Fungal Pneumonia: Diagnostic Criteria

I. General Diagnostic Framework

II. EORTC/MSGERC Diagnostic Categories (Invasive Fungal Disease)

Proven IFD

• Histopathologic, cytopathologic, or direct microscopic evidence of fungal elements from a needle aspiration or biopsy specimen with associated tissue damage, OR
• Positive culture from a normally sterile site

Probable IFD

• At least one host factor + one clinical criterion + one mycologic criterion

Possible IFD

• Host factor + clinical criterion, but no mycologic evidence

III. Host Factors (Risk Criteria)

• Prolonged neutropenia (>10 days, ANC <500/μL)
• Allogeneic HSCT recipient
• Prolonged corticosteroid use (>0.3 mg/kg/day prednisone equivalent for >3 weeks)
• Treatment with T-cell immunosuppressants (calcineurin inhibitors, anti-TNF agents)
• Inherited severe immunodeficiency (e.g., chronic granulomatous disease, SCID)
• AIDS (particularly for Cryptococcus, PCP, Histoplasma)

IV. Clinical Criteria (Radiologic)

V. Mycologic Criteria

Serum/BAL Biomarkers

Culture

• Sputum culture: low sensitivity, poor specificity for most fungi
• BAL culture: preferred for most fungal pathogens
• Blood culture: positive in disseminated candidiasis and cryptococcosis

Molecular / PCR

• Aspergillus PCR (serum or BAL): emerging role; ESCMID guidelines support use alongside GM
• PCP PCR (BAL): highly sensitive but interpretation requires clinical context (colonization vs. infection)

Histopathology

• Gold standard for proven IFD
• Cytology and/or histopathology distinguish fungal infection from pulmonary hemorrhage, rejection, or other infiltrates
• Tissue obtained via:
  • Transbronchial biopsy (BAL)
  • CT-guided transthoracic needle aspiration (pleural-based lesions)
  • Open lung biopsy (when less invasive tests are unrevealing)

VI. Organism-Specific Diagnostic Highlights

VII. Practical Diagnostic Algorithm

Suspected fungal pneumonia
        │
        ├── Obtain: CT chest + serum biomarkers (GM, β-D-glucan, CrAg per host)
        │
        ├── Immunocompromised?
        │       YES → Apply EORTC/MSGERC criteria
        │              Probable = Host + CT finding + positive biomarker
        │
        ├── BAL bronchoscopy if CT abnormal
        │       → Culture, GM, cytology, PCR
        │
        └── Biopsy if:
                - BAL unrevealing
                - Accessible lesion (CT-guided)
                - Diagnosis remains uncertain → Open lung biopsy

• Diagnosis is almost never based on a single test; it requires host + clinical + mycologic convergence.
• Galactomannan and β-D-glucan are adjuncts, not standalone tests — they are most useful in high-pretest-probability (immunocompromised) patients.
• Histopathology remains the gold standard for proven disease.
• CT imaging, particularly the halo sign (early) and air crescent sign (late), is central to the clinical diagnosis of invasive aspergillosis.

Etiologic Factors of Fungal Pneumonia

I. Causative Organisms (Etiology by Pathogen)

1. Aspergillus spp. (most common cause of invasive fungal pneumonia)

• Species: A. fumigatus (most common), A. flavus, A. niger, A. terreus
• Habitat: Ubiquitous in soil, decaying organic matter, construction dust, hospital air
• Route: Inhalation of airborne conidia
• Pathogenesis: In immunocompetent hosts, conidia are cleared by alveolar macrophages and neutrophils. In immunocompromised hosts, conidia germinate into hyphae, invade pulmonary vasculature (angioinvasion), causing hemorrhagic infarction and dissemination
• Key risk factors (Harrison's, p. 6263): profound neutropenia, glucocorticoid use, underlying respiratory disease, CGD, advanced HIV, relapsed leukemia; risk increases with longer duration and higher steroid doses

2. Pneumocystis jirovecii (PCP)

• Classification: Previously classified as a protozoan; now recognized as an atypical fungus
• Habitat: Ubiquitous environmental organism; asymptomatic colonization common in humans
• Route: Airborne transmission; reactivation of latent infection
• Pathogenesis: Organism attaches to type I pneumocytes via surface glycoprotein A → alveolar damage → impaired gas exchange → progressive hypoxia
• Key risk factors: HIV/AIDS (CD4 <200 cells/μL), solid organ transplant, hematologic malignancies, prolonged steroids, primary immunodeficiencies

