Aspergillosis

01Pathophysiology

Aspergillosis is caused by Aspergillus species, most commonly A. fumigatus, a ubiquitous mold found in the environment. The organism produces small conidia (2 to 3 micrometers) that are easily inhaled deep into the airways. The clinical syndrome that develops depends entirely on the host immune status and pre-existing lung architecture, which is why the same organism produces vastly different diseases.

In immunocompetent individuals with pre-existing lung cavities (such as old tuberculosis cavities, sarcoidosis, or emphysematous bullae), the inhaled conidia colonize the cavity without invading tissue. The fungus proliferates within the cavity, forming a dense mass of hyphae, mucus, and cellular debris called an aspergilloma (fungus ball). Because the fungus does not invade the parenchyma, patients are typically asymptomatic unless the fungus ball erodes into a bronchial vessel, causing hemoptysis, which can be life-threatening.

In atopic patients (those with asthma or cystic fibrosis), the immune system mounts an exaggerated type I (IgE-mediated) and type III (immune complex) hypersensitivity response to Aspergillus antigens colonizing the airways. This is allergic bronchopulmonary aspergillosis (ABPA). The allergic inflammation causes bronchial wall damage, mucus impaction, and progressive central bronchiectasis. The hallmark is recurrent asthma exacerbations that are difficult to control, peripheral eosinophilia, and markedly elevated serum IgE. The fungus does not invade tissue in ABPA; the damage is immunologically driven.

In severely immunocompromised patients, particularly those with prolonged neutropenia (>10 days) or those on high-dose immunosuppressive therapy after transplantation, the innate immune defenses (neutrophils and macrophages) fail to kill the inhaled conidia. The hyphae then invade blood vessel walls (angioinvasion), causing thrombosis, tissue infarction, and hemorrhagic necrosis. This is invasive pulmonary aspergillosis (IPA), the most lethal form. Angioinvasion explains the characteristic CT findings: the "halo sign" (ground-glass opacity surrounding a nodule, representing hemorrhage around the infarct) seen early in the disease, and the "air crescent sign" (a crescent of air between the necrotic lung and surrounding parenchyma) seen later as neutrophils recover and the necrotic tissue retracts. From the lungs, the fungus can disseminate hematogenously to the brain, kidneys, skin, and other organs.

The histopathological hallmark across all invasive forms is septate hyphae with dichotomous branching at 45-degree angles (acute angles), which distinguishes Aspergillus from Mucor (which has ribbon-like, pauciseptate hyphae branching at 90-degree angles).

02Classification and Clinical Manifestation

03Diagnostic Workup

ABPA Workup: When a patient with poorly controlled asthma or cystic fibrosis presents with recurrent exacerbations, peripheral eosinophilia, and fleeting pulmonary infiltrates, ABPA should be suspected. The best initial step is to measure serum total IgE (a level >1000 IU/mL is a key threshold). The diagnosis is then supported by demonstrating Aspergillus sensitization via positive skin prick test or elevated Aspergillus-IgE, along with positive Aspergillus-IgG (precipitins), elevated peripheral eosinophils, and central bronchiectasis on HRCT. There is no single confirmatory test; ABPA is diagnosed by meeting a constellation of criteria (the Rosenberg-Patterson or revised ISHAM criteria). The total IgE level is also used to monitor treatment response and detect flares.

Aspergilloma Workup: A patient with a known lung cavity (particularly from prior TB) who presents with hemoptysis should raise suspicion. Chest CT is the best initial test and characteristically shows a solid, round mass within a cavity with a surrounding crescent of air (air crescent sign). The mass classically moves with changes in patient position ("rolling ball" on decubitus imaging). Aspergillus IgG (precipitating antibodies) is positive in over 90% of cases, making it a highly reliable serologic marker. Sputum cultures may grow Aspergillus, but this is not always reliable. Biopsy is rarely needed because the imaging and serology are usually sufficient.

Invasive Pulmonary Aspergillosis Workup: In a neutropenic patient with persistent fever despite broad-spectrum antibacterial therapy, the best initial step is to obtain a CT chest, which may show the halo sign (a ground-glass halo surrounding a pulmonary nodule, representing hemorrhage around an area of infarction) in early disease. This finding in the right clinical context is highly suggestive and should prompt empiric antifungal therapy even before culture confirmation. Serum galactomannan (a cell wall component of Aspergillus released during hyphal growth) should be sent as a biomarker; it has good sensitivity in neutropenic patients but performs less well in non-neutropenic hosts. Be aware that piperacillin-tazobactam can cause false-positive galactomannan results, a frequently tested fact. BAL with culture and galactomannan improves diagnostic yield. The most accurate test (gold standard) is tissue biopsy demonstrating septate hyphae with acute-angle branching invading tissue, but this is often impractical in critically ill, thrombocytopenic patients.

