Histoplasma isolates may also resemble species of Sepedonium and Chrysosporium. Traditionally, positive identification required conversion of the mould form to the yeast phase by growth at 37C on enriched media, however culture identification by the exoantigen test is now the method of choice.

Culture of Histoplasma capsulatum
Culture of Histoplasma capsulatum.

rounded tuberculate macroconidia
Large, rounded, single-celled, tuberculate macroconidia and small microconidia of H. capsulatum.

WARNING: RG-3 Organism.  Cultures of H. capsulatum represent a severe biohazard to laboratory personnel and must be handled with extreme caution in an appropriate pathogen handling cabinet.

 

MIC data is limited.  Antifungal susceptibility testing of individual strains is recommended.

Antifungal MIC ug/mL Antifungal
MIC ug/mL
Range
MIC90
Range
MIC90
Fluconazole
0.125-64
16
Amphotericin B
0.03-2
0.5-1
Itraconazole
0.03-8
0.06 (1)
Posaconazole
0.25-1
0.25 (2)
Voriconazole
0.03-2
0.25 (1)
     

 

Clinical significance:

Histoplasmosis is an intracellular mycotic infection of the reticuloendothelial system caused by the inhalation of conidia from the fungus Histoplasma capsulatum. Approximately 95% of cases of histoplasmosis are inapparent, subclinical or benign. Five percent of the cases have chronic progressive lung disease, chronic cutaneous or systemic disease or an acute fulminating fatal systemic disease. All stages of this disease may mimic tuberculosis.

Histoplasma capsulatum has a world wide distribution, however, the Mississippi-Ohio River Valley in the U.S.A. is recognized as a major endemic region. Environmental isolations of the fungus have been made from soil enriched with excreta from chicken, starlings and bats.

Mycosis: Histoplasmosis

Further reading:

Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.