Fungal Tissue Responses
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Direct answer
Broad, aseptate hyphae branching at right angles in the necrotic turbinate of a patient in diabetic ketoacidosis is mucormycosis; narrow, septate hyphae branching at acute angles invading a lung vessel is Aspergillus; budding yeasts with pseudohyphae are Candida; and a single encapsulated yeast on India ink preparation is Cryptococcus. Fungal morphology in tissue is organism-specific enough that histology often guides treatment days before culture, which matters because these infections kill immunosuppressed patients quickly. India's experience of the post-COVID mucormycosis wave made fungal tissue responses a mainstream examination topic rather than a specialist footnote.
What you must remember
- Candida: the most common fungal infection overall — oral thrush, vaginal and oesophageal disease, and disseminated infection from neutropenia or lines; budding yeasts with pseudohyphae, visible on periodic acid-Schiff or silver stains.
- Aspergillus: septate hyphae branching at acute angles; allergic bronchopulmonary aspergillosis in asthmatics, aspergilloma in old cavities, and angioinvasive disease in neutropenia with haemoptysis, halo sign and air-crescent sign on imaging.
- Mucorales: broad, ribbon-like, aseptate hyphae branching at right angles; rhinocerebral disease in diabetic ketoacidosis and steroid-treated patients; treated with aggressive surgical debridement plus amphotericin — antifungals alone do not clear devitalised tissue.
- Cryptococcus: encapsulated budding yeast, 5–10 micrometres; India ink on cerebrospinal fluid shows unstained halos, mucicarmine stains the capsule; brain lesions have a soap-bubble appearance; the classic meningitis of acquired immunodeficiency syndrome.
- Dimorphic fungi: mould in the environment, yeast in tissue at 37°C — Histoplasma capsulatum lives inside macrophages and is reported from the Gangetic plain of India; Coccidioides shows spherules packed with endospores.
- Stains and settings: Grocott methenamine silver and periodic acid-Schiff on biopsy; Sabouraud agar in culture; the dictum that tissue invasion, not colonisation, is proven only when hyphae invade viable tissue.
- Aflatoxin: Aspergillus flavus contaminant of stored groundnuts and maize, a well-documented dietary hepatocarcinogen in parts of Africa and Asia, acting with hepatitis B to multiply liver cancer risk.
The diabetic with facial pain
A 48-year-old man presents with periorbital swelling, facial pain and a black crust on the nasal turbinate, three days into poorly controlled diabetic ketoacidosis. The clinical picture is rhinocerebral mucormycosis until proven otherwise, and the sequence that follows is fixed: urgent endoscopic debridement with frozen-section histology, amphotericin started the same day, and simultaneous correction of the ketoacidosis, because acidosis disables the transferrin-bound iron sequestration that normally restrains the fungus — the reason iron-overloaded and deferoxamine-treated patients are also susceptible. The biopsy shows the broad aseptate ribbons branching at right angles, often pauciseptate and folded so that a naive eye miscounts them as Aspergillus.
The counter-image is the neutropenic haematology patient with a pleural-based nodule and a halo of ground-glass around it on computed tomography: septate, acute-angle hyphae invading a pulmonary artery, thrombosing it and producing wedge-shaped infarction. The angle is not pedantry — voriconazole treats Aspergillus but Mucorales are intrinsically resistant to it, so the histology reading changes the drug. India's 2021 experience welded this lesson into practice: COVID-19 with high-dose steroids and diabetes produced the world's largest reported mucormycosis surge, and the pathology report often arrived as the decisive piece of management information.
Where students slip
Angle and septation are recited correctly and then abandoned at the first wet smear; practise reading them as a drug decision — acute angle, septate, voriconazole; right angle, aseptate, amphotericin and surgery. Second, Candida on a swab is treated as disease; it is a commensal, and true oesophageal or disseminated candidiasis requires compatible endoscopic or histological invasion. Third, the India ink test is written up as a cryptococcal stain; the ink does not stain anything — the capsule excludes the ink and appears as a clear halo around the budding yeast, a distinction vivas enjoy.
Frequently asked questions
Which fungus causes rhinocerebral disease in diabetic ketoacidosis and what is its morphology?
Mucor and related Mucorales — broad, aseptate hyphae branching at right angles, requiring surgical debridement with amphotericin.
How are Aspergillus hyphae identified in tissue?
Septate hyphae of uniform width branching at acute angles, frequently invading and thrombosing blood vessels in angioinvasive disease.
Why does India ink show a halo around Cryptococcus?
The polysaccharide capsule excludes the ink, so the budding yeast is surrounded by a clear unstained halo; mucicarmine and the cryptococcal antigen test confirm.
Which dimorphic fungus is reported from the Gangetic plain of India?
Histoplasma capsulatum, an intracellular yeast of macrophages in tissue, acquired from bat and bird guano-enriched soil.
Which stains demonstrate fungi on histology?
Grocott methenamine silver and periodic acid-Schiff stains; culture on Sabouraud agar remains the reference for speciation.
Why did mucormycosis surge after COVID-19 in India?
Uncontrolled diabetes, high-dose corticosteroids and COVID-19 itself combined to disable innate immunity and iron sequestration, producing the world's largest reported wave of post-COVID mucormycosis.