[Marty]

(Mycoses)


Most agents of systemic mycoses exist as saprophytes in soil, in decaying vegetation and dung, and on keratinized animal tissues. The soil reservoir is the primary source of most infections, which can be acquired by inhalation, ingestion, or traumatic introduction of fungal elements.


Pathogenic fungi establish infection in apparently normal hosts, and such diseases as histoplasmosis, coccidioidomycosis, and blastomycosis are regarded as primary systemic mycoses. Opportunistic fungi usually require a host that is debilitated or immunosuppressed (eg, by such stresses as captivity, metabolic acidosis, malnutrition, viral infections, or neoplasia) 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, mucormycosis, cryptococcosis, and candidiasis, which may be focal or systemic. Cryptococcosis has been recorded in apparently healthy animals, possibly due to some subtle host defect (possibly in cell-mediated immunity).


Clinical findings and gross lesions are not definitively diagnostic of systemic mycoses; microscopical identification or culture of the organism, or both, are required. Identification of the fungus and the tissue reaction via microscopical examination of exudates and biopsy material is adequate for diagnosis of histoplasmosis, cryptococcosis, blastomycosis, coccidioidomycosis, and rhinosporidiosis. Other diseases, such as candidiasis, aspergillosis, and mucormycosis, require both cultural isolation and microscopical evaluation for a definitive diagnosis. 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 histoplasmosis, blastomycosis, cryptococcosis, and coccidioidomycosis.