Diagnosis and Prevention of Invasive Fungal Infections in the Immunocompromised Host

Abstract

Topic Importance

The prevalence of invasive fungal infections (IFIs) has risen in the past 3 decades, attributed to advancements in immune-modulatory therapies used in transplantation, rheumatology, and oncology.

Review Findings

Organisms that cause IFI evade the host’s natural defenses or at opportunities of immunologic weakness. Infections occur from inhalation of potentially pathogenic organisms, translocation of commensal organisms, or reactivation of latent infection. Organisms that cause IFI in immunocompromised populations include Candida species, Cryptococcus species, environmental molds, and endemic fungi. Diagnosis of these infections is challenging due to slow organism growth and fastidious culture requirements. Moreover, fungal biomarkers tend to be nonspecific and can be negatively impacted by prophylactic antifungals. Antibody-based tests are not sensitive in immunocompromised hosts making antigen-based testing necessary. Prevention of IFI is guided by pathogen avoidance, removal or minimization of immune-suppressing factors, and pharmacologic prophylaxis in select hosts.

Summary

Understanding the complex interplay between the immune system and opportunistic fungal pathogens plays a key role in early diagnosis and prevention.

Opportunistic fungi exploit vulnerabilities in host defenses to cause life-threatening infections in immunocompromised individuals. The clinical presentation of invasive fungal infection (IFI) can vary widely depending on the organism and immunologic vulnerability. In this review, we will explore immunologic risk factors for infection, common clinical manifestations of IFI in the immunocompromised host without HIV, diagnostic approaches, and preventative measures.

Immunologic Risk Factors for IFIs

Fungal organisms cause infection by evading natural host defenses or exploiting immunologic vulnerability. Infections occur from inhalation of potentially pathogenic organisms (eg, Cryptococcus species, molds, Pneumocystis jirovecii, endemic fungi), translocation of commensal organisms (eg, Candida species), or reactivation of latent infection (eg, Histoplasma capsulatum, Cryptococcus species). Host defenses and immunogenetics influence the manifestation and severity of disease. For example, the clinical spectrum of pulmonary aspergillosis can range from chronic disease in otherwise immunocompetent individuals with underlying structural lung disease to angioinvasive pneumonia in immunocompromised hosts.

Both the innate and adaptive immune systems play a key role in antifungal immunity. The innate immune system protects against fungal organisms through a combination of barrier defenses,¹ mechanical clearance (eg, cilia, cough reflex),² direct pathogen killing,³ and adaptive immune stimulation. Neutrophils, as part of the body’s innate immune system, play a pivotal role against IFI.³ Neutrophils form swarms, a coordinated clustering, which stimulates factors like nicotinamide adenine dinucleotide phosphate oxidase, myeloperoxidase, and the formation of neutrophil extracellular traps. Neutrophils adapt their response depending on the type and severity of the fungal burden, aiding in both direct pathogen killing and modulating the cellular immune response.³ For these reasons, neutropenia is a primary immunologic risk factor for the development of IFI.

The adaptive immune response is centered around CD4+ T-cell activation.⁴ T-helper (Th)1 cells secrete tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma enhancing the function of phagocytic cells, stimulating granulocyte formation, and promoting B-cell production of opsons.⁵ Th17 cells affect chemokine production to recruit additional phagocytic cells to the site of immunologic activation. Antibodies to fungal components prevent fungal adherence, neutralize toxins, and assist with opsonization.⁶

Applying these concepts to clinical care, patients with acute leukemia and hematopoietic stem cell transplant (HCT) recipients are at the highest risk for IFI due to prolonged periods of profound neutropenia. Patients on prolonged moderate- to high-dose corticosteroids (prednisone ≥ 20 mg/d for > 4 weeks) are also at increased risk of IFI because corticosteroids impair neutrophil migration and phagocytosis without decreasing neutrophil counts. Solid organ transplant (SOT) recipients are at increased risk of IFI not only because of neutrophil dysfunction from chronic corticosteroids but also secondary to T-cell lymphopenia and cytokine suppression from T-cell depleting induction agents, calcineurin inhibitors, and antimetabolite medications. For most SOT recipients, the risk of IFI is greatest within the first year after transplant because the degree of immunosuppression and risk of rejection is greatest during this time.⁷ In addition to the risk factors, lung transplant recipients also have reduced barrier defenses secondary to dysfunctional cilia, airway ischemia, and alveolar macrophage dysfunction, which increase the risk of IFI beyond that of other SOT recipients. Another notably vulnerable ICH population includes those individuals treated with TNF-alpha inhibitors. TNF-alpha inhibitors dampen the communication between macrophages and Th1 cells preventing effective granuloma formation.⁸ This renders the host particularly vulnerable to endemic fungal infections.⁹

Evidence Review

IFI: Clinical Manifestations

To further explore immunologic risk and host vulnerability, we will discuss IFI syndromes by pathogen. Clinical presentation and diagnostic tools for common fungal syndromes in ICHs are summarized in Table 1.

