Invasive Aspergillosis: Causes, Symptoms, and Treatment

Invasive aspergillosis is a life-threatening fungal infection caused mainly by Aspergillus fumigatus, a mold whose microscopic spores are virtually everywhere in the outdoor environment. Healthy people inhale hundreds of these spores daily without consequence, but in people with severely weakened immune systems the fungus can take root in the lungs and spread through the bloodstream to other organs. The infection carries high mortality even with treatment, which makes understanding who is at risk, how the disease is caught early, and what drugs work best a matter of genuine urgency.

The Fungus Behind the Disease

Aspergillus fumigatus is the dominant species in invasive disease, responsible for most cases in people with compromised immunity.¹ Other Aspergillus species do show up in clinical samples. A large survey of cultures from Southern California found that Aspergillus niger was the most frequently isolated species overall, but that was driven by skin, nail, and scalp samples; in respiratory cultures, A. fumigatus accounted for about 40% of isolates, with A. niger a distant second at roughly 26%.² Species like A. flavus, A. terreus, and A. niger can occasionally cause invasive disease, but A. fumigatus dominates because its spores are the right size to reach deep into the lungs and it thrives at human body temperature.

The spores, called conidia, are tiny enough to travel to the smallest airways. In a healthy person, the immune system eliminates them before they can germinate into the thread-like filaments (hyphae) that cause tissue damage. When that immune defense is impaired, spores germinate and hyphae grow into lung tissue and, critically, into blood vessel walls. This invasion of blood vessels is a hallmark of the disease and contributes directly to tissue death and the potential for spread to distant organs.³

Who Gets Invasive Aspergillosis

The classic at-risk group includes people with severely low white blood cell counts, particularly those undergoing chemotherapy for blood cancers or receiving stem cell transplants. A systematic review and meta-analysis found that prolonged low neutrophil counts roughly quadrupled the odds of invasive aspergillosis, while corticosteroid use in transplant recipients increased the risk nearly fivefold.⁴ In allogeneic stem cell transplant recipients specifically, graft-versus-host disease, cytomegalovirus infection, and the type of stem cell product used all shape risk, especially in the weeks after the new marrow engrafts.⁵

But the risk profile has expanded beyond classic hematology patients. People in intensive care with severe viral pneumonia can also develop invasive aspergillosis, even if their immune system was previously normal.⁶ During the COVID-19 pandemic there was widespread concern about “COVID-associated pulmonary aspergillosis.” A nationwide cohort study, however, found that COVID-19 itself did not significantly raise the risk of invasive pulmonary aspergillosis compared to influenza or other ICU admissions. What did matter was corticosteroid use lasting a week or more, regardless of dosage.⁷ Chronic lung disease, chronic liver disease, and prolonged hospital stays also independently increased risk in that study. The upshot is that while blood cancers and transplants remain the core risk categories, any critically ill patient on prolonged steroids or with damaged lungs deserves attention.

How the Immune System Normally Stops It

Two cell types do most of the heavy lifting against inhaled Aspergillus spores, and their relative contributions are not what you might expect. Animal studies showed that depleting alveolar macrophages (the resident immune cells in the lung air sacs) did not lead to invasive disease, because neutrophils still arrived on schedule and contained the infection. By contrast, depleting neutrophils before or within a few hours of exposure led to high mortality.⁸ Even a brief window of neutrophil presence was enough to prevent spores from germinating into invasive hyphae.

Macrophages still play a role, but a more nuanced one. Research has shown that macrophages cluster around spores and actually inhibit their germination, which paradoxically can shield the fungus from neutrophil-mediated killing.⁹ In a healthy person this containment strategy works fine. But if neutrophils are absent or delayed, the macrophage “cocoon” can become a reservoir where spores survive and eventually escape.

Genetic variation in the immune system also matters. Systematic reviews have found that inherited differences in toll-like receptors and dectin-1, which are pattern-recognition molecules the immune system uses to detect fungi, are associated with susceptibility to invasive aspergillosis.¹⁰ In stem cell transplant recipients, the relevant genetics may come from the donor rather than the patient, since the transplanted immune system carries the donor’s genetic blueprint. One study found that certain donor gene variants in toll-like receptor 4 and interferon-gamma were more common among transplant patients who developed the infection.¹¹

Symptoms and Where It Spreads

The lungs are the primary site of infection. Early symptoms tend to be frustratingly nonspecific: fever that does not respond to antibiotics, cough, chest pain, and sometimes shortness of breath. In people with very low white blood cell counts, fever alone may be the only initial sign, because the inflammatory response needed to produce other symptoms is suppressed along with the immune system.

