Histoplasmosis other diagnostic studies

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Aravind Kuchkuntla, M.B.B.S[2]

The gold standard for diagnosis of histoplasma infection is direct visualization of the oviod yeast cells in the tissue samples and body fluid specimen. Antigen testing is an useful method for rapid diagnosis of histoplasma infection.

• The gold standard for diagnosis of histoplasma infection is direct visualization of the oviod yeast cells in the tissue samples and body fluid specimen.^[1]
• Histoplasma capsulatum is characterized by a budding yeast connected with a narrow base and is mostly identified within the macrophages and monocytes.^[2]
• In immunocompetent people, immune response by the macrophages results in the formation of a granuloma and the yeast forms are demonstrated within the histiocytes in the granuloma. However in patients with disseminated infection the yeast forms can be demonstrated in the histiocytes scattered throughout the organ and are not confined to the granulomas alone.
• Different stains such as the gram stain, Giemsa stain, Hematoxylin-eosin stain, Mucicarmine stain, PAS stain and Wright-Giemsa stain are useful for demonstration of the granulomas and the yeast forms in the tissue specimen or body fluid samples.

• Culture is positive 50% to 85% patients with disseminated histoplasmosis and chronic pulmonary histoplasmosis.^[2]
• Culture takes 4 weeks to grow and the mold appears as a suede-like or cottony mycelium. Microscopic examination of the mold demonstrate characteristic large, rounded tuberculate macroconidia.

• Antigen testing is an useful method for rapid diagnosis of histoplasma infection. Galactomannan antigen can be detected in the blood or urine samples and is positive in majority of patients with acute pulmonary histoplasmosis and disseminated histoplasmosis.^[3]
• Histoplasma antigen detection in urine and/or serum is the most widely used and most sensitive method for diagnosing acute histoplasmosis.

• Antibody tests: Because development of antibodies to Histoplasma can take two to six weeks, antibody tests are not as useful as antigen detection tests in diagnosing acute histoplasmosis or in immunosuppressed persons, who may not mount a strong immune response.
  • Immunodiffusion (ID): It can help differentiate between chronic or severe acute infection.
  • Complement Fixation (CF): Complement-fixing antibodies may take up to 6 weeks to appear after infection. Complement fixation is more sensitive but less specific than immunodiffusion.

• Polymerase chain reaction can help in the diagnosis of histoplasma infection, but the role is uncertain as the results are not consistent with the infection status.^[4][5]

• 
  Magnified 125X, “digested”, then stained using a fluorescent antibody-staining technique, this photomicrograph reveals the presence of Histoplasma capsulatum antigens in this human lung tissue specimen. From Public Health Image Library (PHIL). ^[6]
• 
  Magnified 125X, “digested”, then stained using a fluorescent antibody-staining technique, this photomicrograph reveals the presence of Histoplasma capsulatum antigens in this human lung tissue specimen. From Public Health Image Library (PHIL). ^[6]
• 
  Magnified 1250x, this H&E-stained liver tissue specimen reveals the presence of yeast-staged Histoplasma capsulatum fungal organisms inside a number of phagocytic macrophages. From Public Health Image Library (PHIL). ^[6]
• 
  This Giemsa-stained photomicrograph reveals a histiocyte within which numerous Histoplasma capsulatum fungal organisms in their yeast-stage of development were contained. From Public Health Image Library (PHIL). ^[6]
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  This photomicrograph reveals the presence of Histoplasma capsulatum fungal organisms in any unknown specimen, which were in the organism’s yeast phase, displaying numerous microconidia. From Public Health Image Library (PHIL). ^[6]