Table 1.

Clinical Presentation and Diagnostic Evaluation for Common Fungal Syndromes

Syndrome	Typical Host	Presentation	Diagnostic Tests
Candida species
Candidemia with hematogenous seeding	Severe T-cell immune suppression or neutropenia, critically ill patients on broad-spectrum antibiotics, or those receiving parenteral nutrition	Fever or septic shock—indistinguishable from bacteremia Pustular (or macular in severe neutropenia) rash Vitritis and/or chorioretinitis MSK abscess	Blood culture: -Slow growth, low sensitivity unless actively fungemic at time of collection β-1,3-d-glucan: -High false-positive rate -Cannot differentiate from other fungi -Sensitivity may be reduced with antifungal prophylaxis T2Candida Panel: -Rapid result with high sensitivity and specificity but not widely available
Chronic disseminated candidiasis	Hematologic malignancy with prolonged severe neutropenia	Develops after neutrophil recovery Upper abdominal pain Fever, nausea, vomiting, anorexia Microabscesses in spleen and/or liver	Culture or histopathology from deep tissue: -Slow growth, invasive procedure β-1,3-d-glucan: -High false positive rate -Cannot differentiate from other fungi -Sensitivity may be reduced with antifungal prophylaxis
Deep-seated surgical site infection	Solid organ transplant recipients	Fever Pleural effusion Intraabdominal fluid collection	Culture or histopathology from deep tissue: -Slow growth, invasive procedure β-1,3-d-glucan: -High false positive rate -Cannot differentiate from other fungi -Sensitivity may be reduced with antifungal prophylaxis
Cryptococcus species
Pneumonia	Pulmonary sarcoidosis, deficient cell-mediated immunity (SOT, HCT)	Asymptomatic nodule to fever, cough, chest pain Radiographic findings: cavitary nodule, scattered or clustered nodules, lobar pneumonia	Culture (sputum, BAL or bronchial washings): -Growth within 7 d -BAL specimens have better yield than sputum Histopathology: -Easily identified via morphologic characteristics -Involves invasive procedure Serum antigen: -High sensitivity (unless isolated pulmonary nodule)
Meningitis	Pulmonary sarcoidosis, deficient cell-mediated immunity (SOT, HCT)	May have respiratory symptoms Usually abnormal chest radiograph No CNS symptoms or headache, neck stiffness	Culture (sputum, BAL or bronchial washings): -Growth within 7 d -BAL specimens have better yield than sputum Histopathology: -Easily identified via morphologic characteristics-Involves invasive procedure Serum antigen -High sensitivity (unless isolated pulmonary nodule) CSF antigen and culture: -All immunocompromised patients with Cryptococcus infection should have CNS evaluation
Aspergillus species
Tracheobronchitis	Lung transplant recipient with airway ischemia	Necrosis and erythema of trachea and mainstem bronchi Shortness of breath, difficulty mobilizing secretions Drop in FEV1, plateauing of expiratory flow-volume loop	Culture (bronchial washings or BAL): -Low sensitivity and does not distinguish infection from colonization Histopathology: -Endobronchial biopsy may help solidify diagnosis but low sensitivity Galactomannan (BAL): -Likely to be positive but cannot distinguish between fungal organisms -Impact of antifungal agents on sensitivity is unknown
Rhinosinusitis	Hematologic malignancy, prolonged neutropenia, HCT	Fever, nasal congestion, and pain around the eyes Involvement of the orbit and extension to the vasculature and brain can lead to additional manifestations	-CT scan may show focal soft tissue lesions, bony erosions, and hypodense areas; MRI scan may also show focal enhancement -Biopsy for histopathology and cultures
Aspergilloma	COPD or structural lung disease, corticosteroids	Hemoptysis, fever, chest pain CT chest scan: rounded mass inside a cavity; may have surrounding ground glass or consolidation	Histopathology: - Confirms diagnosis; CT scan guided or at time of excisional treatment
Invasive pulmonary aspergillosis	HCT and SOT recipients (particularly lung), hematologic malignancies	Fever, shortness of breath, cough, hemoptysis Radiographic findings: diffuse nodules, consolidation, halo sign	Histopathology: -Biopsy (transbronchial) can confirm diagnosis but may not be safe Galactomannan (serum): -Unlikely to be positive unless host is profoundly neutropenic Galactomannan (BAL): -High sensitivity and specificity
Disseminated Aspergillus	Profound neutropenia, HCT recipients	The infection can spread to various organs, and patients with disseminated infection have poor prognosis	Galactomannan (serum): -Likely to be positive in disseminated disease
Mucorales
Rhinocerebral disease	Poorly controlled diabetes mellitus, high-dose corticosteroids	Headache, sinus pressure, fever	Histopathology: -Criterion standard for diagnosis; friable nature of organism makes it difficult to grow on culture -Fungal biomarkers (β-1,3-d-glucan or galactomannan) are usually negative
Pulmonary mucor	HCT and SOT recipients, neutropenia	Fever, shortness of breath, cough, hemoptysis Radiographic findings: diffuse nodules, consolidation, halo sign, or reverse halo sign	Histopathology: -Criterion standard for diagnosis; friable nature of organism makes it difficult to grow on culture -Fungal biomarkers (β-1,3-d-glucan or galactomannan) are usually negative
Endemic fungi
Disseminated histoplasmosis	HCT and SOT recipients, TNF-alpha inhibitors	Sepsis, adrenal insufficiency Miliary pattern on chest imaging	Culture (bronchial washings or BAL, serum): -Slow turnaround time Serum or urine antigen: -High sensitivity and specificity, prognostic value -Cross-reactivity with other endemic fungi
Blastomyces pneumonia	Immunocompetent host or with any degree of immunodeficiency	Nonresolving pneumonia Fever, shortness of breath, cough Skin ulcerations	Urine antigen: -Improved sensitivity over serum antigen testing
Disseminated Coccidioidomycosis	Immunocompetent host or impaired T-cell immunity	Infection can spread from the lungs to other organs, including skin, joints, bones, brain, etc Symptoms vary to the extent of disease severity and organ involvement	Histopathology: -Direct visualization of spherules filled with endospores the criterion standard for diagnosis Culture: -Can grow on most fungal or bacterial media Antigen detection: -In serum, urine, or CSF, helpful in rapid diagnosis: limited evidence and high cross reactivity with histoplasma

