Dimorphic Systemic Mycoses
These are fungal infections of the body caused by fungal pathogens which can overcome the physiological and cellular defences of the normal human host by changing their morphological form. They are geographically restricted and the primary site of infection is usually pulmonary, following the inhalation of conidia.
|Penicilliosis marnefffei||Penicillium marneffei||Rare*|
Histopathology is especially useful and is one of the most important ways of alerting the laboratory that they may be dealing with a potential pathogen.
Tissue morphology of dimorphic pathogens:
|Blastomycosis||Large broad base unipolar budding yeast cells (8-10um).|
|Coccidioidomycosis||Spherules (10-80um) with endospores (2-5um).|
|Histoplasmosis||Small narrow base budding yeast cells (1-5um; 5-2um in var. duboisii)|
|Paracoccidioidomycosis||Large narrow base, multi-budding yeast cells (20-60um).|
|Sporotrichosis||Small narrow base budding yeast cells (2-5um).|
WARNING: Cultures of Blastomyces dermatitidis, Coccidioides immitis and Histoplasma capsulatum represent a severe biohazard to laboratory personnel and must be handled with extreme caution in an appropriate pathogen handling cabinet.
In the past microscopic morphology, conversion from the mould form to the yeast or spherule form, and animal pathogenicity have all been used; however exoantigen tests are now the method of choice, at least for Blastomyces dermatitidis, Coccidioides immitis and Histoplasma capsulatum.