As the disease progresses and hyphae invade blood vessels, patients can develop hemoptysis (coughing up blood) and areas of lung tissue can die off. The blood vessel invasion is also what allows the fungus to spread to other organs. The central nervous system is one of the most feared destinations. Brain involvement usually occurs through blood-borne spread in immunocompromised patients, though in rare cases it can happen through direct extension from the sinuses or following head trauma.¹² Skin, kidneys, and the heart valves are other potential sites when the infection disseminates.

Diagnosis

Diagnosing invasive aspergillosis early is one of the biggest challenges in managing the disease, because the symptoms overlap with bacterial pneumonia and other fungal infections, and cultures are slow and often negative even when the fungus is present. Clinicians rely on a combination of imaging, blood and lung-fluid biomarkers, and molecular testing.

CT Imaging

Chest CT scans are usually the first clue. In patients with very low neutrophil counts, the classic early finding is the “halo sign”: a pulmonary nodule or mass surrounded by a haze of ground-glass opacity, representing bleeding around the infected tissue.¹³ Early CT studies showed that these halos appeared before cavitation developed, and that cavities and air crescent signs typically formed later, often coinciding with bone marrow recovery from chemotherapy.¹⁴ The halo sign is not unique to aspergillosis, but in a severely immunosuppressed patient it is highly suggestive and should prompt immediate further workup.

Galactomannan and Other Biomarkers

Galactomannan is a sugar molecule released by growing Aspergillus hyphae, and detecting it in blood or lung-washing fluid (bronchoalveolar lavage, or BAL) is one of the cornerstone diagnostic tests. A Cochrane systematic review of BAL galactomannan testing found that at the standard cutoff, sensitivity was about 88% and specificity about 81%.¹⁵ In practical terms, a very low value essentially rules the disease out, while a very high value rules it in. One study found that a galactomannan index below 0.5 had high enough sensitivity to virtually exclude the diagnosis, while a value above 3.0 reached 100% specificity, confirming the diagnosis regardless of how likely it seemed beforehand.¹⁶

Galactomannan testing is more sensitive in lung-washing fluid than in blood, making bronchoscopy valuable when the clinical suspicion is high. Compared to other methods, BAL galactomannan also outperforms serum testing in sensitivity while maintaining similar specificity.¹⁷

PCR and Lateral-Flow Testing

Molecular testing has increasingly entered clinical use. Combining an Aspergillus-specific lateral-flow device (a rapid point-of-care test) with PCR testing of BAL fluid achieved 100% sensitivity in one study, with specificity around 86%.¹⁸ Another comparison found that pairing the galactomannan test with PCR on BAL fluid reached 100% sensitivity and 95–98% specificity for probable or proven disease, outperforming any single test alone.¹⁹ Conventional fungal culture, meanwhile, suffers from low sensitivity: it often misses cases, even when the infection is present. The trend in clinical practice is toward combining biomarkers rather than relying on any single test.

Blood-based tests are less sensitive than lung-fluid tests, but newer approaches show promise. One study found that serum interleukin-8 was the most reliable blood biomarker, and that combining it with either BAL lateral-flow testing or BAL PCR yielded excellent sensitivity and specificity for distinguishing probable invasive aspergillosis from other diagnoses.²⁰

Treatment

Antifungal drugs are the backbone of therapy, and the choice of agent has become more nuanced as newer options have arrived.

First-Line Antifungal Therapy

Voriconazole has been the standard first-line treatment since a landmark trial showed it was superior to conventional amphotericin B.²¹ Isavuconazole has since been shown to be noninferior to voriconazole in a randomized trial, with the added advantage of fewer drug interactions and a more predictable blood-level profile.²² In real-world use, both drugs show comparable treatment completion rates and similar durations of therapy, though voriconazole tends to be more commonly prescribed and may be more cost-effective.²³ Posaconazole is another option: a global database study found six-week mortality rates were comparable between voriconazole and either posaconazole or isavuconazole, suggesting these newer agents are effective alternatives even in higher-risk patients.²⁴

Liposomal amphotericin B remains an important alternative, particularly for patients who cannot tolerate azole antifungals or whose infections are resistant to them. It is also used for secondary prevention in patients who have survived a prior episode and face renewed immunosuppression.²⁵

Drug Level Monitoring

Voriconazole is metabolized unpredictably. The same dose can produce vastly different blood levels in different patients, affected by genetics, liver function, other medications, and even age. Too-low levels risk treatment failure; too-high levels risk liver toxicity and neurological side effects like visual disturbances and confusion. Therapeutic drug monitoring, where blood levels are measured and doses adjusted accordingly, is now considered standard practice.²⁶ One retrospective study found that subtherapeutic initial voriconazole levels were associated with worse early treatment response, reinforcing why monitoring matters in the first days of therapy.²⁷ Isavuconazole, by contrast, has more linear pharmacokinetics, which is one reason some clinicians favor it in patients on complex drug regimens.

