THE MERCK VETERINARY MANUAL
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Overview of Fungal Infections

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Systemic mycoses are infections with fungal organisms that exist in the environment, enter the host from a single portal of entry, and disseminate within the host usually to multiple organ systems. The soil reservoir is the primary source of most infections, which can be acquired by inhalation, ingestion, or traumatic introduction of fungal elements. (Also see Dermatophilosis.)

Pathogenic fungi establish infection in apparently normal hosts, and such diseases as histoplasmosis, coccidioidomycosis, blastomycosis, and cryptococcosis are regarded as primary systemic mycoses. Opportunistic fungi usually require a host that is debilitated or immunosuppressed to establish infection. Prolonged administration of antimicrobials or immunosuppressive agents appears to increase the likelihood of infection by the opportunistic fungi that cause diseases such as aspergillosis and candidiasis, which may be focal or systemic.

Clinical findings and gross lesions are often suggestive of systemic mycoses, but definitive diagnosis requires microscopic identification, culture of the organism, or PCR. Identification of the fungus and the tissue reaction via microscopic examination of exudates and biopsy material is adequate for diagnosis of histoplasmosis, cryptococcosis, blastomycosis, coccidioidomycosis, and rhinosporidiosis. Other diseases, such as candidiasis, aspergillosis, zygomycosis, phaeohyphomycosis, hyalohyphomycosis, and oomycosis (pythiosis and lagenidiosis), usually require more than microscopic evaluation for a definitive diagnosis. Some of these fungi are also common contaminants of cultures; thus, tissue invasion and reaction must be demonstrated for the culture isolation to be considered significant. Serology may be useful for diagnosis (and prognosis) of some mycotic diseases such as coccidioidomycosis, pythiosis, and lagenidiosis. Antigen titers have proved useful for cryptococcosis, histoplasmosis, and blastomycosis.

For treatment, see discussions of specific systemic mycoses (below) and see Systemic Pharmacotherapeutics of the Integumentary System.

Last full review/revision May 2014 by Joseph Taboada, DVM, DACVIM

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