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Fungal Infections

Species name/ common name:

Histoplasma capsulatum

Histoplasma duboisii (previously Histoplasma capsulatum var. duboisii)

Natural habitat

Bird and bat droppings;

Soil and dust.

Geography

H. capsulatum is worldwide.

  • Common cause of chronic pneumonia in the USA.
  • Particularly around Ohio River Valley and lower Mississippi River.
  • Other central and South  American countries have endemic areas of histoplasmosis
  • African Histoplasmosis common in caves in Southern and East Africa.

H. duboisii has a more limited distribution.

  • Limited to tropics of Capricorn and Cancer, as well as Madagascar.   

Frequency

Positive histoplasmin test in 90% of individuals living in high-risk areas such as the Eastern & Central US.

Diseases

Histoplasmosis (Cave disease/Caver’s disease; Darling's disease; Ohio Valley disease; Reticuloendotheliosis; Spelunker’s Lung):

·         Primary pulmonary histoplasmosis;

·         Progressive disseminated histoplasmosis;

·        Chronic cavitary pulmonary histoplasmosis

·         African histoplasmosis

Culture peculiarities

It is thermally dimorphic i.e. grows in mould form at 25°C and yeast form at 37°C. At 25°C the mould is slow-growing, grainy-to-cottony in appearance and becomes increasingly brown in colour from white over time. Yellow/yellow-orange reverse on SAB agar.

At 37°C the yeast form is creamy coloured, slow growing and it will grow on enriched media such as horse blood agar.

Antifungal resistance (intrinsic and acquired)

Susceptible to all systemic antifungal agents except the echinocandins, especially itraconazole and amphotericin B. One noted case of an AIDS patient in which H. capsulatum showed acquired resistance to fluconazole. This is due to decreased sensitivity towards ergosterol synthesis.

Biosafety level 3

This fungal species must be managed in a laboratory with safety containment level 3.

Industrial use:

None reported

Images

Histoplasma
Example of intracellular (monocyte) yeasts of H. capsulatum in disseminated histoplasmosis. Wrights stain of blood.

Histoplasma
Example of intracellular (monocyte) yeasts of H. capsulatum in disseminated histoplasmosis. Wrights stain of blood

Histoplasma
A few Histoplasma cells inside a liver macrophage (x2000)

Histoplasma

An example of Histoplasma in tissue, stained with a silver (GMS) stain.

Histoplasma

Microscopic morphology of Histoplasma capsulatum


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