Aspergillosis

• Spore Inhalation and Germination Dynamics: Aspergillus fumigatus produces small spores (2-3 μm diameter) that readily reach the alveoli upon inhalation. In healthy individuals, resident alveolar macrophages rapidly recognize and kill spores through pattern recognition receptors (PRRs) including dectin-1 (β-glucan sensor) and toll-like receptors (TLRs), preventing germination. However, in immunocompromised states or when overwhelming inocula are present, spores escape this innate recognition and germinate into filamentous hyphae. The transition from resting spore to hyphal growth involves upregulation of germination-associated genes and metabolic shift to aerobic metabolism. Once germinated, hyphae physically damage alveolar epithelium through mechanical invasion and produce tissue-damaging enzymes (elastases, serine proteases), generating acute pulmonary inflammation.

• Allergic Sensitization and Type 2 Immune Response (ABPA Pathogenesis): In individuals with underlying asthma or cystic fibrosis, repeated inhalation of Aspergillus spores and hyphae triggers an exaggerated Th2-skewed immune response rather than appropriate Th1/Th17 killing response. This occurs through presentation of aspergillal antigens (particularly high-molecular-weight allergens) to naive CD4+ T cells by dendritic cells in a context that favors IL-4/IL-5 production. The resulting cascade produces specific IgE antibodies to aspergillal antigens, creating a state of allergic sensitization. Upon re-exposure, cross-linking of IgE on mast cells and basophils triggers degranulation releasing histamine, tryptase, and leukotrienes, causing immediate hypersensitivity reactions (bronchospasm, urticaria). Simultaneously, elevated IgG (especially IgG4) creates immune complexes depositing in airways and lung parenchyma, triggering complement activation and local inflammation. Additionally, enhanced local IL-4, IL-5, and IL-13 production by Th2 cells drives eosinophil recruitment and mucus hypersecretion, creating an environment where hyphae proliferate within airway mucus and inspissated secretions rather than being cleared. The critical pathophysiologic defect in ABPA is failure of appropriate Th1/Th17 responses that would normally clear fungal organisms, leaving allergic and inflammatory responses unopposed.

• Chronic Pulmonary Aspergillosis (CPA): Cavitation and Saprophytic Colonization: In patients with underlying chronic lung disease (prior tuberculosis, COPD, bronchiectasis), or less commonly in immunocompromised states with granulomatous responses, Aspergillus colonizes pre-existing cavities and damaged airways. In this microenvironment, hyphae grow saprophytically (as a commensal) within cavities where local immune responses are impaired due to poor vascularization and limited T cell penetration. The hyphal mass induces a granulomatous response at cavity margins, characterized by epithelioid macrophages and multinucleated giant cells, but this walls off rather than eliminates the infection. Chronic antigenic stimulation induces persistent IgG antibody production with very high titers; immune complex deposition causes chronic inflammation with IL-6 production driving progressive fibrosis. The fibrinolytic activity of aspergillal proteases and host inflammatory mediators causes cavity enlargement and, critically, angioinvasion leading to hemoptysis through erosion into adjacent pulmonary vessels. In chronic cavitary pulmonary aspergillosis (CCPA), continuous hyphal shedding into airways occurs, distinguishing it from simple colonization.

• Invasive Aspergillosis: Angioinvasion and Dissemination Mechanism: In severely immunocompromised patients (neutrophil count <500/μL, prolonged corticosteroid use >2 weeks, hematologic malignancy, HSCT recipients), aspergillal pathogenesis shifts dramatically. Germinated hyphae physically invade through the pulmonary epithelium and into blood vessel walls (angioinvasion), a pathologic hallmark distinguishing invasive from allergic disease. This angioinvasion is enabled by multiple factors: (1) absence of effective fungal-specific T cell responses allowing uncontrolled hyphal proliferation, (2) depleted neutrophil populations unable to perform hyphal killing through reactive oxygen species and extracellular trap formation, and (3) aspergillal production of gliotoxin (immunosuppressive toxin) that directly inhibits T cell proliferation and neutrophil function. The angioinvasion causes thrombosis and tissue infarction, manifesting clinically as hemoptysis and consolidation on imaging. Hyphal invasion through vessel walls enables hematogenous dissemination to brain, heart, kidneys, and other organs, with CNS involvement occurring in 5-15% of IA cases and carrying particularly poor prognosis. Unlike bacterial pneumonia, the inflammatory response in IA is paradoxically minimal initially due to immunocompromise, explaining why imaging findings may lag behind clinical deterioration and why recovery of immune function during treatment improves outcomes.

