Paper I
Question
Enumerate FOUR fungi causing systemic mycosis. Write briefly the laboratory diagnosis of any one of them.
Answer
Four fungi causing systemic mycosis: Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis, and Paracoccidioides brasiliensis (all dimorphic fungi causing endemic systemic mycoses).
Laboratory diagnosis of Histoplasma capsulatum (histoplasmosis):
- Direct microscopy — Giemsa/Wright stain of bone marrow, blood buffy coat, or tissue biopsy, showing small (2–4 μm) intracellular budding yeast forms within macrophages.
- Culture — on Sabouraud dextrose agar at two temperatures: 25–30°C yields the mould form (with characteristic large, tuberculate macroconidia) and 37°C yields the yeast form — demonstration of this temperature-dependent dimorphism confirms the diagnosis; culture, however, can take several weeks and requires biosafety precautions given infectivity of the mould-phase conidia.
- Antigen detection — Histoplasma urinary/serum/CSF antigen assay, particularly valuable in disseminated disease and in immunocompromised (e.g., HIV/AIDS) patients, offering rapid results.
- Serology — complement fixation test and immunodiffusion, detecting antibody; useful in subacute/chronic disease but may be negative in acute or severely immunocompromised cases.
- Histopathology — biopsy showing granulomatous inflammation with characteristic intracellular yeast forms on special stains (PAS, Gomori methenamine silver).
- Molecular methods (PCR) — increasingly used for rapid, specific confirmation.