04Management and Treatment

ABPA: The cornerstone of ABPA management is systemic corticosteroids to suppress the immunologic response driving airway damage. Prednisolone is started at 0.5 mg/kg/day for 1 to 2 weeks, then gradually tapered over 3 to 5 months. Itraconazole is added as a steroid-sparing agent, reducing the fungal antigen burden in the airways and allowing lower steroid doses. Total IgE should be monitored regularly; a doubling of the baseline IgE level (after initial decline) suggests a flare requiring retreatment. In patients with steroid-dependent or refractory ABPA, omalizumab (anti-IgE monoclonal antibody) has shown benefit and may be considered.

Aspergilloma: The key management principle is that asymptomatic aspergillomas do not require treatment. Antifungal drugs do not penetrate the avascular fungus ball effectively. Intervention is warranted only for symptomatic hemoptysis. For life-threatening hemoptysis, bronchial artery embolization provides temporizing control of bleeding. Definitive treatment is surgical resection (lobectomy or segmentectomy) in patients with adequate pulmonary reserve. The decision to operate must weigh the risk of surgery against the risk of recurrent fatal hemorrhage.

Invasive Pulmonary Aspergillosis: This is a medical emergency. The next best step whenever IPA is suspected (neutropenic fever + halo sign on CT) is to start empiric voriconazole immediately, even before culture results return. Delay in antifungal therapy increases mortality. Voriconazole is the first-line agent based on landmark trial data (superior survival compared to amphotericin B deoxycholate). Isavuconazole is an acceptable alternative with fewer drug interactions and no need for cyclodextrin vehicle (therefore safer in renal impairment). Liposomal amphotericin B is the alternative for patients who cannot tolerate or have contraindications to azoles. Echinocandins (caspofungin, micafungin) are used as salvage combination therapy but are not first-line monotherapy. Equally important is immune reconstitution: reducing immunosuppression and using G-CSF to recover neutrophil counts whenever possible. Treatment continues for a minimum of 6 to 12 weeks and should not stop until imaging shows resolution and the patient's immune status has recovered.

Voriconazole monitoring: Therapeutic drug monitoring is essential because of its nonlinear pharmacokinetics. Target trough levels are 1 to 5.5 mcg/mL. Common adverse effects include transient visual disturbances (photopsia), hepatotoxicity, skin photosensitivity (long-term use increases risk of squamous cell carcinoma), and QT prolongation. Voriconazole is a potent CYP2C19 and CYP3A4 inhibitor, leading to numerous drug interactions (e.g., with sirolimus, which is contraindicated with concurrent voriconazole).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get questions on aspergillosis wrong is by confusing the different clinical forms and applying the wrong management. Test-writers exploit this by presenting a neutropenic patient with IPA and offering itraconazole as an answer choice (wrong: itraconazole is for ABPA, not invasive disease) or by presenting an aspergilloma and offering voriconazole (wrong: antifungals do not penetrate the fungus ball). Always match the clinical form to the correct treatment.

A second frequent trap involves the halo sign versus air crescent sign timeline. The halo sign appears early in IPA (first 1 to 2 weeks) and may disappear as the disease progresses. The air crescent sign appears later, typically as neutrophils recover. A vignette showing a neutropenic patient whose CT initially showed a halo sign and now shows an air crescent sign after G-CSF administration is testing whether you recognize that this represents immune recovery and tissue cavitation, not worsening disease.

Another tested concept is the false-positive galactomannan with piperacillin-tazobactam. A vignette may describe a neutropenic patient on pip-tazo with a positive galactomannan, and the question tests whether you know this could be a false positive and whether you would still pursue further workup (the answer is yes: you should still treat empirically and obtain CT/BAL rather than dismissing the result).

For ABPA, the classic trap is a patient with "difficult-to-control asthma" where the vignette buries the clue of central bronchiectasis or brown mucus plugs in the history. Students who focus on adjusting asthma medications miss the diagnosis entirely. The core competency being tested is recognizing that refractory asthma with eosinophilia and elevated IgE should trigger evaluation for ABPA.

Finally, the histologic distinction between Aspergillus and Mucor is one of the most reliably tested facts in mycology. Remember: Aspergillus = septate, 45-degree acute-angle branching; Mucor = pauciseptate (ribbon-like), 90-degree wide-angle branching. If a biopsy description mentions "non-septate" or "ribbon-like" hyphae, the answer is mucormycosis regardless of how "Aspergillus-like" the clinical scenario appears.