• Fungal Virulence Factor Contributions: Aspergillus fumigatus produces multiple virulence factors enhancing pathogenesis across disease phenotypes. Gliotoxin, a epipolythiodioxopiperazine metabolite, suppresses antigen presentation, T cell proliferation, and neutrophil function through mitochondrial disruption and death pathway activation. Proteases (serine proteases, aspartic proteases, metalloproteinases) degrade complement components (C3, C5), immunoglobulins, and tissue matrix proteins, facilitating invasion and immune evasion. Melanin in the spore cell wall absorbs reactive oxygen species, protecting against macrophage and neutrophil killing. Calmodulin and other calcium-binding proteins regulate hyphal thermotolerance allowing growth at 37°C. Aspergillolides and other secondary metabolites contribute to local tissue damage. Genetic variation among Aspergillus isolates, particularly in calcineurin pathway genes, influences azole susceptibility and pathogenicity—an increasingly important consideration as antifungal resistance emerges.

• Aspergillus Species Selection: While over 20 Aspergillus species can cause human disease, A. fumigatus accounts for 70-90% of all aspergillosis cases due to its ability to grow at 37°C (thermotolerant), small spore size enabling alveolar deposition, and production of virulence factors. A. flavus and A. niger represent the next most common causative organisms, with A. niger particularly associated with otomycosis and external auditory canal infection. A. terreus and A. nidulans are less common but increasingly recognized, with A. terreus showing intrinsic amphotericin B resistance and A. nidulans associated with specific immunodeficiencies (STAT3 mutations). Environmental sampling reveals ubiquitous Aspergillus spores in soil, decaying vegetation, and indoor environments including hospital HVAC systems, making exposure nearly universal; symptomatic disease reflects host factors rather than exposure differences.

• Allergic Bronchopulmonary Aspergillosis (ABPA) Risk Factors: The strongest risk factor is underlying asthma, present in 7-10% of asthmatics but reaching 10-15% in severe asthma and 7-10% in cystic fibrosis populations. Cystic fibrosis is the second strongest association, where defective CFTR protein impairs mucociliary clearance and alters airway glucose levels, creating optimal growth conditions for Aspergillus. Prior atopic dermatitis and allergic rhinitis increase risk through Th2-skewed immune responses. Male predominance (approximately 1.5:1) suggests testosterone's immunosuppressive effects. Geographic factors influence prevalence, with higher rates in tropical and subtropical climates (higher mold exposure). Immunoglobulin E (IgE) deficiency paradoxically increases ABPA risk through loss of allergic sensitization control. Systemic corticosteroid therapy worsens ABPA outcomes by suppressing Th1 responses.

• Chronic Pulmonary Aspergillosis (CPA) Predisposing Conditions: Prior tuberculosis represents the strongest risk factor, with post-tuberculous cavitary lung disease providing the ideal microenvironment for aspergillal colonization; up to 30% of TB cavity patients develop CCPA. Chronic obstructive pulmonary disease (COPD) with emphysematous changes creates damaged airways where Aspergillus colonizes. Bronchiectasis from any cause (cystic fibrosis, post-infectious, non-tuberculous mycobacterial infection) increases CPA risk 10-fold. Prior fungal infections (particularly histoplasmosis or coccidioidomycosis in endemic areas) leaving cavitary scars predispose to aspergillal superinfection. Diabetes mellitus impairs local airway immunity. Chronic kidney disease stages 3-5 increase susceptibility through multiple mechanisms. History of prolonged chemotherapy with neutropenia increases risk for IA that subsequently transitions to CPA with immune recovery. Cigarette smoking impairs macrophage function and ciliary clearance.

