Abstract
Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis/posadasii, and Cryptococcus spp. (both C. neoformans and C. gattii) are invasive pulmonary fungal pathogens that infect both immunocompetent and immunocompromised hosts and have typical yeast or yeast-like morphology on histologic examination. All have defined endemic regions except for Cryptococcus neoformans, which is globally distributed, and C. gattii, which has a more limited distribution, including an outbreak strain in Western North America. Although definitive diagnosis is best accomplished with fungal culture that can also render antifungal sensitivity data, clinical scenarios do occur wherein histologic diagnosis is needed. Each fungus has characteristic morphologic and histochemical findings that allow for identification. The relative toxicity of many antifungal medications underscores the importance of appropriate pathogen-targeted therapy.
Histologically, Histoplasma can be identified by its small, ovoid size and narrow budding; Blastomyces by its larger double-contoured spherical forms with broad-based budding; Coccidioides by its large, endospore-laden mature spherules; and Cryptococcus by its varied size and Mucicarmine-staining mucoid capsule. Degenerating or variant forms can cause challenges in rendering a confident diagnosis, and in the absence of microbiologic confirmation, clinical and radiographic correlation is often needed.
Access provided by Autonomous University of Puebla. Download chapter PDF
Similar content being viewed by others
Keywords
- Lung
- Fungal infection
- Yeast morphology
- Special stains
- Histoplasma
- Blastomyces
- Coccidioides
- Cryptococcus
Case Presentation
A 62-year-old man presents to clinic with a 4-month history of persistent malaise, shortness of breath, and nonproductive cough. He has a past medical history of psoriatic arthritis for which he is taking a monoclonal antibody immunosuppressive medication. He states that he has felt this way since an extended trip that included visits to Arizona, Mississippi, and Texas. A CT scan shows a 2.2 cm peripheral lung nodule in the right upper lung that has been slowly increasing in size over the past 3 months (Fig. 52.1). Serologic tests are equivocal, and he agrees to a biopsy of the pulmonary nodule. Histologic sections show a nodule consisting predominantly of caseating necrosis with rare scattered poorly staining spherules and degenerated forms that are highlighted by a GMS stain (Fig. 52.2). These forms appear to range markedly in size from <10 μm to approximately 100 μm. Rare spherules contain many small endospores, while smaller spherules do not contain these structures (Fig. 52.2). A histologic diagnosis of coccidioidomycosis is made, and the patient is prescribed a 3-month course of fluconazole.
Final Pathologic Diagnosis: Coccidioidomycosis of the Lung
Clinical Considerations
Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides immitis/posadasii are frequently encountered pulmonary pathogens in immunocompetent hosts. Cryptococcus neoformans is more frequently encountered in immunocompromised patients such as those with HIV/AIDS, organ transplant, and prolonged courses of immunosuppressive medications. However, C. gattii has been shown to cause clinically severe infections in immunocompetent hosts [1, 2]. While all are environmental soil fungi, the geographic distribution varies. Histoplasma is most frequently found in the distribution of the Ohio and Mississippi rivers [3]; Blastomyces shares these river valleys but also extends throughout the Great Lakes, St. Lawrence River, and parts of Canada [4]. Coccidioides is an endemic pathogen in the southwestern United States, Mexico, parts of Central and South America [5], and arid regions of the northwestern United States [6]. Cryptococcus neoformans has a global distribution [7], while C. gattii has a more limited distribution with outbreak- [1] and non-outbreak-related infections occurring in the Western and Northwestern regions of North America [2], as well as tropical and subtropical regions [8].
These mycoses all have a wide range of associated clinical severity. Histoplasmosis is most frequently asymptomatic and self-limiting [9] but can produce an acute presentation, a chronic infection akin to mycobacterial disease, or disseminated disease. A similar range of severity and time course exists for blastomycosis, which can have a varied clinical presentation and frequently presents with concurrent skin involvement [10, 11]. Coccidioides most frequently presents as a flu-like illness but, like the others, can present with more chronic, fibrocavitary, or disseminated disease [12]. Unlike Blastomyces which produces disseminated disease in 20–50% of patients [13, 14], dissemination occurs in ~1% of cases of coccidioidomycosis, and patients of African and Pacific Islander descent are at increased risk [15, 16]. Cryptococcus most frequently has a subacute or chronic clinical presentation but has been shown to asymptomatically colonize the respiratory tract in hosts with prior pulmonary disease [17] and can present as disseminated disease in profoundly immunocompromised patients. The most common and feared clinical complication of cryptococcosis is cerebromeningitis. In all of these mycoses, the manner of clinical presentation alone is generally insufficient to rule out any given fungus. However, an appropriate travel history, immunocompetency status, and certain symptoms (e.g., neurological involvement for Cryptococcus and skin involvement for Blastomycosis) can markedly influence the differential diagnosis.
