Paper I — 2024 June (Supplementary) (2019 Scheme) — Q9
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Paper I
2024 June (Supplementary) (2019 Scheme) · 100 marks · 180 min
Question
Laboratory tests for detecting infections by dimorphic fungi
Q94 marksShort Answers
Answer
Dimorphic fungi: fungi that exist in two distinct morphological forms depending on temperature/environment — the mould (mycelial) form at room temperature (25–30°C, in the environment/culture) and the yeast form at body temperature (37°C, in host tissue) — examples include Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and Sporothrix schenckii.
Laboratory tests for detecting dimorphic fungal infections:
Direct microscopy — KOH mount or special stains (e.g., Giemsa, PAS, silver stain) of clinical specimens (sputum, tissue biopsy, bone marrow, skin scraping) to visualize the characteristic yeast form in tissue (e.g., small intracellular budding yeasts of Histoplasma capsulatum within macrophages).
Culture — on Sabouraud dextrose agar, incubated at two temperatures: 25–30°C (yielding the mould/mycelial form with characteristic macroconidia/microconidia) and 37°C (yielding the yeast form) — demonstration of this temperature-dependent dimorphism on culture is the definitive confirmatory test.
Serology — detection of specific antibody (complement fixation test, immunodiffusion) or antigen (e.g., Histoplasma urinary/serum antigen assay, useful in disseminated disease and immunocompromised patients).
Skin (delayed hypersensitivity) tests — e.g., histoplasmin/coccidioidin skin test, indicates past exposure/infection (epidemiological rather than acute diagnostic value, and can be negative in active disseminated disease due to anergy).
Histopathology — biopsy of affected tissue showing granulomatous inflammation with characteristic yeast forms on special stains (PAS, Gomori methenamine silver).
Molecular methods (PCR) — increasingly used for rapid, specific identification, particularly in culture-negative or slow-growing cases.
Culture demonstrating temperature-dependent dimorphism remains the gold-standard confirmatory laboratory test for this group of fungi.