• Invasive Aspergillosis Risk Factors and Immunocompromised States: Neutropenia (absolute neutrophil count <500/μL for >10 days) represents the most critical risk factor for IA; both degree and duration of neutropenia correlate with infection risk. This occurs in acute leukemia (particularly acute myeloid leukemia), lymphoma during intensive chemotherapy, and myeloablative conditioning for hematopoietic stem cell transplantation (HSCT). Hematologic malignancies per se increase IA risk beyond chemotherapy-induced neutropenia through additional immunodefects. Solid organ transplant recipients, particularly lung transplant (highest risk due to direct exposure), liver transplant, and heart transplant recipients, develop IA at 1-12% incidence depending on prophylaxis use. Prolonged corticosteroid therapy (>2 weeks at ≥20 mg daily prednisone equivalent) impairs T cell function and is present in 20-40% of IA cases; this includes corticosteroid use in COPD exacerbation management and ABPA. HIV/AIDS with CD4 count <50/μL increases IA risk substantially, though less common than with other Candida and Pneumocystis. Chronic granulomatous disease (CGD) and STAT3-deficient hyper-IgE syndrome increase IA risk through defective neutrophil oxidative burst despite normal numbers. Autoimmune conditions treated with TNF-α inhibitors (infliximab, adalimumab) substantially increase IA risk (5-6 fold) through suppression of Th1 and granulomatous responses. Graft-versus-host disease (GVHD) after HSCT, particularly chronic GVHD requiring corticosteroids, dramatically increases IA incidence. Diabetes mellitus is an independent risk factor even without other immunodeficiency. Previous aspergillosis increases recurrence risk.

• Environmental and Occupational Exposures: Exceptionally high spore exposure in occupational settings (farming, grain handling, composting) can overcome normal immune defenses; outbreaks of IA have been documented in hospitalized patients during building construction or water damage events associated with massive spore aerosolization. Hospital HVAC system contamination poses particular risk in HSCT units. Marijuana inhalation (even in immunocompetent individuals) has caused aspergillosis through contamination of the product with Aspergillus spores.

ABPA Clinical Manifestations

• Asthma exacerbation pattern: Acute episodes of wheezing, dyspnea, chest tightness, and nighttime cough that fail to respond optimally to standard asthma therapy or show corticosteroid dependence (requiring ≥15 mg daily prednisone to control symptoms). The exacerbations often follow a cyclic pattern with spontaneous remissions, reflecting antigenic fluctuations with aspergillal proliferation. Sputum production is characteristic and may contain brownish mucus plugs or frank "fungal balls" visible in sputum (rare but diagnostic when present).
• Fever and systemic symptoms: Unlike typical asthma, ABPA often includes low-grade fever (37.5-38.5°C), malaise, and arthralgias mimicking systemic inflammatory conditions. Night sweats may occur during exacerbations. Weight loss can develop with chronic disease.
• Pulmonary symptoms: Dyspnea on exertion progressing to dyspnea at rest in advanced stages. Hemoptysis occurs in 10-30% of cases, usually minor (blood-tinged sputum) but potentially massive if angioinvasion occurs. Chest pain suggests pleural involvement.
• Physical examination findings: Bilateral scattered crackles/rales on auscultation (reflecting bronchitis and pulmonary infiltrates), prolonged expiratory phase suggesting small airway disease, and occasionally wheezes. Fever on vital signs. Clubbing can develop with chronic ABPA (similar to bronchiectasis pathophysiology). Peripheral eosinophilia may be evident on examination if significant hypersensitivity reaction is occurring.