CSF = cerebrospinal fluid; HCT = hematopoietic stem cell transplant; MSK = musculoskeletal; SOT = solid organ transplant; TNF-alpha = tumor necrosis factor-alpha.

Candidemia and Invasive Candidiasis

Candida species are commensal organisms that can cause cutaneous, mucosal, or invasive infections. Invasive candidiasis (IC) comprises candidemia and deep-seated candidiasis. Deep-seated candidiasis results from hematogenous seeding or direct inoculation. Intraabdominal candidiasis, a form of deep-seated candidiasis, is commonly caused by a breach in the gastrointestinal tract.¹⁰ The most common pathogenic Candida species are Candida albicans, Candida glabrata, Candida tropicalis, Candida parapsilosis, and Candida krusei. Species distribution can vary geographically.¹¹ Candida auris has emerged as a multidrug-resistant pathogen that has been associated with outbreaks in the health care setting.¹² The virulence traits of pathogenic Candida species are associated with the ability of the organism to alter its morphology from yeast to filamentous forms (hyphae and pseudohyphae).¹³ The hyphal forms play a role in the invasion and destruction of host tissues.¹³ The production of extracellular matrix (biofilm) further protects the organism from host defenses.¹⁴

The clinical presentation of candidemia can range from fever to septic shock and is indistinguishable from bacterial bloodstream infection (Table 1). Risk factors for candidemia include indwelling central venous catheters (particularly those infusing high dextrose-containing fluids such as parenteral nutrition), hemodialysis, critical illness, recent abdominal surgery, and broad-spectrum antibiotic use. Immunocompromised host who are particularly vulnerable to invasive candidiasis are abdominal transplant recipients, those suffering from prolonged periods of neutropenia, and those with prolonged hospitalization due to critical illness. Chronic disseminated candidiasis (also known as hepatosplenic candidiasis) is a rare entity most encountered in patients with hematologic malignancy after recovery from profound neutropenia.¹⁵ Candidemia can cause metastatic ocular infection. The incidence of endogenous fungal endophthalmitis ranges from < 2% to 20% in various studies and has declined in recent years due to the use of effective antifungal agents.¹⁶

Blood cultures are sensitive at detecting viable Candida cells; however, the sensitivity is hindered by the fact that these cells are rapidly eliminated from the circulation. Another challenge is that deep-seated candidiasis may be missed in noncandidemia patients.¹⁷ Time to positivity with the current automated blood systems typically ranges from 24 to 72 hours with longer incubation period required for C glabrata.

