5.4 Invasive Fungal Infections (Aspergillus, Candida, Mucor) & PJP Prophylaxis

Invasive Fungal Infections (Aspergillus, Candida, Mucor) & PJP Prophylaxis

Core Clinical Principle: Invasive fungal infections (IFIs) represent one of the leading causes of non-relapse mortality following hematopoietic cell transplantation. The risk trajectory is bimodal: Candida and Aspergillus predominate during initial neutropenic aplasia (Phase 1), while angioinvasive molds (Aspergillus, Mucorales, Fusarium) and Pneumocystis jirovecii dominate during Phase 2 and Phase 3 in patients receiving high-dose corticosteroids for acute or chronic GVHD. Early biomarker screening, rapid high-resolution imaging, aggressive surgical consultation, and meticulous management of CYP3A4 drug-drug interactions are vital.

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1. Classification & Epidemiology of Fungal Pathogens in HCT

Fungal pathogens in the immunocompromised transplant host are broadly divided into three major categories:

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|                               CLASSIFICATION OF POST-TRANSPLANT FUNGI                             |
|                                                                                                   |
|   YEASTS & YEAST-LIKE                   MOLDS (FILAMENTOUS)               DIMORPHIC & ATYPICAL    |
|   * Candida albicans (test isolate)      * Aspergillus (fumigatus/flavus)  * Histoplasma capsulatum|
|   * Candida glabrata (reduced azole)    * Mucorales (Rhizopus, Mucor)     * Coccidioides immitis  |
|   * Candida krusei (fluconazole-R)      * Fusarium species                * Blastomyces           |
|   * Cryptococcus neoformans             * Scedosporium species            * Pneumocystis jirovecii|
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1. Invasive Candidiasis

• Pathophysiology: Arises primarily from endogenous gastrointestinal flora that translocates across ulcerated, chemoradiation-damaged mucous membranes into the bloodstream during pre-engraftment aplasia.
• Species Nuances:
  • Candida albicans: Often azole-susceptible, but species identification and susceptibility plus prior exposure guide therapy.
  • Candida glabrata: Azole susceptibility is variable; an echinocandin is commonly used initially while susceptibility is assessed.
  • Candida krusei: Intrinsically fluconazole-resistant; select an active agent from susceptibility, infection site and current guidance.
• Management: An echinocandin is a common initial candidemia therapy, with drug/dose adjusted for species, susceptibility, prior azole exposure, organ function and clinical severity. Evaluate central-line removal individually based on source, access and stability; obtain clearance cultures and assess metastatic infection.

2. Invasive Aspergillosis

• Pathophysiology: Inhalation of ubiquitous airborne conidia (Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger, Aspergillus terreus). Conidia germinate into septate hyphae that exhibit acute 45-degree branching and aggressively invade pulmonary arterial blood vessels (angioinvasion), causing thrombosis, tissue infarction, and hematogenous dissemination to the brain, liver, and skin.
• Risk Windows:
  • Window 1: Pre-engraftment neutropenia (Day 0 to +30).
  • Window 2: Late phase post-transplant (> Day +30 to Day +180+) during systemic corticosteroid therapy for acute or chronic GVHD.

3. Mucormycosis (Zygomycosis)

• Pathophysiology: Caused by saprophytic environmental molds of the order Mucorales (Rhizopus, Mucor, Lichtheimia / Absidia, Cunninghamella). Characterized microscopically by broad, ribbon-like, non-septate or sparsely septate hyphae with wide 90-degree right-angle branching.
• Clinical Behavior: Hyper-aggressive, rapidly destructive angioinvasion causing extensive tissue infarction, vascular thrombosis, and necrotic black eschars in the rhino-orbital-cerebral, pulmonary, or cutaneous distributions.
• Drug activity: Voriconazole lacks reliable Mucorales activity, and echinocandins are not effective monotherapy. Use a guideline- and species-directed active regimen.

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2. Diagnostic Biomarkers & Radiographic Hallmarks

Galactomannan supports evaluation for invasive aspergillosis in appropriate serum or BAL settings, while beta-D-glucan can support several fungal infections and PJP but is usually not detected with Mucorales and is not reliable for Cryptococcus. Positivity thresholds and false-positive/negative causes are assay-, specimen-, host-, and treatment-dependent. A biomarker does not independently prove or exclude disease.

High-resolution CT can reveal nodules, halo, cavitation, air crescent, or reverse-halo patterns, but none is pathognomonic. Obtain cultures, fungal PCR or antigen testing, bronchoscopy/BAL, and tissue when safe and useful. Do not delay therapy in an unstable high-risk patient while pursuing confirmation.

3. Pharmacology of Mold-Active Triazoles & Critical Drug Interactions

Mold-Active Triazoles Overview

Doses, loading, route, therapeutic-drug-monitoring targets, and adjustments must come from the current label and infectious-disease/pharmacy plan. Oral formulations are not automatically interchangeable.

Critical Azole–Immunosuppressant Interactions

Voriconazole, posaconazole, isavuconazole and itraconazole differ in enzyme/transporter effects and labeling. Tacrolimus, cyclosporine, sirolimus and other narrow-therapeutic-index drugs require a pharmacist-led plan using the specific pair's prescribing information, baseline organ function, electrocardiographic risk and frequent concentrations. Some combinations require avoidance; others require an initial adjustment followed by measured titration. When an azole starts, stops, or changes route, anticipate delayed concentration changes rather than automatically restoring a prior dose at 48–72 hours.

4. Mucormycosis Emergency

Suspected mucormycosis requires immediate infectious-disease and site-specific surgical evaluation. Obtain tissue for histopathology, culture and molecular identification when feasible, but do not delay active therapy in a rapidly progressive case. Liposomal amphotericin B is a common initial agent; dose reflects CNS involvement, disease severity and renal risk. Isavuconazole or posaconazole can be used in selected first-line, step-down or salvage plans. Remove devitalized tissue when feasible and reverse modifiable risks such as uncontrolled hyperglycemia, acidosis, neutropenia or excessive immunosuppression. Hydration and electrolyte replacement must be individualized to renal and cardiac status.

5. Pneumocystis jirovecii Pneumonia (PJP) Prevention and Recognition

PJP risk persists after HCT and with corticosteroids, GVHD therapy, lymphopenia, or delayed immune recovery. Start and stop dates are driven by engraftment, marrow tolerance, immune suppression and the transplant protocol rather than a calendar alone. Dry cough, fever, exertional dyspnea, hypoxemia and diffuse ground-glass changes can evolve despite a relatively quiet examination. Beta-D-glucan can support but does not establish the diagnosis; respiratory PCR or staining must be interpreted with host and specimen context. Do not delay treatment in an unstable high-risk patient.

TMP-SMX is preferred for prophylaxis when tolerated because it is effective and also covers selected bacterial and parasitic pathogens. Monitor cytopenias, renal function, potassium, allergy and interactions. Alternatives include dapsone, atovaquone, and aerosolized or IV pentamidine, but they are not equivalent for every patient. Check G6PD status before dapsone because hemolysis and methemoglobinemia can occur; monitor symptoms even with normal testing. Give atovaquone with food to improve absorption and assess whether intake is reliable. Aerosolized pentamidine requires respiratory-administration precautions and may have less extrapulmonary coverage. Choose prophylaxis and treatment with the infectious-disease/transplant team and reassess after intolerance or breakthrough infection.