Serologic testing for Coccidioides usually begins with enzyme immunoassays and are confirmed by immunodiffusion and complement fixation, titers of which track with disease states [18]. Antigenic testing for Cryptococcus is available in serum and CSF and is an important adjuvant test, particularly to evaluate for cryptococcal cerebromeningitis [19]. Antigenic tests are available for Coccidioides, Blastomyces, and Histoplasma, but antigens are cross-reactive across these and other yeasts [20, 21].
Radiologic Features
Because these different mycoses can have varied clinical presentations and distributions, radiographic findings can vary and overlap. However, there are still characteristic findings in typical presentations of disease. In acute histoplasmosis, radiographs most often show patchy pneumonia in one or more lobes with frequently noted hilar or mediastinal lymph nodes [22]. Severe disease can show diffuse pulmonary reticulonodular pulmonary infiltrates. In blastomycosis, acute presentations are more likely to present with airspace consolidation, and chronic presentations are more likely to present with mass-forming lesions [23]. In coccidioidomycosis, many patients have generally unremarkable radiographic findings; less than half show patchy areas of consolidation, and a minority show hilar lymph node enlargement [12]. In an immunocompetent patient, Cryptococcus can show multiple small well-defined nodules or focal areas of consolidation with an upper lobe predominance [24, 25].
Histologic and Immunophenotypic Features
In invasive disease, the shared morphology of these four mycoses is that of yeast or yeast-like forms; it is this feature that groups them as a common differential diagnosis. Upon the detection of candidate yeast forms in a biopsy or surgical specimen, delineation between the different mycoses can be made by several features, including size, shape (oval, round, or pleomorphic), pattern of budding (narrow, broad, or varied), and certain characteristic morphologic or staining features. A summary table of morphologic features is presented in Table 52.1.
Histoplasma spp. infects alveolar macrophages and is predominantly seen as an intracellular pathogen within phagocytic cells [26] with small, ovoid yeast forms (2–5 μm) that are the smallest of the yeasts forms discussed in this chapter (Fig. 52.3a). These typically stain poorly in routine preparations but can be observed as phagocytosed forms in the cytoplasm of macrophages. While other yeasts such as Cryptococcus can overlap in size with Histoplasma, the overall population of other yeasts is typically pleomorphic with larger forms present. Histoplasma shows narrow-based budding that contrasts with Blastomyces’ broad-based budding.
Blastomyces are double-contoured, refractile, and typically 8–15 μm in size, creating morphologic overlap with either Cryptococcus (smaller forms) or Coccidioides (larger forms). Occasionally, very small forms mimicking Histoplasma in size can occur, but, as with Cryptococcus, these will typically exist in a continuum of size, including forms with more classic morphology [27]. The broad-based attachment of blastoconidia to their parent cells is the most reliable feature to distinguish Blastomyces from similar sized mimics (Fig. 52.3b, inset). Blastomyces (Fig 52.3b) will also be larger than Histoplasma (Fig 52.3a) if measurements with a calibrated micrometer can be made. The double cell wall is also a helpful finding (Fig. 52.3b).
The most characteristic finding of Coccidioides is that of a large, mature spherule greater than 30 μm containing multiple endospores (Fig. 52.2a). Smaller, immature spherules can be seen, which are generally ovoid and varied in size (Fig. 52.2c, d); these spherules can be confused with the yeast forms of other mycoses. Endospores are small (2–5 μm) (Fig. 52.2b) and may be difficult to identify in the absence of spherules [28]. Unlike other invasive yeasts, occasional hyphae and arthroconidia can occasionally be seen. When mature endospore-laden spherules are present, a confident diagnosis can be made, but if only immature spherules or degenerated forms are present, then a diagnosis is less certain and should be correlated with serology, immunofluorescence, or fungal culture.
Cryptococcus has intermediate-sized (most typically 4–7 μm) yeast forms with narrow budding (Fig. 52.3c), surrounded by a mucoid capsule that characteristically stains with a Mucicarmine stain (Fig. 52.3d). These yeasts may be pleomorphic, ranging up to 15 μm. Capsule-deficient forms can be present. Particularly in caseous or necrotic nodules, cryptococcal cells may be small, nonviable, distorted, and acapsular, becoming potentially confused with Histoplasma. Cryptococcal forms are typically not easily seen in an H&E stain, especially in the case of degenerated cells or resolving/resolved infections. In the case of unsuccessful fungal culture, immunofluorescence or molecular studies may be undertaken in order to make a histologic diagnosis.
