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Invasive fungal infections are associated with significant morbidity and mortality, occurring most frequently in immunocompromised patients. Advances in medical technology, including organ and bone marrow transplantation (BMT), cytotoxic chemotherapy, the widespread use of indwelling intravenous (IV) catheters, and the increased use of broad-spectrum antibiotics have contributed to the increase of fungal infections.

Fungi are eukaryotic organisms that exist in two basic forms, yeasts and molds. Figure 29-1 displays how pathogenic fungi can be grouped based on their morphological characteristics. Yeasts are unicellular in nature whereas molds are filamentous. Some fungal organisms exist in both forms and are referred to as dimorphic fungi. Dimorphic fungi exist as molds in the environment and convert to parasitic yeast in the human body. Common dimorphic fungi that cause invasive infection are Blastomyces dermatitidis, Histoplasma capsulatum, and Coccidioides immitis. Exposure to these organisms occurs in certain regions of the country and as such is termed endemic fungi.


Morphologically, pathogenic fungi can be grouped as either filamentous molds or unicellular yeasts. Molds grow as multicellular branching, thread-like filaments (hyphae) that are either septate (divided by transverse walls) or coenocytic (multinucleate without cross walls).

In human infection, the most common invasive fungal infections are caused by yeasts (Candida species). Examples of Candida species include C. albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. Auris, and C. krusei. Candida are normal inhabitants of mucocutaneous surfaces of the human body and frequently colonize the female genital tract, gastrointestinal (GI) tract, and skin. These organisms cause infections if they overwhelm host defenses and invade sterile areas. A classic example is seen with a proliferation in the number of Candida in the GI tract after treatment with broad-spectrum antibiotics. Common invasive infections include bloodstream infections (candidemia) and/or invasive candidiasis (eg, peritonitis and hepatosplenic). Risk factors for invasive candidiasis include neutropenia, diabetes, immunodeficiency diseases, high-dose corticosteroids, immunosuppressants, antineoplastic agents, total parenteral nutrition, antimicrobials, surgery, and burns.

Cryptococcus neoformans is typically an encapsulated yeast found in soil or bird excrement that causes invasive infections less frequently than Candida. C. neoformans most often causes meningitis in immunocompromised patients.

Molds cause fewer infections than yeasts because they affect patients with severely suppressed immune systems. The most common molds that cause clinical infections are the Aspergillus species. Aspergillus is a ubiquitous mold that grows well on a variety of substrates, including soil, water, decaying vegetation, and organic debris. Common species causing aspergillosis infections are: (1) A. fumigatus, (2) A. flavus, and (3) A. niger. The term aspergillosis may be broadly defined as a spectrum of diseases attributed to allergy, colonization, or tissue invasion. Invasive infections with Aspergillus are associated with a high mortality.


Invasive fungal infections ...

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