Opportunistic Fungal Infections

Intro & Risks - Unwelcome Guests

• Opportunistic Fungal Infections (OFIs): Caused by low-virulence fungi (commensals, environmental) becoming pathogenic in hosts with impaired immunity or breached defenses.
• Key Risk Factors:
  • Immunosuppression: HIV (CD4 < 200/µL), corticosteroids, chemotherapy, transplant recipients.
  • Systemic Disease: Diabetes (esp. DKA), malnutrition, chronic organ failure.
  • Barrier Disruption: Burns, trauma, indwelling catheters, broad-spectrum antibiotics.
  • Neutropenia: Severe (< 500/µL neutrophils), prolonged.

⭐ Candida spp. and Aspergillus spp. are the most common opportunistic fungal pathogens worldwide.

Major Players 1 - Candida & Aspergillus

• Candida species (e.g., C. albicans)
  • Dimorphic: Yeast & pseudohyphae/hyphae in tissue.
  • Normal flora; opportunistic.
  • Risk: Immunosuppression (HIV, DM, steroids), antibiotics, catheters.
  • Cutaneous:
    • Intertrigo (axillae, groin): Erythematous plaques, satellite pustules.
    • Onychomycosis, paronychia.
    • Chronic Mucocutaneous Candidiasis (CMC).
  • Dx: KOH (budding yeasts, pseudohyphae), culture (SDA).
  • Rx: Topical (clotrimazole, nystatin); Systemic (fluconazole).
• Aspergillus species (e.g., A. fumigatus)
  • Mold: Septate hyphae, acute angle (45°) branching.
  • Risk: Profound neutropenia, corticosteroids, CGD.
  • Cutaneous (often 2° to systemic/primary inoculation):
    • Papules/pustules → necrotic ulcers, black eschar.
    • Otomycosis (A. niger).
  • Dx: Biopsy (histopathology), culture (SDA), galactomannan.
  • Rx: Voriconazole (invasive DOC), Amphotericin B, debridement.

⭐ Candida albicans forms germ tubes at 37°C in serum, a rapid identification test.

Major Players 2 - Zygomycetes & Crypto

• Zygomycetes (Mucormycosis)

  • Agents: Rhizopus (most common), Mucor, Lichtheimia.
  • Risks: Diabetes (esp. DKA), neutropenia, iron overload (Deferoxamine 📌 "Fe-eds fungi").
  • Clinical: Rhinocerebral (most common; black necrotic eschar), pulmonary, cutaneous.
  • Patho: Angioinvasion → thrombosis & tissue necrosis.
  • Dx: Biopsy: broad, pauciseptate (aseptate) hyphae, wide-angle (90°) branching.
  • Rx: Aggressive surgical debridement + IV Liposomal Amphotericin B. Step-down to Posaconazole.

• Cryptococcosis

  • Agents: Cryptococcus neoformans (soil, pigeon droppings), C. gattii (eucalyptus trees).
  • Risks: HIV/AIDS (CD4 < 100 cells/µL), organ transplant, long-term steroids.
  • Clinical:
    • Meningoencephalitis: Most common; headache, fever, altered mental status.
    • Pulmonary: Asymptomatic to pneumonia.
    • Cutaneous: Papules, pustules, nodules, often umbilicated (molluscum-like).
  • Dx:
    • CSF: India ink stain (capsular halos).
    • Cryptococcal Antigen (CrAg) test: Highly sensitive & specific (CSF, serum).
    • Culture: Sabouraud Dextrose Agar (SDA).
  • Rx:
    • Induction: Amphotericin B + Flucytosine.
    • Consolidation & Maintenance: Fluconazole.

  ⭐ In HIV patients, initiation of ART is often deferred for 2-4 weeks after starting antifungal therapy for cryptococcal meningitis to avoid IRIS (Immune Reconstitution Inflammatory Syndrome).

Rarer Foes & Tx - Niche Invaders & Fightback

• Key Infections & Features:
  • Chromoblastomycosis: Fonsecaea. Chronic verrucous plaques. Histo: Sclerotic bodies (Medlar/"copper pennies"). Tx: Itraconazole, Terbinafine, heat.
  • Eumycetoma: Madurella. Tumefaction, sinuses, coloured grains. Tx: Surgery + Itraconazole.
  • Phaeohyphomycosis: Dematiaceous fungi (Exophiala). Subcutaneous cysts/plaques. Tx: Excision, Itraconazole.
  • Hyalohyphomycosis: Hyaline fungi (Fusarium). Necrotic lesions (immunocompromised). Tx: Voriconazole, Ampho B.
  • Cutaneous Zygomycosis: Rhizopus/Mucor. Rapidly progressive necrotic eschars (diabetics). Tx: Aggressive debridement + Ampho B.
• Diagnosis Overview:
  • Deep biopsy (histopathology: H&E, PAS, GMS), fungal culture.
  • Direct microscopy (KOH: grains/hyphae).
• Treatment Principles:
  • Combination: surgical debridement + prolonged systemic antifungals (species-specific).

⭐ Sclerotic bodies (Medlar bodies, "copper pennies") on histology are pathognomonic for Chromoblastomycosis.

High‑Yield Points - ⚡ Biggest Takeaways

• Immunocompromised states (HIV, uncontrolled diabetes, immunosuppression) are key risk factors.
• Cutaneous cryptococcosis often mimics molluscum contagiosum or shows ulceronodular lesions.
• Mucormycosis (rhino-orbital-cerebral) is critical in DKA, with black necrotic eschars.
• Disseminated histoplasmosis in AIDS causes umbilicated skin papules.
• Talaromycosis (Penicilliosis) presents with necrotic umbilicated papules, especially in AIDS.
• Cutaneous aspergillosis can show necrotic ulcers or nodules, often at IV sites.
• Biopsy with fungal stains and culture is essential for diagnosis.