Molecular Testing
Fungal culture should be performed in all clinical scenarios where invasive fungal disease is suspected. However, invasive fungal disease is often an unexpected finding, and fungal culture can sometimes produce isolates that are of uncertain clinical significance. Histologic evaluation, including assessment of host response, adds valuable information in these cases [28]. There are, however, cases where morphologic findings are not entirely specific for an etiology, and fungal culture has either failed or was not performed. In these cases, PCR-based methods performed on the formalin-fixed, paraffin-embedded tissue are becoming increasingly common and useful diagnostic tools [29, 30]. While molecular-based methods are largely more sensitive than histologic ones, a negative result does not definitively rule out invasive fungal disease in cases with strong histologic evidence or clinical suspicion. Degradation of DNA in nonviable organisms or after formalin fixation can cause a sample’s total fungal DNA content to be below the threshold of detection for that assay, and caution in such cases is warranted. For cultured isolates of Coccidioides, DNA hybridization probes are available.
Key Points for Differentiating Coccidioides, Cryptococcus, Blastomyces, and Histoplasma
Each of These Pathogens Has Yeast or Yeast-Like Forms in Tissue: What Morphologic Features Distinguish Them?
Size is an important feature distinguishing these organisms (see Table 52.1), but other key morphologic features are often visible. A unique feature of Coccidioides distinct from Histoplasma, Blastomyces, and Cryptococcus is the formation of spherules, generally greater than 30 μm diameter; mature spherules are laden with endospores (2–5 μm). Cryptococcus spp. usually have a capsule that stains with mucicarmine. Blastomyces has a thick, double-contoured cell wall and divides by broad-based budding, whereas Histoplasma lacks the double-contoured wall and divides by narrow-based budding.
What Are Important Pitfalls in the Identification of These Organisms?
The pathologist should be aware of capsule-deficient Cryptococcus, small-variant Blastomyces, and the presence of free Coccidioides endospores in the absence of spherules. Free endospores are small and may hide in the background. Small-variant Blastomyces typically has a range of sizes, helping to distinguish from Histoplasma capsulatum. Serologic tests such as cryptococcal antigen may help diagnose capsule-deficient Cryptococcus. Other yeasts, such as Candida spp. and Sporothrix spp. may appear similar in histologic section.
What Additional Diagnostic Testing Is Important/Should Be Recommended?
Fungal culture and molecular microbiologic testing are far more specific than histologic assessment for species identification. Culture should be taken in lesions with a high suspicion, and the laboratory should be alerted to concern for Coccidioides or Histoplasma due to the significant risk of occupational exposure. Fungal PCR can be performed on the paraffin block in cases without a microbiologic diagnosis. Serologic testing for either antibodies or antigens is often helpful, but cross-reactivity among these and other yeasts is an important concern.
References
Kidd SE, Hagen F, Tscharke RL, Huynh M, Bartlett KH, Fyfe M, et al. A rare genotype of Cryptococcus gattii caused the cryptococcosis outbreak on Vancouver Island (British Columbia, Canada). Proc Natl Acad Sci. 2004;101(49):17258–63.
Harris JR, Lockhart SR, Sondermeyer G, Vugia DJ, Crist MB, D’Angelo MT, et al. Cryptococcus gattii infections in multiple states outside the US Pacific Northwest. Emerg Infect Dis. 2013;19(10):1620–6.
Manos NE, Ferebee SH, Kerschbaum WF. Geographic variation in the prevalence of histoplasmin sensitivity. Dis Chest. 1956;29(6):649–68.
Bradsher RW, Chapman SW, Pappas PG. Blastomycosis. Infect Dis Clin N Am. 2003;17(1):21–40, vii.
Edwards PQ, Palmer CE. Prevalence of sensitivity to Coccidioidin, with special reference to specific and nonspecific reactions to Coccidioidin and to Histoplasmin. Dis Chest. 1957;31(1):35–60.
Litvintseva AP, Marsden-Haug N, Hurst S, Hill H, Gade L, Driebe EM, et al. Valley fever: finding new places for an old disease: Coccidioides immitis found in Washington State soil associated with recent human infection. Clin Infect Dis. 2015;60(1):e1–3.
Levitz SM. The ecology of Cryptococcus neoformans and the epidemiology of Cryptococcosis. Rev Infect Dis. 1991;13(6):1163–9.
Kwon-Chung KJ, Bennett JE. High prevalence of Cryptococcus neoformans var. gattii in tropical and subtropical regions. Zentralbl Bakteriol Mikrobiol Hyg [A]. 1984;257(2):213–8.
Goodwin RA, Loyd JE, Des Prez RM. Histoplasmosis in normal hosts. Medicine (Baltimore). 1981;60(4):231.