β-1,3-d-glucan (BDG) is abundant in the cell wall of Candida species and can be detected in serum. The sensitivity of serum BDG for invasive candidiasis is approximately 80%, and the specificity is lower at 64%.¹⁸ We consider BDG testing helpful in excluding IC given its negative predictive value. T2Candida Panel (T2 Biosystems) is a US Food and Drug Administration-approved test that detects Candida species directly from blood and may be useful in immunocompromised hosts because it is not affected by prophylactic antifungals or immunosuppressed states. It is currently not widely available for use but has a high sensitivity and results within a couple of hours.¹⁹

Cryptococcosis

Cryptococcus neoformans and Cryptococcus gattii are the causative agents of severe infections in individuals with compromised cell-mediated immunity.²⁰ Although C neoformans predominantly affects immunocompromised hosts, C gatti more commonly infects healthy individuals. Cryptococcus species is found in decaying wood, tree hallows, soil, and bird excreta.²¹ Cryptococcus species is transmitted via inhalation of spores and can disseminate into other organs, particularly the central nervous system.²² The primary virulence factor is the polysaccharide capsule that disrupts phagocytosis.²³ Primary cryptococcal infection can be asymptomatic or mildly symptomatic and persists in a latent state within the lungs of an immunocompetent host.²⁴ Pulmonary cryptococcal infections commonly present with cough, dyspnea, chest pain, and fever (Table 1). Headaches or neck stiffness may indicate CNS disease. All immunocompromised hosts with cryptococcal infection should have cerebrospinal fluid (CSF) testing regardless of symptoms.²⁵

Diagnostic tests for cryptococcal infection include direct visualization, culture, and antigen detection. The sensitivity of these testing modalities is not decreased in the setting of immunosuppression or prophylactic antifungal use. Cryptococcus species capsule components can be detected using latex agglutination, lateral flow, or enzyme immunoassays. These assays are known as cryptococcus antigen testing. Antigen testing can be performed on serum and CSF.²⁶ Higher antigen titer correlates with disease burden and is a poor prognostic indicator.²⁷ BDG is not a major cell wall component of most strains of Cryptococcus species, and testing is expected to be negative in cryptococcosis.

Invasive Aspergillosis

Aspergillus species can be found in soil and decaying vegetation, and in poorly maintained ventilation and water systems. Aspergillus fumigatus is the most common species to cause disease; other notables are Aspergillus niger, Aspergillus flavus, and Aspergillus terreus.²⁸ Aspergillus species cause infection by inhalation of conidiospores. Aspergillus species infections have a spectrum of clinical presentations that, in part, are determined by host factors (Table 1). Lung transplant recipients who have reduced barrier defenses (airway anastomosis ischemia, nonfunctional cilia) can present with Aspergillus tracheobronchitis. Patients on chronic inhaled or systemic corticosteroids can develop a localized mycetoma. Patients with severe immunosuppression, particularly those with prolonged neutropenia, can develop invasive pulmonary aspergillosis (IPA) or, less commonly, disseminated disease.²⁹

The initial clinical presentation of IPA may be similar to bacterial pneumonia; however, certain radiographic findings such as pulmonary nodules or the halo sign (a solid pulmonary nodule with surrounding ground glass opacities) are suggestive of IPA.²⁹ Histopathologic diagnosis with direct visualization of septate hyaline hyphae with dichotomous acute angle branching is the criterion standard but requires tissue sampling which may not be feasible.³⁰ In immunocompromised hosts, respiratory specimen cultures (sputum, endotracheal aspirates, bronchoalveolar lavage [BAL]) have a positive predictive value of 72% to 90% for IPA.³¹ BAL cultures have a specificity of approximately 97%, but sensitivity is around 50%.³¹

When evaluating immunocompromised hosts, clinicians often have to distinguish between airway colonization and disease. Because galactomannan is released during fungal growth, its detection supports the diagnosis of active infection. Galactomannan can be detected by enzyme immunoassay. In patients with neutropenia, serum galactomannan detection has a sensitivity of 71% and specificity of 89%.³⁰ In nonneutropenia patients, serum galactomannan is rapidly cleared by neutrophils, reducing the sensitivity to 25%.³⁰ Detection of galactomannan in BAL fluid can be helpful because it may increase sensitivity to 88%.³⁰ However, galactomannan is a food stabilizer in many dairy products.³² As a result, false-positive results on BAL can occur after aspiration. Prophylactic azole antifungals may decrease the sensitivity of serum galactomannan.³³ Of note, Aspergillus species fungal wall contains BDG, and testing may be positive in cases of aspergillosis.

