HISTOPLASMA

SPECIES - DIMORPHIC FUNGUS

Histoplasma capsulatum var. capsulatum
Histoplasma capsulatum var. duboisii

MICROSCOPIC APPEARANCE

at 25 degrees C.:
Hyphae septate, hyaline, often sterile. Macroconidia (macroaleurioconidia) unicellular, hyaline, thick-walled, smooth or warty. Microconidia (microaleurioconidia) unicellular, hyaline, with wall smooth or rough.

at 37 degrees C. on a rich medium:
Budding yeasts.

MACROSCOPIC APPEARANCE

at 25 degrees C.:
Surface: Texture wooly to granular; White becoming brownish on surface.
Reverse: Yellowish.
Growth Rate: Rapid to very rapid growth.

at 37 degrees C. on rich medium or in infected tissues:
Surface: Texture creamy; Cream colored on surface.
Reverse: Cream.
Growth Rate: Slow growth.

DISTINGUISHING FEATURES

Histoplasma capsulatum is a pathogenic fungus which should only be manipulated in culture in a biological safety cabinet in a containment laboratory. It is distinguished from Chrysosporium by its production of tuberculate macroconidia, and from Sepedonium by the production of microaleurioconidia. Additionally, it converts to a yeast phase at 35 degrees C. on rich media. The transition from the filamentous phase to the yeast phase may sometimes require several weeks of incubation. To speed the identification process and to reduce the risk of laboratory-acquired infection, most mycology laboratories today prefer to identify the organism by specific exoantigen testing or nucelic acid hybridization studies. H. capsulatum encompasses two varieties: the variety capsulatum , the more widespread of the two, which produces small yeasts 2-4um in length, and the variety duboisti , present primarily in Africa, which characteristically produces much larger yeasts measuring 12-15um.

HABITAT

Mostly isolated from nitrogen-rich soils contaminated by excrement of birds and bats. Despite its world-wide distribution, the organism is often encountered in tropical or subtropical regions, as well as in several large river basins in temperate regions. The valleys of the Mississippi and the Ohio rivers in the United States are known for their high degree of endemicity.

PATHOGENICITY

Histoplasma capsulatum is the etiologic agent of histoplasmosis in man and other mammals, an infection most often presenting in a benign pulmonary form, but occasionally progressing to a life-threatening, disseminated form particularly affecting the reticuloendothelial system. Histoplasma capsulatum var. duboisii causes a clinically distinct disease, African histoplasmosis, in which large yeast cells with thicker walls are found in tissues, in contrast to the small yeast cells of Histoplasma capsulatum var. farciminosum , which causes epizootic lymphangitis.

RECOMMENDED MEDIA

Mycelial phase:
Potato Dextrose Agar
Sabouraud Dextrose Agar

Incubate aerobically at 25 degrees C. for 15-20 days.

Yeast phase:
Brain Heart Infusion Agar

Incubate aerobically at 35 degrees C. for 10-14 days.

REFERENCES

1. Hensyl, William R., et al. 1990. Stedman's Medical Dictionary , 25th ed. Williams & Wilkins, Baltimore, MD.

2. St. Germain, Guy and Summerbell, Richard, Ph.D. 1996. Identifying Filamentous Fungi , 1st ed. Star Publishing Company, Belmont, CA.

3. Larone, Davise, H. 1995. Medically Important Fungi , A Guide to Identification . 3rd ed. American Society for Microbiology Press, Washington, D.C.

4. The Oxoid Vade-Mecum of Microbiology . 1993. Unipath Ltd., Basingstoke, UK.


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