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Fungi previously considered to be harmless colonizers, including Pseudallescheria boydii (Scedosporium apiospermum), Scedosporium prolificans, Fusarium, Paecilomyces, Trichoderma longibrachiatium, and Trichosporon, are now significant pathogens in immunocompromised patients. This occurs most often in patients being treated for hematopoietic malignancies and in those receiving broad-spectrum antifungal prophylaxis. Infection may be localized in the skin, lungs, or sinuses, or widespread disease may appear with lesions in multiple organs. Fusariosis should be suspected in severely immunosuppressed persons in whom multiple, painful skin lesions develop; blood cultures are often positive. Sinus infection may cause bony erosion. Infection in subcutaneous tissues following traumatic implantation may develop as a well-circumscribed cyst or as an ulcer.

Nonpigmented septate hyphae are seen in tissue and are indistinguishable from those of Aspergillus when infections are due to S apiospermum or species of Fusarium, Paecilomyces, Penicillium, or other hyaline molds. Spores or mycetoma-like granules are rarely present in tissue. The differentiation of S apiospermum and Aspergillus is particularly important, since the former is uniformly resistant to amphotericin B but may be sensitive to azole antifungals (eg, voriconazole). Infection by any of a number of black molds is designated as phaeohyphomycosis. These black molds (eg, Exophiala, Bipolaris, Cladophialophora, Curvularia, Alternaria) are common in the environment, especially on decaying vegetation. In tissues of patients with phaeohyphomycosis, the mold is seen as black or faintly brown hyphae, yeast cells, or both. Culture on appropriate medium is needed to identify the agent. Histologic demonstration of these organisms is definitive evidence of invasive infection; positive cultures must be interpreted cautiously and not assumed to be contaminants in immunocompromised hosts.

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