Pulmonary fungal infection in the immunocompromised patient

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Pulmonary fungal infection in an immunocompromised host is not one disease. Invasive aspergillosis is common after prolonged neutropenia, allogeneic transplantation or potent immunosuppression. Mucorales are a critical alternative in uncontrolled diabetes, iron overload, profound immune suppression or breakthrough infection on some mould-active prophylaxis. Cryptococcus, endemic dimorphic fungi and yeasts require exposure- and host-specific reasoning. Candida in respiratory secretions usually represents colonisation rather than Candida pneumonia, so treatment should not be driven by a sputum label alone.

No single test rules invasive fungal disease in or out. Diagnosis combines the probability conferred by the host, a compatible clinical and CT syndrome, and mycological evidence from blood, BAL or tissue. Galactomannan sensitivity changes with specimen type, neutrophil state and prior antifungals. Beta-D-glucan is a broad fungal cell-wall marker, not a species test; false positives occur and Mucorales may not elevate it. Culture permits identification and susceptibility testing but is slow and insensitive. Histological invasion is compelling yet often unsafe to obtain in thrombocytopenic or hypoxic patients.

Treatment is time-sensitive and technically demanding. The presumptive regimen must match likely species and previous prophylaxis. Voriconazole has established use in invasive aspergillosis but no activity against Mucorales. Liposomal amphotericin B covers Mucorales and many moulds but brings renal and electrolyte toxicity. Isavuconazole may be used for selected aspergillosis or mucormycosis pathways and shortens rather than prolongs QT, yet still has important interactions. Duration is measured in weeks to months and depends on immune recovery, radiology and microbiological response, not fever resolution alone.

Key points

Prolonged neutropenia, allogeneic stem-cell transplantation, solid-organ transplantation, high-dose or prolonged corticosteroids and major T-cell suppression create different fungal risks.
Persistent or recurrent fever despite broad antibacterial therapy may be the only early sign; cough, pleuritic pain or haemoptysis can appear late.
Urgent thin-section CT is more sensitive than a chest radiograph: nodules, halo sign, wedge-shaped infarcts, cavitation or reversed halo should accelerate specialist review.
Serum and BAL galactomannan support invasive aspergillosis in the right host; beta-D-glucan is nonspecific and may be negative in mucormycosis.
Send BAL or tissue for microscopy, fungal culture, molecular testing and biomarkers where safe, but do not delay treatment in a high-risk deteriorating patient.
Voriconazole is a usual first-line medicine for invasive aspergillosis; suspected Mucorales requires a mould-active alternative such as liposomal amphotericin B and urgent expert input.
Azoles have major CYP interactions, variable exposure and QT effects; therapeutic drug monitoring and pharmacy reconciliation are core treatment, not optional extras.
Reverse modifiable immune suppression, recover neutrophils where possible and consider surgical source control for selected localised Mucorales disease or life-threatening haemoptysis.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Prolonged neutropenia

Haematological malignancy and cytotoxic treatment reduce neutrophil defence, permitting invasive mould disease from ordinarily contained airborne spores.

Transplant and immune therapy

Solid-organ or stem-cell transplantation, corticosteroids and targeted immune treatments impair cellular and phagocyte responses against fungi.

Advanced HIV

Severe cellular immune deficiency predisposes to Pneumocystis and, according to geography and exposure, cryptococcal or endemic fungal infection.

Structural and environmental factors

Damaged lung, construction dust and high environmental spore burden can compound host immune risk, with pathogen spectrum varying by setting.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Spore inhalation
Airborne fungal propagules reach distal bronchi and alveoli continuously but are normally cleared by macrophages and neutrophils.
2
Failure of innate containment
Neutropenia or phagocyte dysfunction permits germination and hyphal growth within lung tissue, with effects that increase as the pathological process progresses.
3
Angioinvasion
Invasive moulds penetrate blood vessels, causing thrombosis, haemorrhage, infarction and characteristic but non-specific CT lesions, which helps produce the characteristic physiological impairment.
4
Dissemination
Organisms spread through blood to brain, skin, kidneys and other organs when local and systemic immunity cannot contain infection.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

High-risk febrile hostRed flag

Persistent or recrudescent fever during prolonged neutropenia, after allogeneic transplantation or under intensive corticosteroid or T-cell suppressive therapy should trigger urgent fungal-risk review even with a normal chest radiograph.

