Dematiaceous fungi

Introduction

• Dematiaceous fungi are a heterogeneous group of melanized fungi characterized by the presence of dark, pigmented hyphae and spores due to the production of melanin in their cell walls.
• These fungi cause various infections, collectively termed phaeohyphomycosis, which range from superficial to deep, invasive, and disseminated diseases.
• Pigmentation: The dark color (brown to black) is due to the deposition of dihydroxynaphthalene melanin. This pigment contributes to the fungi’s pathogenicity by enhancing resistance to oxidative stress and immune defenses.
• Habitat: Widely distributed in nature, especially in soil, decaying organic matter, and plant debris.

 

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Epidemiology

1. 1. Global Distribution:
      • Found worldwide, particularly in tropical and subtropical regions.
      • Infections are more common in areas with warm, humid climates.
   2. Reservoir:
      • Soil, decaying wood, plant material, and water.
   3. Mode of Transmission:
      • Entry into the host occurs via:
        • Traumatic inoculation (e.g., splinters, thorns, or cuts).
        • Inhalation of fungal spores, leading to pulmonary or systemic infections.
      • No person-to-person transmission.
   4. At-Risk Populations:
      • Immunocompetent individuals (especially with localized cutaneous infections).
      • Immunocompromised patients (HIV/AIDS, organ transplant recipients, or those on immunosuppressive therapy) are at risk of severe invasive disease.
   5. Prevalence of Specific Infections:
      • Subcutaneous infections: Common in rural agricultural workers.
      • Invasive infections: More common in immunosuppressed individuals.

 

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Pathogenesis

The pathogenesis of dematiaceous fungi is primarily driven by their ability to evade host immune defenses and establish infection.

1. Traumatic Inoculation or Inhalation:
   • • Infections begin with introducing fungal elements (conidia or hyphae) into the host via wounds or the respiratory tract.
2. Melanin Production:
   • • Melanin in fungal cell walls provides resistance to:
       • Oxidative stress (produced by macrophages and neutrophils).
       • Antifungal agents.
3. Immune Evasion:
   • • Dematiaceous fungi resist phagocytosis by immune cells.
     • Chronic infections may lead to granuloma formation.
4. Invasive Growth:
   • • In immunocompromised patients, fungi may invade blood vessels, leading to systemic dissemination (e.g., brain, heart, or other organs).
5. Host Response:
   • • Immunocompetent hosts typically mount a granulomatous inflammatory response containing the infection.
     • Immunosuppression can lead to widespread tissue necrosis and dissemination.

 

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Clinical Manifestations

The spectrum of diseases caused by dematiaceous fungi depends on the host’s immune status and infection site.

1. Superficial and Cutaneous Infections:

• • Tinea nigra: A superficial fungal infection presenting as dark macules on the palms or soles.
  • Black piedra: A superficial hair infection with black nodules along hair shafts.

2. Subcutaneous Infections:

• • Chromoblastomycosis:
    • Chronic, progressive subcutaneous infection caused by traumatic inoculation.
    • Presents as verrucous (wart-like) lesions, plaques, or nodules.
    • Common in rural, tropical regions.
  • Eumycetoma:
    • A chronic granulomatous infection involving skin, subcutaneous tissue, and sometimes bone.
    • Presents with draining sinuses and discharge of black fungal grains.

3. Phaeohyphomycosis:

• • A broad term encompassing infections caused by dematiaceous fungi.
  • Includes:
    • Localized infections: Cutaneous, subcutaneous abscesses.
    • Invasive infections: Sinusitis, pulmonary infections, brain abscesses, or disseminated disease.

4. Central Nervous System (CNS) Infections:

• • Occur via hematogenous spread or direct extension (e.g., from paranasal sinuses).
  • Manifestations:
    • Brain abscess.
    • Meningitis.

5. Disseminated Infections:

• • Seen in severely immunosuppressed individuals.
  • Multisystem involvement with poor prognosis.

 

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Laboratory Diagnosis

Accurate diagnosis relies on clinical presentation, microscopy, culture, histopathology, and molecular techniques.

1. Microscopy

• • Specimens: Skin scrapings, tissue biopsies, sputum, or cerebrospinal fluid (CSF).
  • Direct Examination:
    • Use 10–20% KOH preparation or stains like Gomori methenamine silver (GMS) or Periodic acid-Schiff (PAS).
    • Findings:
      • Darkly pigmented, septate hyphae.
      • Yeast-like cells may also be observed in some infections.

2. Culture

• • Specimens: Tissue biopsies, pus, or respiratory specimens.
  • Growth Conditions:
    • Media: Sabouraud dextrose agar or potato dextrose agar.
    • Temperature: Grows at 25°C (mold phase) and 37°C (yeast phase, in some species).
  • Colony Morphology:
    • Colonies are slow-growing and darkly pigmented (gray, brown, or black).

3. Histopathology

• • Specimens: Biopsied tissue.
  • Stains: H&E, GMS, or PAS.
  • Findings:
    • Pigmented hyphae or yeast-like cells in tissue.
    • Granulomatous or necrotizing inflammation.

4. Molecular Diagnostics

• • PCR-based assays: Used to detect fungal DNA in clinical specimens.
  • Offers rapid and specific identification.

5. Serology

• • Rarely used, as serologic tests for dematiaceous fungi are not widely available or standardized.

6. Imaging

• • Used for invasive infections (e.g., brain abscesses or pulmonary involvement).
  • CT or MRI may reveal abscesses, cavities, or sinus involvement.

Common Dematiaceous Fungi

• • Exophiala spp.
  • Cladophialophora spp.
  • Fonsecaea spp.
  • Bipolaris spp.
  • Alternaria spp.
  • Curvularia spp.