Chronic Pulmonary Aspergillosis (CPA) Clinical Features

• Chronic cavitary aspergillosis (CCPA): Insidious onset of productive cough with progressive dyspnea over weeks to months, often attributed initially to chronic bronchitis or inadequate TB treatment. Sputum is typically purulent and may contain fungal particles. Constitutional symptoms include low-grade fever, night sweats, weight loss, and malaise—mimicking TB reactivation. Hemoptysis is a cardinal feature (40-80% of patients) and ranges from blood-tinged sputum to life-threatening massive hemoptysis when aspergillomas erode into pulmonary vessels; this distinction between minor and massive hemoptysis (>200 mL in 24 hours) drives urgency of management. Fatigue and decreased exercise tolerance progress as lung function declines. In advanced stages, cor pulmonale develops with lower extremity edema and jugular venous distention.
• Aspergilloma (fungal ball): Often asymptomatic when discovered incidentally on imaging; however, when symptomatic, presents with cough (particularly productive of dark sputum), dyspnea, and recurrent minor hemoptysis (blood-tinged sputum, usually managed conservatively) with occasional episodes of massive hemoptysis (1-3% per year risk). The hemoptysis mechanism involves erosion of the aspergilloma into adjacent cavity walls where hyphal invasion breaches vascular structures.
• Hypersensitivity pneumonitis-like presentation (chronic form): Subacute presentation of dyspnea, cough, fever, and fatigue without cavitary disease, characterized by diffuse nodular infiltrates and granulomatous inflammation. This represents a hypersensitivity response to chronic aspergillal

Allergic bronchopulmonary aspergillosis (ABPA)

• Screening: *Aspergillus*-specific IgE (or immediate cutaneous hypersensitivity to Aspergillus antigen) — a negative result essentially excludes ABPA, since Type I sensitization is obligatory.
• Total serum IgE: markedly elevated, with the ISHAM working-group criteria using a threshold above 1000 IU/mL; the value also serves as the follow-up marker of disease activity.
• Supportive: *Aspergillus*-specific IgG/precipitins, peripheral eosinophilia (ISHAM uses >500 cells/µL off steroids), and CT showing central (proximal) bronchiectasis, fleeting infiltrates, and high-attenuation mucus plugging.

Chronic pulmonary aspergillosis / aspergilloma

• CT chest: intracavitary mass with the air crescent sign; mobility of the fungus ball with repositioning is the Monod sign.
• Aspergillus IgG (precipitating antibody): the single most useful serologic test; elevated titer plus a cavity plus ≥3 months of symptoms establishes chronic cavitary disease.

Invasive aspergillosis

• CT chest is the first test in febrile neutropenia unresponsive to antibiotics: early halo sign (nodule with ground-glass rim = perilesional hemorrhage from angioinvasion), later air crescent sign as neutrophils recover and infarcted tissue retracts.
• Serum galactomannan: cell-wall galactofuranose antigen; an optical-density index of ≥0.5 is the usual positive cutoff. Sensitivity is best in neutropenic/hematologic patients and falls with mold-active prophylaxis or in solid-organ transplant. BAL galactomannan outperforms serum for pulmonary disease.
• Serum (1,3)-β-D-glucan: pan-fungal and non-specific; notably negative in mucormycosis and cryptococcosis.
• Confirmatory/gold standard: tissue histopathology (GMS/PAS) showing septate hyphae branching at acute ~45° angles with tissue invasion, plus culture; culture also enables azole-susceptibility testing.
• Classification: the EORTC/MSGERC consensus definitions grade cases as proven, probable, or possible using host factors, clinical/radiologic features, and mycologic evidence.

Immediate stabilization (invasive disease or massive hemoptysis)

• Massive hemoptysis is an airway emergency: secure the airway, position bleeding side down, and proceed to bronchial artery embolization; definitive control is surgical resection once stabilized.
• Reverse the host defect: taper corticosteroids and reduce immunosuppression where possible; per IDSA, G-CSF or granulocyte transfusions may be considered in persistent neutropenia.

Invasive aspergillosis — first-line (IDSA 2016 Aspergillosis guideline)

• Triazoles (voriconazole): primary therapy; inhibits fungal lanosterol 14-α-demethylase, blocking ergosterol synthesis. Standard is an IV loading regimen (6 mg/kg q12h ×2 doses) then 4 mg/kg q12h, with therapeutic drug monitoring of troughs because CYP2C19 polymorphisms cause wide exposure variability.
• Isavuconazole: IDSA-endorsed alternative primary agent; no cyclodextrin vehicle and less QT effect (it actually shortens QT).
• Liposomal amphotericin B: alternative primary therapy, preferred when azole resistance or intolerance is a concern.
• Combination voriconazole plus an echinocandin (e.g., caspofungin): may be considered in severe disease; echinocandins are not recommended as monotherapy first-line because they are only fungistatic against Aspergillus (they hit β-1,3-glucan, not the hyphal core).