Angioinvasive phenotypeRed flag

Pleuritic pain, haemoptysis, hypoxaemia and CT nodules with surrounding ground glass or wedge-shaped infarcts suggest vascular invasion. Major bleeding or rapidly rising oxygen need is an emergency.

Airway-invasive phenotype

Cough, wheeze, centrilobular nodules, tree-in-bud change and bronchial wall abnormalities may occur, particularly in non-neutropenic transplant recipients, and can mimic bacterial or viral infection.

Possible Mucorales disease

A reversed-halo pattern, tissue necrosis, sinus-orbital disease, breakthrough mould infection, uncontrolled diabetes or failure on voriconazole should raise suspicion. Imaging is not diagnostic, so call specialists rather than switching casually.

Different hosts, different clues

Advanced HIV raises cryptococcosis and PJP; travel or residence may raise Histoplasma or Coccidioides; solid-organ type and time from transplant alter likelihood. Ask about prophylaxis because it changes both risk and test performance.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Urgent thin-section CT chestFirst step
Why
Detect early nodules, halo or reversed-halo patterns, infarcts, cavitation, airway lesions and a target for BAL or biopsy.
Interpretation and limitations
Patterns change with immune recovery and are not species-specific. A normal early CT lowers but does not eliminate risk in a profoundly immunocompromised patient.
02
Serum galactomannan
Why
Seek circulating Aspergillus antigen, especially in prolonged neutropenia or haematological malignancy, alongside serial clinical assessment.
Interpretation and limitations
A positive result increases probability in the right host; prophylaxis may reduce sensitivity and some products or fungi cause false positives. It does not detect Mucorales.
03
Serum beta-D-glucan
Why
Provide a broad adjunct for invasive fungal disease and PJP when available.
Interpretation and limitations
It is nonspecific, susceptible to false positives and may remain low in mucormycosis or cryptococcosis. Never use it to identify a species.
04
Bronchoscopy with BAL
Why
When physiologically safe, send targeted fluid for fungal microscopy and culture, Aspergillus galactomannan, molecular assays and concurrent bacterial, mycobacterial, viral and PJP tests.
Interpretation and limitations
BAL often yields more direct evidence than serum, but colonisation is possible. The risk of precipitating ventilation in severe hypoxaemia must be weighed against how results will change therapy.
05
Tissue histology and culture
Why
Biopsy an accessible lesion when platelet count, anatomy and physiology allow and the distinction would alter treatment or surgery.
Interpretation and limitations
Hyphal invasion establishes invasive disease; morphology may suggest septate Aspergillus or broad Mucorales hyphae but culture or molecular identification is still needed.
06
Baseline treatment safety and interaction screen
Why
Check FBC, renal and liver profile, potassium, magnesium and ECG; reconcile immunosuppressants, anticoagulants, anticonvulsants, antiretrovirals and prophylaxis.
Interpretation and limitations
Results determine formulation, dose adjustment and monitoring. Azole interactions may dangerously raise or lower co-medicines, while amphotericin toxicity can be amplified by other nephrotoxins.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Bacterial pneumonia

Bacterial infection remains common in neutropenia and may coexist; cultures, CT pattern and fungal biomarkers guide but do not independently prove causation.

Viral pneumonitis

Respiratory viruses and cytomegalovirus cause similar fever and ground-glass change, requiring targeted molecular testing, with targeted examination and testing used to resolve the uncertainty.