Polymerase chain reaction (PCR)-based techniques can detect Aspergillus species DNA in the blood or BAL specimen and may provide another diagnostic tool in immunocompromised hosts. Testing on whole blood or serum has a modest sensitivity and specificity for a single positive test, and low sensitivity and high specificity when two consecutive positive tests are performed.³⁴ The major limitations of PCR-based tests are the lack of standardized testing protocol and widespread availability, and reduced sensitivity when patients are on antifungal prophylaxis or treatment.³⁵

Mucormycosis

Mucormycosis is caused by filamentous fungi in the order Mucorales. Most pathogenic species belong to the genera Rhizopus, Mucor, Lichtheimia, and Rhizomucor.³⁶ Mucorales are present in the soil and decomposing organic matter and spread by inhalation of fungal conidia that become lodged in the sinuses or directly inoculate terminal alveoli. Alveolar macrophages and neutrophils are the primary defense against Mucorales.³⁷ Patients with hematologic malignancies, patients with prolonged corticosteroid use, and HCT and SOT recipients are at increased risk for mucormycosis.³⁶ Patients with neutropenia and SOT most often present with pulmonary disease rather than rhino-orbital-cerebral disease (Table 1).³⁶ Symptoms of pulmonary mucormycosis are nonspecific. Most common radiographic findings are consolidation, pulmonary nodules, or reverse halo sign (ground glass opacity surrounded by consolidation).³⁸

Definitive diagnosis requires the identification of Mucorales in tissue specimens. Characteristic morphologic features of Mucorales on direct visualization include large (6-25 μm) hyphae at a right angle with minimal septations.³⁶ Invasive infection is suggested histopathologically when associated necrosis and angioinvasion are seen. BDG and galactomannan are typically negative. PCR-based testing on histologic samples showing fungal elements can assist in the diagnosis of mucormycosis; however, the usefulness of molecular testing on tissue samples without demonstrable fungal elements is uncertain.³⁹ In a prospective trial of quantitative PCR in serum targeting Mucor/Rhizopus, Rhizomucor, and Lichtheimia, the sensitivity and specificity were 85.2% and 89.8%, respectively.⁴⁰ Despite the benefits of noninvasive molecular diagnostics, practical application and widespread availability are still limited.

Fusariosis

Fusarium species are found in soil, plant debris, and water systems as biofilms. Infections are caused by Fusarium solani, Fusarium oxysporum, and Fusarium fujikuroi.⁴¹ Fusarium species evade host defenses with the production of mycotoxins and with their ability to sporulate in tissue and blood.⁴¹ The disease spectrum varies based on the host immune system and portal of entry. For example, patients may present with cellulitis caused by a cutaneous portal of entry, or sinusitis caused by inhalation and mucosal invasion. Typical hosts for invasive fusariosis are patients with hematologic malignancies and allogenic HCT and SOT recipients. Severe neutropenia increases the risk of hematogenous spread with subsequent disseminated disease including multiple skin lesions.⁴¹ In patients with severe neutropenia, skin involvement is common, characterized by painful, erythematous papules or nodules on the skin that rapidly progress to develop central necrosis.⁴² Skin lesions can be biopsied and cultured. Lungs are affected in most disseminated cases. Radiologic findings include pulmonary nodules, nodular consolidations, and areas of lung infarction.⁴²

On microscopic examination Fusarium species hyphae resemble those of Aspergillus, with hyaline, septate filaments branching at acute and right angles. The presence of both hyphae and yeast-like structures in tissue is highly indicative of fusariosis, especially in high-risk individuals. Unlike other molds, disseminated fusariosis is associated with a high frequency of positive blood cultures secondary to the ability of the organism to sporulate in vivo. Fusarium species grow in aerobic bottles within 1 to 4 days during fungemia.^41,42 In cases of pulmonary fusariosis, respiratory samples (sputum, BAL, lung biopsy) may yield positive cultures. Like other fungi, Fusarium species contain BDG and galactomannan on their cell wall, and these tests can be positive in patients with fusariosis.⁴¹

Scedosporiosis

Scedosporium species are typically found in sewage and saltwater environments and can be transmitted through cattle, bats, and poultry excreta. Scedosporium apiospermum and Scedosporium aurantiacum are the most clinically relevant species.⁴³ Patients with neutropenia, patients with hematologic malignancies, and lung transplant recipients are at greatest risk for infection. Pulmonary disease is the most common manifestation, with disseminated disease occurring in approximately 25% of immunocompromised hosts.⁴³ Radiologic findings include hilar lymphadenopathy and tree-in-bud nodules progressing to nodular consolidation.⁴³ Diagnosis is based on histopathology of infected tissue showing both hyphae and conidia. Irregular branching of Scedosporium species hyphae and intravascular or intratissue conidia help differentiate it from other molds.⁴³

