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The human respiratory tract serves as the principal route of entrance for most major fungal pathogens. These fungi are classified as endemic mycoses, fungal opportunists, and emerging pathogens (Table 38–1). Fungal pneumonia poses a unique problem for clinicians in that diagnosis is often made late in the course of a patient’s illness. It is common for fungal etiologies to be considered only after a patient has failed to respond to treatment for bacterial pneumonia. The ability of clinicians to recognize and effectively treat invasive fungal infections impacts on the morbidity and mortality associated with these diseases. Enhanced knowledge of the epidemiology, clinical pattern, diagnostic methodologies, and therapy of fungal pneumonia may enable clinicians to more effectively identify and manage these types of infections.

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Table 38–1. Common and Emerging Pathogens Associated with Fungal Pneumonia.
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Essentials of Diagnosis

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  • • Endemic to Ohio and Mississippi River valley regions; the causative fungus is found in soil contaminated by bird or bat droppings.
  • • Acute infection is usually asymptomatic or flu-like; chronic cavitary pneumonia occurs in smokers with emphysema; it is a disseminated disease, especially in human immunodeficiency virus (HIV)-infected individuals.
  • • Diagnosis is by bronchoalveolar lavage (BAL) or tissue biopsy, by blood and bone marrow cultures for disseminated infection, and by urine Histoplasma antigen for severe pulmonary or disseminated disease.

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General Considerations

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Histoplasmosis, a disease caused by the dimorphic fungus Histoplasma capsulatum, is the most common mycosis occurring in the United States. H capsulatum is found predominantly in the mid- and south-central United States, especially in the Ohio and Mississippi River valley regions. Histoplasmosis also occurs in other areas of the world, including Africa, Central and South America, and the Caribbean.

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Pathogenesis

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Because H capsulatum grows well in soil enriched with bird and bat guano, exposure may occur in or around caves (spelunking), barns, bird roosts, and chicken coops. H capsulatum spores are aerosolized, especially with soil excavation, and inhaled. Once inhaled, spores are deposited into pulmonary alveoli where they revert to yeast forms. This genetically controlled switch from mold form at ambient temperature to yeast form at body temperature is called thermal dimorphism and is a property of H capsulatum and other endemic fungi. The presence of H capsulatum in the lung causes an influx of inflammatory cells (primarily lymphocytes and macrophages) that attempt to control and contain infection. Macrophages phagocytose and in turn disseminate H capsulatum throughout the reticuloendothelial system of the body during primary infection. T cell immunity develops in immunocompetent hosts ...

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