Sarosi GA, Davies SF. Blastomycosis. Am Rev Respir Dis. 1979;120(4):911–38.
Vanek J, Schwarz J, Hakim S. North American blastomycosis: a study of ten cases. Am J Clin Pathol. 1970;54(3):384–400.
Kim K-I, Leung AN, Flint JD, Muller NL. Chronic pulmonary coccidioidomycosis: computed tomographic and pathologic findings in 18 patients. Can Assoc Radiol J. 1998;49(6):401–7.
Anderson JL, Meece JK, Hall MC, Frost HM. Evidence of delayed dissemination or re-infection with Blastomyces in two immunocompetent hosts. Med Mycol Case Rep. 2016;13:9–11.
Saccente M, Woods GL. Clinical and laboratory update on blastomycosis. Clin Microbiol Rev. 2010;23(2):367–81.
Brown J, Benedict K, Park BJ, Thompson GR. Coccidioidomycosis: epidemiology. Clin Epidemiol. 2013;5:185–97.
Mease L. Pulmonary and extrapulmonary coccidioidomycosis, active component, U.S. Armed Forces, 1999-2011. MSMR. 2012;19(12):2–4.
Pappas PG. Cryptococcal infections in non-HIV-infected patients. Trans Am Clin Climatol Assoc. 2013;124:61–79.
McHardy IH, Dinh B-TN, Waldman S, Stewart E, Bays D, Pappagianis D, et al. Coccidioidomycosis complement fixation titer trends in the age of antifungals. J Clin Microbiol. 2018;56(12):e01318-18.
Rajasingham R, Wake RM, Beyene T, Katende A, Letang E, Boulware DR. Cryptococcal meningitis diagnostics and screening in the era of point-of-care laboratory testing. J Clin Microbiol. 2019;57(1):e01238–18.
Frost HM, Novicki TJ. Blastomyces antigen detection for diagnosis and management of blastomycosis. J Clin Microbiol. 2015;53(11):3660–2.
Theel ES, Jespersen DJ, Harring J, Mandrekar J, Binnicker MJ. Evaluation of an enzyme immunoassay for detection of Histoplasma capsulatum antigen from urine specimens. J Clin Microbiol. 2013;51(11):3555–9.
Gurney JW, Conces DJ. Pulmonary histoplasmosis. Radiology. 1996;199(2):297–306.
Fang W, Washington L, Kumar N. Imaging manifestations of blastomycosis: a pulmonary infection with potential dissemination. Radiographics. 2007;27(3):641–55.
Lindell RM, Hartman TE, Nadrous HF, Ryu JH. Pulmonary Cryptococcosis: CT findings in immunocompetent patients. Radiology. 2005;236(1):326–31.
Fox DL, Müller NL. Pulmonary Cryptococcosis in immunocompetent patients: CT findings in 12 patients. Am J Roentgenol. 2005;185(3):622–6.
Garfoot AL, Rappleye CA. Histoplasma capsulatum surmounts obstacles to intracellular pathogenesis. FEBS J. 2016;283(4):619–33.
Tuttle JG, Lightwaedt HE, Altshuler CH. Systemic North American blastomycosis. Report of a case with small forms of blastomycetes. Am J Clin Pathol. 1953;23(9):890–7.
Guarner J, Brandt ME. Histopathologic diagnosis of fungal infections in the 21st century. Clin Microbiol Rev. 2011;24(2):247–80.
Lass-Flörl C, Mutschlechner W, Aigner M, Grif K, Marth C, Girschikofsky M, et al. Utility of PCR in diagnosis of invasive fungal infections: real-life data from a multicenter study. J Clin Microbiol. 2013;51(3):863–8.
McCarthy MW, Walsh TJ. PCR methodology and applications for the detection of human fungal pathogens. Expert Rev Mol Diagn. 2016;16(9):1025–36.
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 2022 Springer Nature Switzerland AG
About this chapter
Cite this chapter
Mays, J.A., Lieberman, J.A., Xu, H. (2022). Histoplasmosis, Blastomycosis, Coccidioidomycosis, and Cryptococcosis in the Lung. In: Xu, H., Ricciotti, R.W., Mantilla, J.G. (eds) Practical Lung Pathology. Practical Anatomic Pathology. Springer, Cham. https://doi.org/10.1007/978-3-031-14402-8_52
Download citation
DOI: https://doi.org/10.1007/978-3-031-14402-8_52
Published:
Publisher Name: Springer, Cham
Print ISBN: 978-3-031-14401-1
Online ISBN: 978-3-031-14402-8
eBook Packages: MedicineMedicine (R0)