Pneumocystis Pneumonia

Pneumocystis jirovecii is a fungus that was previously misclassified as a protozoan. Individuals with T-cell immune deficiencies are at an increased risk of P jirovecii pneumonia (PJP). Infection is transmitted by inhalation of organisms. Pneumocystis can be a colonizing organism in immunocompromised hosts that can potentially lead to pneumonia if the organism burden increases to a threshold that potentiates a dysregulated immune response. In PJP, an influx of CD8+ T-cells and neutrophils into the alveolar and interstitial spaces activates cytokine and chemokines causing direct tissue injury.⁴⁴ This immunologic response is more severe and exaggerated in immunocompromised hosts without HIV than in those with HIV. The prevalence of PJP has been increasing in immunocompromised hosts without HIV, particularly among SOT recipients, patients on prolonged corticosteroids, and those with rheumatologic diseases.⁴⁵ In non-HIV populations, the rapid progression from symptom onset to respiratory failure can be a distinguishing feature of PJP.⁴⁶ Radiographically, it often presents as diffuse ground glass opacities with or without intraparenchymal cyst formation.⁴⁶ Mortality due to PJP is higher in immunocompromised hosts without HIV than in patients with HIV, likely due to underlying comorbid conditions and the exaggerated immune response to infection.⁴⁷

P jirovecii is difficult to culture ex vivo. The diagnosis of PJP can be made based on direct visualization of either the trophic or cystic form by staining. This method has been largely replaced by highly sensitive PCR assays because PCR has a higher diagnostic yield.⁴⁸ PCR assays cannot reliably distinguish airway colonization from infection. Therefore, clinicians should use their judgment when making treatment decisions, particularly in the presence of concomitant opportunistic infections. Serum BDG can be useful for making a presumptive diagnosis of PJP in cases where respiratory specimens are unavailable. Results are nonspecific, but high serum levels of BDG (> 500 pg/mL) in the right clinical context suggest PJP.⁴⁹

Histoplasmosis

Histoplasmosis is caused by Histoplasma capsulatum and is the most prevalent endemic mycoses in the United States. Histoplasma lives as a mold in the soil in the Mississippi and Ohio River Valleys.⁵⁰ Transmission occurs via inhalation of Histoplasma species conidia.⁵⁰ Once in the alveoli, conidia transform into yeast due to temperature shifts. Many individuals who encounter Histoplasma species experience a mild respiratory illness, but the organism may remain latent within the lungs or lymph tissues. ICHs can develop acute pulmonary or progressive disseminated histoplasmosis, typically 2 weeks after exposure (Table 1).⁵⁰ In immunocompromised hosts without HIV, the disease typically affects patients receiving TNF-alpha inhibitors⁵¹ and HCT and SOT recipients.⁵² Donor-derived infection in SOT recipients typically presents with disseminated disease. In disseminated disease, patients may present with ARDS and/or septic shock with a miliary pattern on chest imaging. These patients are also prone to develop adrenal insufficiency due to involvement of the adrenal glands with Histoplasma species.

On microscopic examination, Histoplasma species appear as ovoid, narrow-based budding yeast found free or within necrotizing or nonnecrotizing granulomas.⁵³ In disseminated disease, Histoplasma species can grow from blood and extrapulmonary tissue, usually within 2 to 4 weeks.⁵³ Antigen detection is the preferred diagnostic modality in immunocompromised hosts due to the decreased sensitivity of antibody testing and the long incubation period of culture-based methods. Histoplasma species antigen can be detected on serum, urine, BAL, and CSF.⁵³ Sensitivity and specificity of testing can vary based on the assay and burden of infection. Antigen concentration has prognostic value and typically declines within 2 weeks of treatment.⁵³ Clinicians should be aware of the cross-reactivity between Histoplasma species and Blastomyces species antigen. In patients with pulmonary histoplasmosis, galactomannan may also be detected in BAL fluid.

Coccidioidomycosis

Coccidioidomycosis is caused by Coccidioides immitis and Coccidioides posadasii and is endemic to the Southwestern United States, Mexico, and parts of South America.⁵⁴ Coccidioides species grow as mycelia in the environment and transform into loosely adherent arthroconidia. Infection occurs by inhalation of arthroconidia that transform into spherules releasing endospores. Most of the infections due to Coccidioides species are subclinical and self-limited. However, immunocompromised hosts can develop severe pulmonary and disseminated disease (Table 1). Patients with profoundly impaired T-cell immunity are at the highest risk.⁵⁵ Symptoms start within 1 to 3 weeks of exposure and may be indistinguishable from community-acquired pneumonia.⁵⁵ Progressive disease can cause cavitation and involve the pleural space.

Culture or direct visualization of spherules (20-200 μm) filled with endospores (2-4 μm) is the criterion standard for diagnosis. Cultures are positive only in approximately 50% of immunocompromised hosts.³⁰ Identifying coccioides antigen in serum, urine, or CSF may help establish a rapid diagnosis, especially in immunocompromised hosts who do not develop a sufficient antibody response. Data on the performance on antigen testing are limited, and there is cross-reactivity with Histoplasma species antigen testing.

