Opportunistic Fungal Infections

Opportunistic Fungi - Sneaky Invaders

• Fungi causing disease primarily in immunocompromised individuals; usually harmless in healthy hosts.
• Sources: Endogenous (normal flora) or exogenous (environment).
• Major Risk Factors:
  • Neutropenia (ANC <500/μL)
  • HIV/AIDS (CD4 <200/μL)
  • Immunosuppressive drugs (corticosteroids, chemotherapy)
  • Organ transplantation
  • Broad-spectrum antibiotics
  • Indwelling catheters
  • Diabetes Mellitus

⭐ Candida albicans is the most frequent opportunistic fungal pathogen, capable of causing a wide spectrum of diseases from superficial to systemic infections.

Candida Infections - Yeastie Beasties

• Candida albicans most common; dimorphic (yeast, pseudohyphae, true hyphae).
• Forms:
  • Superficial: Oral thrush (scrapeable white plaques), vulvovaginitis (cottage cheese discharge), intertrigo.
  • Invasive: Candidemia (esp. ICU, central lines), endocarditis, disseminated (liver, spleen).
• Dx: KOH (yeast, pseudohyphae), culture (SDA), Germ tube test (C. albicans). β-D-glucan.
• Rx: Topical (nystatin, azoles); Systemic (fluconazole, echinocandins, Ampho B).
• Risk: Immunosuppression, DM, antibiotics, catheters.

⭐ Candida albicans is germ tube positive at 37°C in serum within 2-4 hours, differentiating it from most other Candida species.

Aspergillus Infections - Moldy Menace

• Ubiquitous mold; spore inhalation. Common species: A. fumigatus, A. flavus (aflatoxin).
• Clinical Forms:
  • Allergic Bronchopulmonary Aspergillosis (ABPA): Asthma, eosinophilia, IgE ↑.
  • Aspergilloma: Fungus ball in pre-existing lung cavities (e.g., TB).
  • Invasive Aspergillosis: Immunocompromised (neutropenia); angioinvasion, fever, cough.
• Diagnosis: Septate hyphae with 45° (acute) angle branching on microscopy. Galactomannan antigen test.
• Treatment: Voriconazole (drug of choice). 📌 Aspergillus = Acute Angle branching.

⭐ Aspergillus flavus produces aflatoxin, a potent hepatocarcinogen strongly linked to hepatocellular carcinoma (HCC).

Cryptococcus Infections - Capsulated Crypt-keeper

• Encapsulated yeast; C. neoformans (immunocompromised, AIDS CD4 <100/µL), C. gattii (immunocompetent).
• Source: Pigeon droppings. Pathogenesis: Inhalation → lungs → hematogenous dissemination, predilection for CNS (meningoencephalitis).
• Virulence: Polysaccharide capsule (antiphagocytic).
• Diagnosis:
  • India ink stain (CSF): Visualizes capsule (halos).
  • Cryptococcal antigen (CrAg) test (serum/CSF): Rapid, highly sensitive & specific.
  • Culture on Sabouraud Dextrose Agar (SDA).
• Treatment: Amphotericin B + Flucytosine (induction); Fluconazole (consolidation/maintenance).

⭐ India ink stain of CSF is a classic rapid diagnostic test revealing the characteristic polysaccharide capsule of Cryptococcus as a clear halo around the yeast cells against a dark background.

Pneumocystis Pneumonia - Lung Parasite Peril

• Agent: Pneumocystis jirovecii (fungus). Airborne transmission.
• Risk: Immunocompromised, esp. HIV (CD4 < 200/µL).
• Sx: Progressive dyspnea, non-productive cough, fever, hypoxemia.
• Dx:
  • BAL/sputum: Cysts/trophozoites (GMS, Giemsa stains).
  • CXR: Diffuse bilateral interstitial infiltrates (ground-glass).
  • ↑ Serum LDH.
• Rx: TMP-SMX. Steroids if severe (PaO2 < 70 mmHg, A-a gradient > 35 mmHg).
• Prophylaxis: TMP-SMX if CD4 < 200/µL or other risks.

⭐ Cell membrane: cholesterol, not ergosterol → most antifungals ineffective.

Mucormycosis - Rhino's Ruin

• Agents: Mucor, Rhizopus, Absidia, Lichtheimia.
• Pathogenesis: Angioinvasion → tissue necrosis (black eschar).
• Risk Factors: Diabetes (DKA), neutropenia, steroids, iron overload (deferoxamine use).
• Clinical: Rhinocerebral (most common; black eschar, facial pain, orbital involvement), pulmonary, cutaneous.
• Diagnosis: Biopsy - broad (5-20 µm), non-septate/pauciseptate hyphae, wide-angle (90°) branching.

  ⭐ Histopathology: Ribbon-like, aseptate hyphae with wide-angle branching is a key finding.

• Treatment: Surgical debridement + Liposomal Amphotericin B; Posaconazole.

High‑Yield Points - ⚡ Biggest Takeaways

• Candida albicans: Most common; germ tube test (+); thrush, vaginitis, systemic in immunocompromised.
• Aspergillus fumigatus: ABPA, aspergilloma, invasive; acute angle (45°) branching septate hyphae.
• Cryptococcus neoformans: Meningitis in AIDS; India ink for capsule; urease (+); pigeon droppings.
• Mucormycosis: Rhino-orbito-cerebral in diabetics (DKA); broad, non-septate hyphae, 90° branching.
• Pneumocystis jirovecii (PCP): Pneumonia in HIV (CD4 < 200); silver stain for cysts.
• Affect severely immunocompromised (HIV, neutropenia, transplants).