Approach Considerations
No clinical presentations or imaging abnormalities exist that provide a definitive diagnosis of blastomycosis. Culture and cytopathology, with direct visualization of B dermatitidis, is the gold standard to provide a definitive diagnosis. In a study of patients with pulmonary involvement, sputum culture was positive for the organism in 86% of cases and 100% of bronchial washings in patients with confirmed pulmonary disease. [44]
The fastest way to diagnose blastomycosis is direct identification of the broad-based budding yeast forms under microscopy. For most specimens, direct visualization should precede culture to confirm the diagnosis. Sputum specimens processed with 10% potassium hydroxide or calcofluor white fungal stain are examined first in adolescent and adult patients, because these specimens have a high overall yield (approximately 80%). Isolation and identification of the organism on sputum culture provides absolute confirmation of the diagnosis. Identification from culture may not be evident for 2 to 4 weeks, and it often requires invasive procedures such as bronchoscopy or tissue biopsy to obtain specimens. [1]
With regard to serologic tests, complement fixation and immunodiffusion tests lack sensitivity and cannot be used to exclude the diagnosis. A commonly used test is a commercially available chemiluminescent DNA probe (AccuProbe), which produces results from culture within hours once there is adequate growth. However, this test also produces a positive result with all Paracoccidioides brasiliensis species, a dimorphic fungus endemic to Central and South America. Nonetheless, P brasiliensis is very rare in the United States, so this probe remains useful. P brasiliensis can be differentiated from B dermatitidis by the appearance of the yeast phase. [2]
In addition, tests have been developed that target Blastomyces virulence factors BAD1 and DRK1 through real-time polymerase chain reaction (RT-PCR) and serologic antibody-based assays, and they have high sensitivity and specificity. The antibodies can be detected in 87.8% of the patients with blastomycosis by the enzyme immunoassay compared to 15.0% by immunodiffusion. [3, 4, 5]
Immunodiffusion and complement fixation of the antigen in serum and urine lacks specificity, and the combination of urine and serum testing does not improve sensitivity significantly. Negative findings should be confirmed with culture and cytology. However, results from one study found serial urine antigen concentrations to be useful in monitoring treatment and relapse from infection. [1]
Skin testing is not reliable for the diagnosis of blastomycosis and is not commercially available.
The diagnosis of blastomycosis is more difficult in children. Children with pulmonary disease who are unable to produce sputum may require invasive procedures, such as bronchoscopy with bronchoalveolar lavage, percutaneous needle biopsy of the lung, and open lung biopsy, for diagnostic confirmation.
A leukocyte and differential count may show leukocytosis with a left shift, particularly in cases with a pneumonic presentation; however, the test has low sensitivity and specificity. Pulse oximetry is appropriate in detecting hypoxemia in cases that present as pneumonia. Arterial blood gases are indicated in patients with tachypnea, pulmonary infiltrates, and hypoxemia by pulse oximetry.
Chest radiography findings vary and lack diagnostic specificity. Other imaging studies or bronchoscopy may be indicated in select situations.
In patients with extrapulmonary disease, percutaneous needle or surgical biopsy of the affected area (eg, skin, subcutaneous nodule, bone) may be helpful. Histology and culture of biopsy specimens may reveal the organism. DNA probe may be useful in identifying B dermatitidis in formaldehyde-fixed tissue samples. [45]
Diagnosis of central nervous system blastomycosis is difficult. Lumbar puncture and cerebrospinal fluid (CSF) analysis may demonstrate a neutrophil predominance, but this is rarely definitive. Ventricular fluid specimens have provided slightly higher rates of culture positivity but are still not sensitive. In one case series, CSF culture identified only 2 of 22 patients with confirmed blastomycosis meningitis. [46]
Prostatic massage may be necessary to facilitate diagnosis in men with blastomycosis of the genitourinary tract. The urine collected after a prostatic massage is likely to have a higher diagnostic yield.
