Histoplasma capsulatum is a true pathogenic, thermal dimorphic fungus, existing in the form of mycelia at 25˚C and in the form of yeast at 37˚C (body temperature). Histoplasma capsulatum var. capsulatum causes the disease most commonly labeled Histoplasmosis mentioned below. Its teleomorph name is Ajellomyces capsulatus,⁸, although the teleomorph is seldom seen.

<img src=“http://botit.botany.wisc.edu/...; align=right alt=“Histoplasma capsulatum at 25 C and 37 C”>

Histoplasma capsulatum at http://botit.botany.wisc.edu/toms_fungi/jan2000.html

Histoplasma capsulatum can be found throughout the world, but is predominant in the United States, specifically found in states of the eastern and Midwest area of the country. Certain regions with a higher concentration of the species include those with higher average temperatures (68-90˚F), and a high humidity (>60%), particularly the region of the Ohio Mississippi Valley.² It may often be found where starlings congregate.

<img src=“http://botit.botany.wisc.edu/...; align=left alt=“starling on top of a sign warning about Histoplasma capsulatum”>

Macroconidia and microconidia, present in the mould form, are both unicellular, round, and hyaline. Macroconidia are large (8-16 um diameter), thick-walled, and often have prevalent fingerlike spines. Microconidia are smooth or rough-walled, abundant, grow from conidiophores produced at right angles to hyphae, and are smaller in diameter (2-5 um). Hyphae are septate and hyaline.⁶ Culture color ranges from white in a fresh sample to a more brownish color as it ages.
  Microconidia are the primary elicitors of the disease. Upon inhalation, the spores germinate, and the mycelium morphs to the invasive yeast at body temperature. Yeast colonization takes place in the human body because the fungus is able to evade lysosome fusion, but continue replicating in phagosomes. At 37˚C, the yeast form reproduces by budding. Distinguishing Histoplasma capsulatum from Blastomyces dermatitidis, H. capsulatum yeast cells form a narrower base when budding.

<img src=“http://botit.botany.wisc.edu/...; align=right alt=“Histoplasma capsulatum”>

See Tom Volk’s Fungi Website: http://tomvolkfungi.net/

From a morphological standpoint, H. capsulatum is similar to a nonpathogenic parasite of mushrooms, Sepedonium and also to a nonpathogenic fungi Renispora flavissima. Sepedonium differs in the fact that the mushroom structure is slightly bigger, contains bright yellow conidia, includes no microconidia, and is not dimorphic. Renispora flavissima is a member of Chrysosporium species and differs from H. capsulatum because it is not dimorphic, includes arthroconidia, and has no macroconidia.²

When Histoplasma capsulatum elicits Histoplasmosis, there are other species that can be similar. Tuberculosis is similar with its flu-like symptoms that both cause in humans upon infection: fever, cough, chills, and general malaise. Otherwise, Cryptococcus neoformans and Chlamydia psittaci are similar because these two species also cause disease by inhalation exposure from infected manure of certain birds.

Although it can be isolated from caves, the soil is the major reservoir for Histoplasma capsulatum. Soil with a high nitrogen content and bird or bat guano are especially favorable environments. Birds do not carry the disease, but the manure acts as a nutrient source for fungi already present, while bats are able to be infected with Histoplasma capsulatum.⁵

Histoplasma capsulatum has no positive benefit use to humans, but has a great relevance to the diseases that are able to infect humans. H. capsulatum is the etiological agent of Histoplasmosis and can cause varying degrees of clinical disease. In its least severe form, acute pulmonary histoplasmosis, it is asymptomatic and the great majority (95%) of people in the endemic areas have already been inoculated with this form at some point in their lives.¹ The other gradually more severe, but extremely rare forms of the disease include: primary cutaneous histoplasmosis and disseminated histoplasmosis. Overall, the fungal species can infect healthy people, but chronic lung disease patients, the older aged, people who work with soil, and young children are more apt to infection.⁴ This is also one of the fungal AIDS associated pathogen besides Candida albicans, Cryptococcus neoformans and Blastomyces dermatitidis in the United States.⁵ Main treatments for the more severe, symptomatic forms of the disease are Amphotercin B, Ketoconazole, or Itraconazole.

Other names to describe the disease it causes, besides Histoplasmosis including: Darling’s disease, reticuloendotheliosis, Ohio Valley disease, Maria fever, and tingo.