Mucormycosis
Mucormycosis, formerly called zygomycosis and popularly known as "black fungus," is a medical emergency: an invasive, tissue-destroying fungal infection caused by molds of the order Mucorales (most often Rhizopus oryzae, Mucor, Lichtheimia and Rhizomucor). The spores are everywhere in soil, decaying plant matter and air. The disease destroys tissue very quickly, is highly aggressive and kills more than 50% of patients if it is not treated as an emergency.
The defining feature of mucormycosis is angioinvasion: the fungus grows into blood vessels, triggers clotting and cuts off blood supply, leaving extensive black necrosis (black eschar) of the skin and mucosa. At Haute-Derma, we treat mucormycosis with maximum urgency: microbiological diagnosis within hours, wide surgical debridement and systemic antifungal therapy at full dose.
Microbiology and Pathogenesis
Ribbon-Like Hyphae With Right-Angle (90°) Branching
- Microscopic appearance: the fungus grows as very broad hyphae, 10–20 µm across, with no cross-walls (aseptate) or only a few (pauciseptate).
- Ribbon-like look: the thin walls fold, twist and collapse on the slide, so the hyphae resemble crumpled ribbons.
- Right-angle (90°) branching: side branches leave the main hypha at a right or obtuse angle. This is a distinctive feature that separates it at once from Aspergillus (narrow, septate hyphae with acute 45° branching).
Angioinvasion and Black Eschar
- Binding to the endothelium: the CotA surface protein of Rhizopus binds specifically to the GRP78 receptor on human vascular endothelial cells.
- Penetrating the vessel wall: the fungus secretes elastase and mucorpepsin and pushes directly through the intima and media of small and medium-sized arteries.
- Thrombosis and ischemic necrosis: hyphae inside the vessel lumen activate coagulation and cause complete thrombosis. Everything downstream, skin and mucosa alike, loses its blood supply, and ischemic gangrenous necrosis (black eschar) develops within a few days.
Key Risk Factors: Ketoacidosis and Free Iron
- Diabetic ketoacidosis (DKA): acidic blood weakens transferrin's grip on iron, releasing large amounts of free iron (Fe3+) into the plasma. Rhizopus uses this free iron to fuel explosive growth.
- Severe immunosuppression: neutropenia, long-term corticosteroid use or organ transplantation.
Clinical Forms Affecting the Skin and Face
Rhinocerebral Form (Nose, Eye and Brain)
A surgical emergency of the highest urgency, seen most often in people with poorly controlled diabetes.
- Early signs: fever, painful swelling of one side of the face, nasal congestion and blood-tinged nasal discharge.
- Characteristic necrosis: a black necrotic ulcer (black eschar) on the hard palate, the nasal turbinates or the skin of the cheek.
- Spread to the orbit and brain: patients rapidly develop drooping eyelids, paralysis of the eye muscles, bulging of the eye and blindness from central retinal artery occlusion. Death follows when the fungus spreads through the sphenoid sinus into the cavernous sinus and brain.
Primary Cutaneous Mucormycosis:
Develops after trauma, burns or penetrating injuries, or at sites of contaminated medical adhesive tape or IV cannulas.
- A firm, swollen red plaque quickly turns into blisters and pustules; the center then breaks down into a deep black necrotic ulcer with a raised violaceous border (gangrenous, ecthyma-like).
Secondary or Disseminated Cutaneous Form: The fungus reaches the skin through the bloodstream from the lungs or gut. Numerous necrotic nodules, hemorrhagic plaques and gangrenous areas appear scattered over the body.
Differential Diagnosis
| Feature | Mucormycosis (Mucorales) | Cutaneous Aspergillosis (Aspergillus) |
|---|---|---|
| Hyphal width | Broad (10–20 µm), ribbon-like | Narrow (3–5 µm), clearly septate |
| Branching angle | Right angle (90°) or obtuse | Acute (45°), fan-like tree pattern |
| Septa | Absent (aseptate) | Regular (septate) |
| Lesion features | Dry black eschar that spreads very fast | Milder black necrosis with a pustular rim |
| Main risk factors | Diabetes with DKA, iron overload | Neutropenia, organ transplantation |
Our Diagnostic Workup at Haute-Derma
KOH Mount With Calcofluor White Fluorescent Stain (Within 1–2 Hours):
- Technique: scrape tissue at the junction of living and dead tissue, not from the fully necrotic center.
- Findings: broad (10–20 µm), aseptate, folded ribbon-like hyphae with 90° branching can be recognized immediately.
Emergency Biopsy for Histopathology:
- Architecture: extensive tissue necrosis with neutrophilic infiltrate (or few inflammatory cells in neutropenic patients).
- H&E, GMS and PAS stains: broad hyphae are seen crossing arterial walls (angioinvasion) and lying within venous or arterial thrombi, causing ischemic necrosis.
Special Points for Culture and Imaging:
- Culture: when biopsy tissue may contain Mucorales, it must not be homogenized. Grinding breaks the aseptate hyphae, killing the fungus so it fails to grow. Instead, cut the tissue into small pieces and place them on SDA agar.
- Imaging (CT/MRI): CT or MRI of the sinuses, orbits and brain is mandatory to map the extent of disease before surgery.
Treatment Protocol at Haute-Derma
Surgical Debridement of Necrotic Tissue
- The surgeon must widely excise all black necrotic tissue until reaching healthy margins that bleed freely.
- In the rhinocerebral form, extensive sinus surgery, maxillectomy or orbital exenteration may be needed if the fungus has invaded these structures.
- Induction therapy:
- Liposomal amphotericin B (LAmB): 5–10 mg/kg/day by IV infusion, a higher dose than for other deep fungal infections because Mucorales destroy tissue so aggressively.
- Step-down / maintenance therapy:
- Once the patient is out of danger and the tissue has been fully debrided, switch to oral isavuconazole (372 mg/day) or delayed-release posaconazole tablets (300 mg/day) for several months.
- Note: voriconazole has NO activity against fungi of the order Mucorales.