Sputum Examination
Sputum microscopy is a simple and inexpensive test to identify Blastomyces, and although the overall sensitivity of this test is modest (< 40%), the potential for rapid identification of the pathogen makes it a reasonable initial option. [44] In patients with pneumonia or acute respiratory distress syndrome, the sensitivity is much higher (approximately 75%).
Sputum microscopy is performed by placing a small sample of freshly expectorated sputum on a slide digested with 10% potassium hydroxide. Under the microscope, yeasts 8-20 μm in size, with single, broad-based buds, double refractile walls, and multiple nuclei, are extremely characteristic of B dermatitidis.
Microscopic examination of a potassium hydroxide wet mount can also be performed on aspirated pus, fistulae, or subcutaneous abscesses. These will reveal the characteristic broad-based budding yeast. Identification of B dermatitidis by calcofluor white staining under a fluorescent microscope is an easy and rapid method of diagnosis. Calcofluor white is a fluorochrome compound that binds to chitin present in the cell walls of B dermatitidis and fluoresces when exposed to short-wavelength ultraviolet light from a fluorescent microscope.
Culture
Culture and cytopathology are the gold standard for the diagnosis of blastomycosis. Isolation and identification of the organism in an appropriate laboratory culture medium provides absolute confirmation of the diagnosis. The organism can be cultured on brain-heart infusion, potato dextrose agar, potato flake agar, and Sabouraud dextrose agar at room temperature. Cultures may become positive in as few as 5 days or many as 30 days when incubated at 25°-30°C (77°-86°F). B dermatitidis colonies are creamy white and transform to a brown-gray color as hyphae grow.
B dermatitidis mold has a distinctive “lollipop” appearance with oval conidia, 2-4 μm in diameter at the tips of thin conidiophores. They also have thin septate hyphae, 1-2 μm in diameter.
Specimens for culture may consist of sputum, tracheal aspirates, bronchoalveolar lavage fluid, tissue biopsy samples, cerebrospinal fluid, or urine. Because colonization with B dermatitidis does not occur, detection of the fungus from any sterile site is diagnostic.
Primary cutaneous blastomycosis has been reported by accidental autoinoculation, therefore, clinical laboratory personnel and pathologists should be notified about the possibility of blastomycosis in the differential diagnosis if they fall ill when handling potential infected tissue or body fluid specimens.
Skin Tests and Serodiagnosis
Skin testing and serodiagnosis of blastomycosis using complement fixation (CF) antibodies and immunodiffusion (ID) precipitin bands currently have very limited roles in diagnosis because of poor sensitivity and specificity, as well as cross-reactivity with other fungi. An enzyme immunoassay with the A antigen of B dermatitidis has been shown to be more sensitive than CF and ID tests [47, 48, 49, 50] ; however, this test is not available in most commercial laboratories.
Detection of (1→3)-β-d-glucan in serum specimens is of limited benefit in patients with blastomycosis. In one case series of four patients with blastomycosis, β-d-glucan was detected in only one patient with disseminated disease. [51]
Chest Radiography
Chest radiograph findings in patients with blastomycosis vary depending on the immune status of the host but, in general, are abnormal in two thirds of cases. In the immunocompetent patient, chest radiography findings can be variable but, in many cases, demonstrate lobar or segmental airspace opacities. A focal mass is common with well-defined margins and can range from 5 to 10 cm in size. These masses may be mistaken for neoplasms. Cavitary lesions are uncommon in immunocompetent patients.
In a single-institution retrospective study (2005-2016) of 36 pediatric patients (age 0-18 years) with pulmonary blastomycosis who underwent chest imaging, the most common pattern of lung involvement was a combination of consolidations (94.4%) with bilateral lung nodules (50%) and reticulonodular opacification (41.2%). [52] The upper (70.6%) and middle (47.1%) lobes were most often affected.