Blastomycosis

Blastomyces dermatitidis and Blastomyces gilchristii are endemic to the United States and Canada. Transmission occurs by inhalation of conidia that transform into yeast after phagocytosis by pulmonary macrophages. Symptoms typically appear 2 to 6 weeks after exposure. Recurrent blastomycosis and reactivation of latent infection have also been reported.⁵⁶ The clinical presentation can range from asymptomatic pulmonary nodules to diffuse pneumonitis and ARDS (Table 1). Radiologic findings of pulmonary blastomycosis include incidental nodules, lobar consolidation, or pulmonary necrosis. Infection can disseminate hematogenously to extrapulmonary organs, especially the skin, joints, and genitourinary tract.⁵⁷

Microscopically, Blastomyces species appear as round, thick-walled, large (8-15 μm) broad-based budding yeast.⁵⁸ Fungal growth may take up to 4 weeks.⁵⁸ Culture sensitivity increases if multiple samples are collected. Of note, B dermatitidis and B gilchristii do not grow in blood culture. As with other endemic fungi, immunocompromised hosts do not mount a significant antibody response to Blastomycosis species, limiting the utility of antibody-based tests. Available antigen tests detect cell wall proteins in the serum, urine, and other body fluids. The sensitivity of antigen testing is higher in the urine (76%-90%) than in the serum (56%-82%). There is high cross-reactivity to Histoplasma species.⁵⁷

Future Directions

Risk Reduction of Invasive Fungal Infections

Given the vulnerability of immunocompromised hosts to develop IFI and the associated morbidity, risk reduction is critical. Risk reduction of IFI is guided by the following three tenets: (1) pathogen avoidance, (2) removal or minimization of immune-suppressing factors, and (3) pharmacologic prophylaxis.

Although complete pathogen avoidance is impossible, some low-risk interventions may prevent IFI in immunocompromised hosts. For example, IC is often nosocomial (ie, catheter-related infections). Nonpharmacologic risk reduction measures include minimizing device use and duration, catheter site care, sterile procedural techniques, and minimizing broad-spectrum antibiotic use.⁵⁹ For other fungi, particularly those that enter the host through the respiratory tract, attention and education on environmental exposure risk reduction is encouraged. Certain activities that are associated with increased pathogen exposure (eg, gardening, construction, farming, spelunking, cannabis inhalation) should be avoided.⁶⁰ If the activity is necessary, inhalation of larger conidia may be reduced by wearing a surgical mask. However, the efficacy of masking for preventing IFI is unknown. Mold exposure in home environments should also be minimized by avoiding any standing, warm water (ie, humidifiers, hot tubs), regularly cleaning air conditioning and heating units, and promptly addressing any water damage or leaks. When feasible in home environments, high-efficiency particulate air filtration and frequent air exchanges can also reduce airborne fungal spores.⁶¹

In general, immune suppression should be minimized to prevent opportunistic infections. Immune suppression minimization and risk for infection looks different for each patient. Net state of immunosuppression is a concept used to assess overall immunosuppression and the risk of infection.⁶² Net state of immunosuppression is composed of several elements: type and intensity of immunosuppressive therapy, cumulative immunosuppressive effects from prolonged duration, immune senescence or other genetic factors weakening the immune system, comorbidity burden, and concurrent infections.⁶² No laboratory test measures net state of immunosuppression. Viral load assessments (eg, Ebstein-Barr virus, cytomegalovirus) may provide some insight into the net state of immunosuppression.⁶² For SOT recipients, the challenge is to maintain pharmacologic immunosuppression at an appropriate level that will facilitate graft tolerance while minimizing risk for infections. In other disease states (eg, hematologic malignancies), the duration and intensity of immunosuppression may not always be modifiable.

Primary pharmacologic prophylaxis is used to mitigate the incidence and severity of IFI in the highest-risk patients, including HCT recipients, patients with cancer with prolonged neutropenia, and SOT recipients. In patients with hematologic malignancy, systemic antifungal prophylaxis reduces the incidence of IFI by 31%.⁶³ Mold-active prophylaxis (itraconazole, voriconazole, or posaconazole) compared with fluconazole (no activity against molds) prophylaxis reduces the risk of IFI and IFI-related mortality.⁶⁴ However, the risk of adverse events leading to antifungal discontinuation is higher in patients taking mold-active agents compared with fluconazole.⁶⁴

