Histoplasma capsulatum: Properties, Diseases and Laboratory diagnosis

Histoplasma capsulatum is an intracellular, thermally dimorphic fungi  (grows as a yeast in body temperature/37°C in humans, mammals or in culture media and as mold in 25°C in environment/culture media) of medical importance that can survive within macrophages for an extended period. This fungal pathogen is associated with birds or bat droppings.

Asexual spores of H.capsulatum
A: Tuberculate macroconidia B: Microconidia

People who visit caves (researchers, spelunkers), farmer, people who work in old buildings or any renovation project are likely to acquire this disease. Contaminated soil can be infectious for many years.

Tuberculate macroconidium (with typical thick walls and radial, finger like projections) is a diagnostic structure of Histoplasma capsulatum. 

Yeast forms of Histoplasma capsulatum are small for fungi (2 to 4 μm) and reproduce by budding (blastoconidia). The mycelia are septate and produce microconidia and macroconidia (tuberculate macroconidia).

Histoplasma capsulatum in bone marrow biopsy
C.H & E stain *1000 D.PAS Stain *1000 Source: ncbi.nlm.nih.gov

Habitat:

Histoplsama capsulatum is found throughout the world. In USA, It is endemic in Midwestern and Central USA, along the Mississippi and Ohio river valley.

Transmission and Disease

Histoplasma capsulatum causes systemic mycoses named Histoplasmosis also known as darling’s disease, cave disease or spelunker’s disease.  People contract this disease by the inhalation of the spore or bird or bat droppings contaminated with spores. It causes profound disease in immune-compromised (like any other fungal infection) but immune-competent person may not know that they are harboring this pathogen.

It causes systemic fungal infection (can invade other parts of the body beside lungs) which can be asymptomatic, acute or chronic. Its acute form resembles pneumonia and chronic disease resembles tuberculosis. Immuno-compromised person can have disseminated infections.

Note: unlike its name; Histoplasma capsulatum is not encapsulated. The designation H.capsulatum is actually a misnomer

Virulence factor:

  1. Ability to survive within the macrophage probably by modulating the pH within the phagolysome is the key virulence factor of Histoplasma capsulatum. Unlike the name suggest, this pathogen does not have capsule.
  2. It produces microconidia and/or small hyphal fragments of size 2-5 μm, an ideal size for alveolar deposition.

Laboratory Diagnosis:

H. capsulatum in Giemsa stained preparation

Sample: Bone marrow, peripheral blood and sputum (organisms are less likely to be detected in the direct microscopic examination of respiratory tract specimens).

Methods:

Histoplasma capsulatum culture in SDA
Source: http://www.mycology.adelaide.edu.au/

*must be handled with extreme caution in a Class II Biological Safety Cabinet (BSCII).