3. Histoplasma capsulatum

• Habitat: Endemic to Ohio and Mississippi River valleys (USA), Central and South America; found in soil enriched with bird or bat droppings (caves, chicken coops)
• Route: Inhalation of microconidia released from disturbed soil
• Pathogenesis: Microconidia convert to yeast form at 37°C in lungs → phagocytosed by macrophages but survive intracellularly → granuloma formation (in immunocompetent) vs. disseminated infection (in immunocompromised)
• Key risk factors: Endemic area exposure, activities like spelunking/construction/farming, HIV (CD4 <150), TNF-α inhibitor use

4. Coccidioides immitis / C. posadasii

• Habitat: Endemic to the arid southwestern USA (San Joaquin Valley), northern Mexico, parts of Central/South America; found in alkaline desert soil
• Route: Inhalation of arthroconidia from disturbed soil (dust storms, construction, agriculture)
• Pathogenesis: Arthroconidia → spherules → endospores released into lung tissue → granulomatous inflammation; in immunocompromised, dissemination to bones, meninges, skin
• Key risk factors: Endemic exposure, African-American or Filipino descent (higher risk of dissemination), pregnancy (3rd trimester), HIV, organ transplant, high-dose steroids

5. Cryptococcus neoformans / C. gattii

• Habitat: C. neoformans — worldwide in pigeon droppings and soil; C. gattii — endemic to tropical/subtropical regions (Pacific Northwest, Australia)
• Route: Inhalation of desiccated yeast or basidiospores
• Pathogenesis: Primary pulmonary infection often subclinical → hematogenous dissemination to CNS (cryptococcal meningitis) in immunocompromised; polysaccharide capsule impairs phagocytosis
• Key risk factors: HIV/AIDS (most common; CD4 <100), solid organ transplant, sarcoidosis, corticosteroid use

6. Mucorales (Mucormycosis)

• Species: Rhizopus, Mucor, Lichtheimia, Cunninghamella
• Habitat: Ubiquitous in soil, decaying vegetation, bread mold
• Route: Inhalation of sporangiospores; occasionally ingestion or direct inoculation
• Pathogenesis: Spores germinate → hyphae invade blood vessel walls → thrombosis → tissue necrosis (angioinvasion); iron availability (e.g., diabetic ketoacidosis, deferoxamine use) is a critical growth factor
• Key risk factors: Diabetic ketoacidosis (DKA), prolonged neutropenia, iron overload/deferoxamine therapy, hematologic malignancy, HSCT, trauma/burns

7. Candida spp.

• Species: C. albicans, C. glabrata, C. tropicalis, C. parapsilosis
• Route: Primarily hematogenous seeding of the lungs (true primary Candida pneumonia from inhalation is rare); aspiration in intubated patients
• Pathogenesis: Part of normal oropharyngeal flora → overgrowth with disruption of mucosal barriers → candidemia → pulmonary seeding
• Key risk factors: Prolonged broad-spectrum antibiotics, central venous catheters, TPN, abdominal surgery, neutropenia, ICU admission

8. Scedosporium / Lomentospora (Rare/Emerging)

• Per Harrison's (p. 6309): Scedosporium apiospermum complex and Lomentospora prolificans are major pathogens in immunocompromised hosts, causing pneumonia, disseminated infection, and brain abscess
• Found in tidal flats, swamps, ponds, manure, and soil in temperate climates
• Route: inhalation of conidia (primary); direct inoculation (secondary)
• Particularly associated with near-drowning and cystic fibrosis

II. Etiologic Factors by Category

A. Pathogen-Related Factors

B. Host-Related Etiologic Factors

C. Environmental / Epidemiologic Factors

III. Summary: Organism-to-Etiology Map

IMMUNOCOMPROMISED HOST
    ├── Neutropenic (hematologic malignancy, HSCT) → Aspergillus, Candida, Mucor
    ├── HIV/AIDS → PCP (CD4<200), Cryptococcus (CD4<100), Histoplasma, Coccidioides
    ├── SOT + steroids → Aspergillus, PCP, Cryptococcus
    └── DKA / iron overload → Mucorales

IMMUNOCOMPETENT HOST (endemic exposure)
    ├── Ohio/Mississippi Valley → Histoplasma
    ├── Southwest USA/desert → Coccidioides
    └── Worldwide (low inoculum) → subclinical Cryptococcus, Histoplasma

STRUCTURAL LUNG DISEASE
    └── COPD, bronchiectasis, TB cavities → Aspergillus (chronic pulmonary aspergillosis, ABPA)