Drug or immune pneumonitis

Cancer therapies can cause inflammatory lung injury; medicine timing and respiratory sampling help avoid inappropriate further immune suppression.

Pulmonary embolic infarction

Peripheral wedge-shaped lesions, pleuritic pain and haemoptysis overlap with angioinvasive fungi; vascular imaging and host context discriminate.

Malignant infiltration

Progressive cancer or lymphoma can produce nodules, consolidation and systemic decline despite anti-infective treatment, while the contrasting pattern directs the next diagnostic test.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01EscalateProbable invasive fungal pneumoniaFirst stepEscalationA high-risk host has persistent fever or respiratory deterioration with compatible CT findings.

1Stabilise oxygenation and sepsis physiology, obtain blood cultures and urgent CT, and involve haematology or transplant, infection or microbiology, respiratory medicine and antifungal pharmacy.
2Send serum biomarkers and pursue BAL or tissue only if the expected diagnostic benefit justifies procedural risk; capture details of prophylaxis and prior antifungal exposure.
3Start prompt mould-active treatment matched to the most likely species and local pathway rather than waiting for culture in an unstable high-probability case.
4Reassess diagnosis, spectrum, drug exposure, interactions and source control within 24 to 72 hours as imaging and mycology evolve.

02AspergillusPresumed invasive aspergillosisCompatible host and imaging with mycological support or sufficiently high clinical probability, without a strong Mucorales signal.

1Use a current specialist regimen, commonly intravenous or oral voriconazole depending on severity and absorption; obtain baseline liver tests, ECG and a complete interaction review.
2Arrange therapeutic drug monitoring at the locally specified time and after formulation, dose or interacting-drug changes; assess visual, neurological, hepatic and skin toxicity.
3Review response clinically and radiologically, noting that lesions may transiently enlarge during neutrophil recovery.
4Continue for an individualised prolonged course until clinical and radiological response and immune recovery are adequate, with secondary prophylaxis if future immunosuppression is planned.

03MucoralesPossible pulmonary mucormycosisCompatible invasive mould syndrome plus reversed halo, tissue necrosis, breakthrough exposure or lack of response to an Aspergillus-only drug.

1EscalationEscalate immediately to a mycology-experienced infection service and start an active regimen, commonly high-priority liposomal amphotericin B according to species, renal status and local guidance.
2Seek tissue diagnosis and culture when safely achievable because species identification and susceptibility can refine therapy.
3Reverse ketoacidosis, neutropenia or unnecessary immunosuppression where possible and obtain early thoracic-surgical opinion for resectable localised disease or haemorrhage.
4Monitor renal function and electrolytes intensively and discuss isavuconazole or posaconazole step-down or salvage only through specialist guidance.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Usual first-line therapy for invasive aspergillosis when appropriate and active.

Voriconazole

For serious invasive aspergillosis, the SmPC describes loading followed by maintenance, commonly intravenous 6 mg/kg every 12 hours for 24 hours then 4 mg/kg every 12 hours, with an oral adult maintenance option; specialists must adjust using exposure, weight, liver function and interactions.

No Mucorales activity. Major CYP interactions, hepatotoxicity, visual or neurological effects, photosensitivity and QT prolongation; use therapeutic drug monitoring and verify the current SmPC, BNF and local protocol.

Broad mould-active treatment central to suspected or proven mucormycosis and an alternative in selected aspergillosis pathways.

Liposomal amphotericin B

Intravenous weight-based specialist regimen; 3 mg/kg daily is used for many systemic fungal infections, while suspected mucormycosis may require a different higher specialist dose. Confirm current SmPC and local mycology advice.

Infusion reactions, nephrotoxicity, potassium and magnesium loss, anaemia and additive toxicity with other nephrotoxins. Formulations are not dose-interchangeable.

Alternative for selected invasive aspergillosis and treatment option in mucormycosis when the specialist pathway supports it.