Posaconazole is the most effective antifungal prophylaxis at reducing IFI in patients with hematologic malignancies, patients with prolonged neutropenia from chemotherapy, and HCT recipients.^65,66 A novel antifungal, rezafungin, is active against Candida species and Aspergillus species.⁶⁷ Its long half-life allows for once-weekly administration, and its lack of hepatotoxicity and significant drug-drug interactions make it a strong candidate for prophylaxis in the future. Our current practice is to use prophylaxis preferentially with posaconazole for patients expected to have prolonged neutropenia (eg, those with acute leukemia, those with preengraftment allogenic HCT recipients). For allogenic HCT, we continue antifungal prophylaxis until day 100 or longer if immunosuppression is being used for graft vs host disease. For autologous HCT recipients with an anticipated shorter duration of neutropenia, we use anti-Candida species prophylaxis with fluconazole as a better tolerated alternative.

Pharmacologic prophylaxis in SOT recipients is dependent on the patient’s net state of immunosuppression and the type of organ transplanted. Transplant centers typically develop their own protocols for type and duration of prophylaxis based on regional environmental risks, center-specific immunosuppression practices, and local experience. Three pharmacologic prophylactic strategies can be used: universal prophylaxis (all patients receive prophylaxis), targeted prophylaxis (only those with certain risk factors receive prophylaxis), or preemptive prophylaxis (those who develop fungal colonization or a positive biomarker receive prophylaxis).

Abdominal organ transplant recipients (kidney, liver, pancreas, and small bowel) are at greatest risk for invasive candidiasis in the first weeks after transplant. Pharmacologic prophylaxis against invasive candidiasis with fluconazole or an echinocandin (anidulafungin, caspofungin, or micafungin) is often used in liver, pancreas, and small bowel transplant recipients and high-risk kidney transplant recipients.⁶⁸

Lung transplant recipients have increased risk of invasive candidiasis in the first month after transplant, but they also experience an increased risk of invasive mold infections, particularly IPA, in the first 6 months after transplant. Antifungal prophylaxis in lung transplant recipients is aimed at preventing invasive aspergillosis.⁶⁹ Although antifungal prophylaxis strategies vary in lung transplant, most transplant centers in the United States use universal prophylaxis, commonly with nebulized amphotericin with or without an azole antifungal for 6 months to 1 year posttransplant.⁶⁹

Antifungal prophylaxis in heart transplant recipients is generally only used in high-risk individuals (eg, those on extracorporeal membrane oxygenation support or renal replacement therapy, those with an open chest).⁷⁰ Prophylaxis is usually targeted at both invasive candidiasis and invasive aspergillosis. Duration is personalized and often depends on the persistence of risk factors.

Pharmacologic prophylaxis in SOT populations is complicated by drug-drug interactions between azole antifungals and calcineurin inhibitors. Alterations in azole antifungal medications (initiation, discontinuation, or dose changes) require careful monitoring of calcineurin inhibitor levels to prevent rejection or toxicity.

Pharmacologic prophylaxis for PJP is distinct from prophylaxis against other fungal infections. PJP prophylaxis with trimethoprim/sulfamethoxazole in non-HIV populations is associated with an 85% reduction in the frequency of PJP.⁷¹ Several guidelines exist for PJP prophylaxis in patients with cancer, HCT recipients, and SOT recipients. Lack of clarity primarily exists regarding when to use prophylaxis for patients with autoimmune diseases who are receiving immunosuppressive therapy. Patients on moderate-dose corticosteroids (prednisone ≥ 20 mg/d) for > 4 weeks benefit from prophylaxis with little associated harm.⁷² Other medications (eg, TNF-alpha antagonists, rituximab) have not been consistently associated with an increased incidence of PJP.⁷³ However, these medications in combination with additional immunosuppressive medications⁷³ or underlying lung disease⁷⁴ may sufficiently increase the risk of PJP to warrant prophylaxis. A study found that patients receiving rituximab with hematologic diseases, rheumatic disease, or peri-SOT with or without additional risk factors had an overall reduced incidence of PJP when trimethoprim-sulfamethoxazole prophylaxis was used.⁷⁵ Erring on the side of prophylaxis seems warranted given the high mortality associated with PJP in non-HIV populations and the overall tolerability of PJP of prophylactic agents.

Summary

IFI present a significant challenge in ICHs, where weakened immune defenses create an environment conducive to fungal colonization and invasion. Understanding the complex interplay between the immune system and fungal pathogens, and common clinical presentations of IFI, is essential for accurate diagnosis. Given the high morbidity associated with IFI, attention should be given to preventive strategies (eg, pathogen avoidance, chemoprophylaxis, minimization of immune suppression).

Funding/Support

This article was supported by the National Heart, Lung, and Blood Institute of the National Institute of Health [Award K23HL151671].

Financial/Nonfinancial Disclosures

None declared.