Disease-related — emergencies first

• Massive hemoptysis (>200 mL/24 h): hyphal/inflammatory erosion of bronchial arteries lining a cavity or aspergilloma. Signals: brisk bright-red expectoration, falling hematocrit, asphyxia rather than exsanguination is the killer. Airway emergency — intubate, position bleeding lung down, embolize.
• CNS aspergillosis: hematogenous spread of angioinvasive hyphae producing septic infarcts, abscess, and mycotic aneurysm with intracranial hemorrhage. Signals: new focal deficit, seizure, or altered mentation in a neutropenic patient; ring-enhancing or hemorrhagic lesions on MRI. Emergency; carries the worst prognosis of any site. Voriconazole penetrates CNS well; amphotericin B does not.
• Pulmonary infarction and cavitation: vessel thrombosis distal to hyphal invasion; the air crescent sign appearing during neutrophil recovery signals sequestration of infarcted lung and heralds a bleeding risk.
• Disseminated disease: endocarditis (especially prosthetic valves, with large friable vegetations and embolic occlusion of major arteries), endophthalmitis, renal and cutaneous lesions.
• Aspergillus tracheobronchitis / anastomotic invasion in lung transplant: pseudomembranes at the anastomosis risk dehiscence and bronchial stenosis.
• Immune reconstitution inflammatory syndrome: paradoxical radiographic and clinical worsening as neutrophils return, despite effective antifungal therapy — do not reflexively call it treatment failure.

Treatment-related

• Voriconazole: reversible visual disturbances/photopsia (classic, early), visual and auditory hallucinations, hepatotoxicity, QT prolongation with torsades risk, severe phototoxicity progressing to cutaneous squamous cell carcinoma with long-term use, and fluoride-associated periostitis causing diffuse bone pain. CYP-mediated calcineurin-inhibitor toxicity presents as rising tacrolimus levels with acute kidney injury.
• Amphotericin B: dose-limiting nephrotoxicity with renal tubular potassium and magnesium wasting, plus infusion-related rigors.
• Corticosteroids for ABPA: hyperglycemia, osteoporosis, adrenal suppression, and reactivation of latent infection.

• The stem's host tells you the disease: asthmatic/CF with recurrent infiltrates → ABPA; old TB cavity with hemoptysis → aspergilloma/chronic pulmonary aspergillosis; prolonged neutropenia or high-dose steroids → invasive aspergillosis.
• Histology buzzword: septate hyphae branching at acute ~45° angles. The classic distractor is mucormycosis — broad, ribbon-like, non-septate (pauciseptate) hyphae branching at wide ~90° angles, in a diabetic with DKA and black nasal eschar. Mucormycosis does not produce galactomannan and is β-D-glucan negative.
• Imaging triad: halo sign (early angioinvasion) → air crescent sign (late, during neutrophil recovery) → Monod sign (fungus ball shifts with patient position, indicating a free intracavitary mass).
• Single best next step in febrile neutropenia with new pulmonary nodules despite broad-spectrum antibiotics: CT chest plus serum galactomannan, then empiric mold-active therapy — do not wait for culture. Voriconazole is the answer for treatment (IDSA), with isavuconazole or liposomal amphotericin B as alternatives.
• The association examiners love: ABPA in asthma and cystic fibrosis, with markedly elevated total IgE, positive *Aspergillus*-specific IgE, eosinophilia, and central bronchiectasis — treat with systemic corticosteroids first, adding itraconazole as a steroid-sparing agent. Antifungal monotherapy for an acute ABPA flare is the wrong answer.
• Asymptomatic simple aspergilloma is observed, not treated; recurrent or massive hemoptysis drives bronchial artery embolization then surgical resection.
• Drug traps: A. terreus is intrinsically amphotericin B–resistant; echinocandins are not first-line monotherapy; voriconazole causes transient visual disturbances, QT prolongation, photosensitivity, and dangerous CYP interactions (rifampin, sirolimus, tacrolimus). Check voriconazole troughs.
• Galactomannan caveat: sensitivity is highest in neutropenic hematology patients and drops sharply in solid-organ transplant recipients and anyone on mold-active prophylaxis — a negative result does not exclude disease in those hosts.