Isavuconazole

Specialist loading and maintenance regimen per current SmPC, species, liver function and interactions; do not improvise from another azole.

CYP3A interactions, hepatic toxicity and pregnancy considerations. It shortens QT but is not interaction-free; exposure and susceptibility need expert interpretation.

Prophylaxis in selected very-high-risk patients and step-down or salvage therapy for selected mould disease.

Posaconazole

Formulation-specific specialist loading and maintenance, with exposure monitoring according to indication and local protocol.

Suspension, tablets and intravenous forms have different pharmacokinetics. Interactions with immunosuppressants, hepatic effects and QT risk require pharmacy oversight.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Respiratory failure

Diffuse or rapidly progressive infection causes severe hypoxaemia, haemorrhage and need for critical-care support, and increasing the burden of otherwise local respiratory disease.

Massive haemoptysis

Angioinvasion erodes pulmonary or bronchial vessels, sometimes bleeding as neutrophils recover or necrotic tissue separates, particularly when baseline cardiopulmonary reserve is limited.

Cerebral dissemination

Bloodstream spread can produce brain abscesses, infarction, seizure or focal neurological deficits, with severity determined by its extent and the patient's underlying reserve.

Antifungal toxicity and interactions

Therapy can impair liver, kidneys, electrolytes or neurological function and interacts substantially with transplant, cancer and anticoagulant medicines.

Persistent or resistant infection

Ongoing immune suppression, poor drug exposure and species resistance can prevent control despite prolonged treatment, and increasing the burden of otherwise local respiratory disease.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Trend oxygen requirement, haemodynamics, fever and pleuritic pain daily; haemoptysis or sudden deterioration requires emergency imaging and airway or bleeding planning.
Check renal function, potassium and magnesium frequently during amphotericin therapy and minimise concurrent nephrotoxins where clinically possible.
Monitor liver tests and ECG for azoles, with adverse-effect review for visual, neurological, dermatological and gastrointestinal toxicity.
Measure voriconazole or other azole levels at locally specified steady state and after dose, formulation, absorption or interaction changes.
Reconcile concentrations and effects of calcineurin inhibitors, mTOR inhibitors, anticoagulants, anticonvulsants and antiretrovirals after every antifungal change.
Repeat CT at a clinically meaningful interval or earlier with deterioration; interpret apparent early enlargement in the context of immune recovery.
Track neutrophil and immune recovery and document a plan for treatment duration or secondary prophylaxis during future immunosuppression.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Candida in sputum is usually a bystander

Respiratory Candida commonly reflects colonisation. Invasive candidiasis is diagnosed through the whole syndrome and evidence from blood or tissue, not sputum growth alone.

Prior prophylaxis reshapes probability

Breakthrough disease may be resistant, outside the prophylactic spectrum or caused by inadequate exposure. Record drug, formulation, adherence, level and interactions.

Immune recovery can look worse first

Lesions can enlarge or cavitate as neutrophils recover. Clinical physiology, mycology and serial imaging help distinguish inflammatory evolution from uncontrolled infection.

Mould identification changes lives

Aspergillus-only therapy is dangerous if the organism is Mucorales. Tissue, culture and molecular identification are worth pursuing when safe and decision-changing.

Antifungal pharmacy is part of source control

An azole level below target because of an interaction or poor absorption is functional undertreatment; an excessive level adds avoidable toxicity.

11Common pitfallsFrequent interpretation and management errors.

01
Waiting for a positive culture before treating a deteriorating, high-risk probable invasive mould infection.
02
Assuming a negative serum galactomannan excludes disease after mould-active prophylaxis.
03
Using voriconazole for suspected mucormycosis.
04
Treating Candida in respiratory secretions as Candida pneumonia without invasive evidence.
05
Starting an azole without reconciling transplant, anticoagulant, antiretroviral and anticonvulsant interactions.
06
Performing bronchoscopy in profound hypoxaemia without deciding whether the result justifies the ventilation risk.