When Surgery Is Needed

Antifungal drugs alone are not always enough. Surgery plays a role in two scenarios: as an emergency procedure to prevent life-threatening bleeding when a fungal mass lies close to a major blood vessel, and as an elective resection of a residual mass before the patient faces another round of immune-suppressing therapy.²⁸ In one case series, the criterion for emergency surgery was direct contact between the aspergillosis lesion and the pulmonary artery on CT. Elective resection was also used diagnostically in patients who had not responded adequately to antifungal therapy within about two weeks.²⁹ These decisions are highly individual and depend on the patient’s overall condition and whether their bone marrow is expected to recover.

The Growing Problem of Azole Resistance

Azole antifungals are the most important drug class for treating invasive aspergillosis, so resistance to them is a serious concern. The best-studied resistance mechanism involves mutations in the cyp51A gene, which codes for the protein that azoles are designed to block. A particularly worrying pattern called TR34/L98H, where a section of the gene’s promoter is duplicated and a single amino acid is swapped, confers resistance to all three major medical azoles: itraconazole, voriconazole, and posaconazole.³⁰ Additional mutations stacked on top of TR34/L98H can make things worse. One study identified a novel combination that drove resistance even higher against all tested azoles, creating what amounts to pan-azole resistance.³¹

What makes this particularly alarming is where the resistance appears to be coming from. The TR34/L98H mutation pattern has been found in A. fumigatus isolates from soil and compost, genetically related to resistant isolates from patients, and these environmental strains are cross-resistant to agricultural azole fungicides.³² Agricultural azoles are sprayed on crops in enormous quantities worldwide to prevent plant fungal diseases. Because the target protein in the fungus is the same one that medical azoles attack, environmental exposure to agricultural fungicides selects for resistance that then compromises human treatment. Studies have identified specific agricultural compounds including difenoconazole, propiconazole, epoxiconazole, bromuconazole, and tebuconazole as drivers of this cross-resistance.³³ All A. fumigatus isolates carrying the TR34/L98H mutation have shown cross-resistance to both medical triazoles and multiple agricultural fungicides, consistent with the hypothesis that environmental fungicide exposure is the primary selection pressure.³⁴

This dynamic creates a problem that individual hospitals cannot solve on their own. Patients can arrive already colonized with resistant strains simply from breathing outdoor air. In regions where environmental resistance rates are high, clinicians may need to consider azole resistance testing before committing to first-line therapy, or use combination regimens that include non-azole antifungals.

Preventing Exposure in Hospitals

Because Aspergillus spores are airborne, reducing the spore burden in hospital air is the most direct prevention strategy. HEPA filtration in patient rooms is widely used in transplant units. One study at a large hospital found that installing portable HEPA filters in high-risk wards cut the incidence of invasive aspergillosis by about half, while wards without filters saw no change during the same period.³⁵ An earlier study during hospital construction, when spore counts surge, found that a dedicated HEPA-filtered ward eliminated invasive aspergillosis completely in neutropenic patients, while low-dose antifungal prophylaxis alone only marginally reduced it.³⁶

That said, the evidence is not uniformly positive. A more recent single-center retrospective study found no significant additional benefit of HEPA filtration on top of antifungal prophylaxis in patients with acute myeloid leukemia, and no effect on ICU admissions or early mortality.³⁷ This likely reflects the fact that modern antifungal prophylaxis is itself quite effective, so layering HEPA on top may produce diminishing returns. In institutions where prophylaxis is standard, HEPA filtration may serve more as a safety net than a standalone measure. During construction or renovation, when spore levels spike, its value remains clearer.

Tissue Diagnosis and the Limits of Culture

One of the underappreciated frustrations in managing invasive aspergillosis is that definitive proof, meaning actual identification of the fungus in tissue, is often impractical. Biopsying lung tissue in a patient with low platelet counts and fragile blood vessels carries significant bleeding risk. When biopsies are obtained, pathologists look for branching filaments that are characteristic of Aspergillus but can resemble other molds. In extremely rare cases, the fungus’s fruiting bodies are visible in tissue sections, which allows a definitive diagnosis at the microscopic level alone without the need for culture.³⁸ In the vast majority of cases, though, clinicians must rely on the combination of imaging features, biomarker results, and clinical context to reach a “probable” diagnosis and start treatment. Waiting for certainty would mean waiting too long.