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Taxonomic Classifications

Kingdom: Fungi
Phylum: Ascomycota
Subphylum: Ascomycotina
Class: Ascomycetes
Order: Onygenales
Family: Onygenaceae
Genus: Ajellomyces (Histoplasma)

Histoplasma capsulatum

Laboratory Culture @ 25C

Laboratory Culture @ 37C

Tissue/Exudate Form

The above culture drawing is courtesy of  College of Veterinary Medicine, Pathobiology

Culture of Histoplasma capsulatum.

Large, rounded, single-celled, tuberculate macroconidia and small microconidia of H. capsulatum.
 Histoplasma capsulatum on culture plates: Mycology Online

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 mold.

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 mold 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.

Description and Habitats

Histoplasma is a thermally dimorphic fungus found in nature. Soil contaminated with bird droppings or excrements of bats is the common natural habitat for Histoplasma. Although it is claimed to exist worldwide, tropical areas are where this fungus is more frequently encountered. It is endemic in the Tennessee-Ohio-Mississippi river basins


Based on the mycological information found in, the genus Histoplasma contains one species, Histoplasma capsulatum. Histoplasma capsulatum has two varieties: Histoplasma capsulatum var. capsulatum and Histoplasma capsulatum var. duboisii. It has a teleomorph referred to as Ajellomyces capsulatus.

Health Effects


Histoplasmosis is a systemic disease, mostly of the reticuloendothelial system, manifesting itself in the bone marrow, lungs, liver, and the spleen. In fact, hepatosplenomegaly is the primary sign in children, while in adults, histoplasmosis more commonly appears as pulmonary disease. This is one of the most common fungal infections, occurring frequently in South Carolina, particularly the northwestern portion of the state. 

          The ecological niche of H. capsulatum is in blackbird roosts, chicken houses and bat guano. Typically, a patient will have spread chicken manure around his garden and 3 weeks later will develop pulmonary infection. There have been several outbreaks in South Carolina where workers have cleared canebrakes which served as blackbird roosts with bulldozers. All who were exposed, workers and bystanders, contracted histoplasmosis.  Histoplasmosis is a significant occupational disease in bat caves in Mexico when workers harvest the guano for fertilizer. 

          In the endemic area the majority of patients who develop histoplasmosis (95%) are asymptomatic. The diagnosis is made from their history, serologic testing or skin test. In the patients who are clinically ill, histoplasmosis generally occurs in one of three forms: acute pulmonary, chronic pulmonary or disseminated. There is generally complete recovery from the acute pulmonary form (another "flu-like" illness). However, if untreated, the disseminated form of disease is usually fatal. Patients will first notice shortness of breath and a cough which becomes productive. The sputum may be purulent or bloody. Patients will become anorexic and lose weight. They have night sweats. This again sounds like tuberculosis, and the lung x- ray also looks like tuberculosis, but today radiologists can distinguish between these diseases on the chest film (histoplasmosis usually appears as bilateral interstitial infiltrates). 

          Histoplasmosis is prevalent primarily in the eastern U.S. In S.C., a histoplasmin skin test survey of lifetime, one county residents, white males, 17 to 21 years old, was performed on Navy recruits. The greatest number of positive skin tests appeared in the northwestern part of the state. A similar study of medical students conducted at Medical University of  South Carolina, about 25 years ago, bore the same distribution (Goodman and Ever, J.S.C.M.A. 67:53-55, 1971).The skin test is NOT used for diagnostic purposes, because it interferes with serological tests. Skin tests are used for epidemiological surveys.

          Clinical specimens sent to the lab depend on the presentation of the disease: Sputum or Bronchial alveolar lavage, if it is pulmonary disease, or Biopsy material from the diseased organ. Bone marrow is an excellent source of the fungus, which tends to grow in the reticulo-endothelial system. Peripheral blood is also a source of visualizing the organism histologically. The yeast is usually found in monocytes or in PMN's. Many times an astute medical technologist performing a white blood cell count will be the first one to make the diagnosis of histoplasmosis. In peripheral blood, H. capsulatum appears as a small yeast about 5-6 microns in diameter. (Blastomyces is 12 to 15 microns). Gastric washings are also a source of H. capsulatum as people with pulmonary disease produce sputum and frequently swallow their sputum.
(Source: Mycology Online)

Macroscopic Features

Being a thermally dimorphic fungus, Histoplasma capsulatum grows in mold form at 25C, and in yeast form at 37C. Below are the macroscopic characteristics at varying temperatures and for both varieties.

At 25C
Colonies are slow growing and granular to cottony in appearance. The color is white initially and usually becomes buff brown with age. The colonies are not sensitive to cycloheximide in the culture media. From the reverse, a yellow or yellowish orange color may be observed. While these features are best observed on Sabouraud dextrose agar (SDA), brain heart infusion agar (BHIA) enhances growth more efficiently.

At 37C
Creamy, slowly growing, moist and yeast-like colonies are formed. This phase is observed in infected tissues and in vitro on enriched media, such as BHIA containing 5-10% blood.

For definitive identification of the fungus, yeast-to-mold conversion should be demonstrated.

Microscopic Features

At 25C
Hyphae are septate and hyaline. Histoplasma capsulatum produces hyphae-like conidiophores which arise at right angles to the parent hyphae. It has both macro- and microconidia. Macroconidia are tuberculate, thick-walled, round, unicellular, hyaline, large and often have fingerlike projections on the surface. These macroconidia are also referred to as tuberculochlamydospores or macroaleurioconidia. Microconidia (microaleurioconidia) are unicellular, hyaline and round, with a smooth or rough wall.

At 37C
Narrow-based, ovoid, budding yeast cells are formed. Yeasts of var. capsulatum are smaller than (2-4 m) those of var. duboisii (12-15 m)

Laboratory Precautions

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


Data on in vitro activity of Histoplasma capsulatum are yet limited. The NCCLS antifungal susceptibility testing methods have not been standardized for testing the activity of this fungus. Amphotericin B, voriconazole, itraconazole, and posaconazole in general yield relatively low MICs for Histoplasma capsulatum. Fluconazole generally appears active, but resistance may develop. The echinocandins, caspofungin and anidulafungin have relatively higher MICs, and one in vivo study found caspofungin to have little activity.

Amphotericin B, itraconazole and fluconazole are currently used in treatment of histoplasmosis. Fluconazole is less active than itraconazoleand is a second-line agent. Ketoconazole is also a second-line drug due to the availability of safer and more efficacious alternatives.

The mycological information gathered and organized in this extensive research on the different Pathogenic Molds was  sourced out from the list of websites below: | | | | | | | | | | |


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