Histoplasmosis pathophysiology

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

Overview

Histoplasmosis is typically acquired via inhalation of airborne microconidia, often after disturbance of contaminated material in the soil. In majority of the patients the infection is asymtomatic and resolves with host's immune response. In few patients inhalation of large amount of inoculum can result in an acute pulmonary infection with symptoms resembling pneumonia. In patients with immunosuppression, they are unable to mount an adequate T-cell mediated immune response resulting in uncontrolled growth of the organism with spread to the surrounding tissue and increasing the morbidity and mortality of the infection.

Pathophysiology

Reservior

• Soil is the reservior for histoplasma microconidia, particularly when heavily contaminated with bird or bat droppings.

Transmission

• The areas contaminated with histoplasma microconidia are called microfoci and disturbance of these microfoci will result in exposure to them.
• The activities which expose the patient to histoplasma microconidia include farming, exposure to chicken coops or caves and sites where black birds have roosted.
• Histoplasmosis is typically acquired via inhalation of airborne microconidia, often after disturbance of contaminated material in the soil.

Pathogenesis

• In majority of the patients the infection is asymtomatic and resolves with host's immune response. In few patients inhalation of large amount of inoculum can result in an acute pulmonary infection with symptoms resembling pneumonia.
• The cell mediated immune response is by the T-lymphocytes which recognize the organism and induce the release of cytokines such as tumor necrosis factor alpha and interferon gamma providing protection aganist re-infection.
• The release of cytokines activates macrophages, inhibiting the growth of the fungus and limit its spread to the surrounding tissue. This results in the formation of a granuloma where in the fungus is present in a nonviable state for life.
• In patients with immunosuppression, they are unable to mount an adequate T-cell mediated immune response resulting in uncontrolled growth of the organism with spread to the surrounding tissue and increasing the morbidity and mortality of the infection.
• Primary cutaneous histoplasmosis and solid organ donor-derived histoplasmosis have been observed although extremely uncommon.

Microscopic pathology

• Histoplasma capsulatum is characterized by a budding yeast connected with a narrow base and is mostly identified within the macrophages and monocytes.
• 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.