However, in the immunocompromised host, cavitary lesions are more common, and disseminated disease can lead to diffuse interstitial infiltrates on the chest radiograph. Pleural effusion is uncommon, but pleural thickening adjacent to an infiltrate may be observed. Hilar or mediastinal lymph node enlargement rarely occurs.
See the image below.
Lateral chest radiograph reveals the ill-defined lingular opacity and an absence of pleural effusions.
Computed Tomography Scanning
Chest computed tomography (CT) scanning is not always necessary in the evaluation of patients with suspected blastomycosis, but this imaging modality can provide better definition of the character and distribution of abnormalities observed on a chest radiograph, and it is helpful in identifying mediastinal abnormalities and loculated pleural effusions. A head CT scan is useful in the detection of brain abscesses.
CT scanning can also be used to detect skeletal involvement in some cases of extrapulmonary blastomycosis. However, magnetic resonance imaging (MRI) is more sensitive for this purpose; radionuclide bone scanning is also an alternative imaging option.
Bronchoscopy
Flexible bronchoscopy has a higher sensitivity than sputum examination for the diagnosis of blastomycosis. One study demonstrated a positive diagnosis in 92% of patients with pulmonary blastomycosis; cultures of bronchial secretions and bronchoalveolar lavage fluid were positive in 100% and 67% of patients, respectively. [44] Bronchoscopy (with washings, brushings, and a biopsy) is indicated in the following situations:
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Absence of sputum
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Nondiagnostic sputum microscopic examination
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Undiagnosed pulmonary mass density, atelectasis, or consolidation
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Hemoptysis
Immune Deficiency Workup
Serious infection with blastomycosis has been increasingly recognized in immunocompromised hosts, especially patients with acquired immunodeficiency syndrome (AIDS). However, other fungal infections, such as progressive disseminated histoplasmosis or cryptococcal meningitis, are more likely to be opportunistic. Blastomycosis is not an AIDS-defining illness and no official recommendations regarding screening for human immunodeficiency virus (HIV) infection in patients diagnosed with blastomycosis are recognized.
Histologic Findings
The yeast forms of B dermatitidis are best visualized with a periodic acid-Schiff (PAS) stain. Methenamine silver and Papanicolaou stains are also reliable.
Demonstration of the yeasts is particularly important in blastomycosis that involves sites with squamous epithelium (eg, skin, larynx, trachea). In these tissues, the fungal infection may provoke a hyperplastic response that simulates squamous cell carcinoma. However, the hyperplastic epidermis lacks the cytologic atypia of squamous cell carcinoma.
Skin lesions of disseminated blastomycosis are characterized by the following histologic features:
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Pseudoepitheliomatous hyperplasia of the epidermis
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Intraepidermal microabscesses
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A suppurating granulomatous reaction in the dermis
Intraepidermal abscesses contain abundant neutrophils and organisms; the organisms are best visualized with the diastase-digested PAS staining procedure or with the methenamine silver stain. The yeasts are present extracellularly in the dermis or intracellularly in multinucleated giant cells. Intracellular yeasts are easily identified on routine hematoxylin and eosin (H&E)–stained sections of skin as punched-out "holes" in the cytoplasm of the giant cells. The inflammatory infiltrate is polymorphous, containing lymphocytes, histiocytes, and neutrophils.
On cytologic examination of specimens such as sputum or tissue aspirates, B dermatitidis typically appears as a round, multinucleate yeast ranging from 8 to 15 μm in diameter, with a broad-based bud.
Granuloma formation is unusual. If it does occur, it generally does not demonstrate caseation or calcification, as is typical of tuberculosis.
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Cutaneous blastomycosis.
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Lateral chest radiograph reveals the ill-defined lingular opacity and an absence of pleural effusions.
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Composite photomicrograph of a tissue specimen from a patient with blastomycosis infection shows an abundance of large budding cells that had been configured in chains. Courtesy of CDC/Dr. Lucille K. George.
