Blastomyces dermatitidis: Morphology, Pathogenesis, and Lab Diagnosis

A hunter from the Ohio River valley has a cough that will not clear and a spreading skin ulcer. A silver stain of the skin biopsy shows the answer: large, round yeast cells, each throwing off a single daughter cell attached by a wide, flat base. That broad-based bud is the signature of Blastomyces dermatitidis. Reading it correctly, and not mistaking its thick wall for the capsule of Cryptococcus, is what points straight to the diagnosis.

Blastomyces dermatitidis is a thermally dimorphic fungus. It grows as a mold (mycelial form) in the environment at 25 to 30°C and converts to a thick-walled, broad-based budding yeast in tissue at 37°C. It causes blastomycosis, a chronic granulomatous and suppurative disease also called Gilchrist disease or North American blastomycosis. The single most useful identifying feature is its appearance in tissue: a large yeast with a broad-based bud and a thick, double-contoured cell wall.

B. dermatitidis grows as a mold in warm, moist wooded terrain rich in decaying wood, leaves or debris. Geographically, it is endemic to North America, including the Ohio and Mississippi River valleys and Canada.  Blastomycosis cases have also been documented in India, Africa, and Central and South America.

Morphology and how to recognize it

In tissue (the yeast form)

In tissue and at 37°C, Blastomyces dermatitidis appears as a large, round to oval yeast, about 8 to 15 micrometers across. Three features identify it:

1. Broad-based budding. The daughter cell (bud) is attached to the parent by a wide, flat base, not a narrow neck. This is the defining feature and the one to look for first.
2. A thick, double-contoured cell wall. The wall is thick and refractile, so it looks like a double outline around the cell.
3. Multiple nuclei. The yeast cells are multinucleate, unlike the single-nucleus yeast of some other fungi.

Important: it is a thick cell wall, not a capsule

Blastomyces does not have a capsule. What looks like a bright rim around the cell is a thick, double-contoured cell wall. This matters because the capsule is the feature of Cryptococcus, and confusing the two is a common and consequential error. The rule to hold onto: broad-based budding with a thick wall is Blastomyces; narrow-based budding with a true clear capsule is Cryptococcus.

In the environment (the mold form)

At 25 to 30°C the mold form shows delicate septate hyphae (1 to 2 micrometers wide) bearing oval or pear-shaped (pyriform) single conidia, 2 to 4 micrometers, held on short stalks. These conidia sitting at the tips of conidiophores are often described as looking like lollipops.

Telling it apart from look-alike yeasts

For a full feature-by-feature key to reading fungi under the microscope, see the guide to microscopic identification of fungi.

Figure: Biology of Blastomycosis

Human infection results following the inhalation of airborne conidia when soil containing microfoci of mycelia is disturbed. Men are affected more often than women, with the ratio of men to women ranging from 4 : 1 to 15 : 1.  Although blastomycosis affects immunocompromised individuals, it can affect healthy individuals as well. Infections often develop in people who have contact with soil, work outside, or frequently engage in outdoor activities such as campers, outdoorsmen and hunters.  Some cases of human-human transmission have been noted.

Pathogenesis

Virulence factors

Blastomyces dermatitidis causes disease through a small set of factors, each tied to what it does for the organism.

1. Thermal dimorphism (mold-to-yeast conversion). What: the switch from the inhaled mold form to the tissue yeast form at body temperature. Why: this conversion is the crucial step in infection. The yeast form is far more resistant to being killed than the inhaled conidia, so the switch is what lets the organism establish disease.
2. Thick cell wall. What: a thick, double-contoured cell wall (rich in alpha-1,3-glucan). Why: it resists phagocytosis and killing by macrophages and neutrophils. This is the antiphagocytic feature, and it is a cell wall, not a capsule.
3. BAD-1 (Blastomyces adhesin-1, formerly WI-1). What: a protein on the yeast cell surface and secreted by it. Why: it is the key virulence factor. It binds the yeast to macrophages and lung tissue, blocks complement from depositing on the yeast surface, and suppresses the host's pro-inflammatory cytokines. Loss of BAD-1 sharply reduces the organism's ability to bind cells and cause disease.

Pathogenesis: putting it together

The sequence starts with inhalation. Airborne conidia are breathed in when soil rich in the mold is disturbed. In the lung, the conidia convert to the yeast form (attach and establish). The thick cell wall and BAD-1 then let the yeast resist phagocytosis and evade complement and cytokine responses (resist defenses). Yeast proliferating in the alveoli signals that natural resistance has failed, and the organism damages lung tissue, producing pneumonitis. If defenses do not contain it, the yeast spreads through blood and lymph (spread) to the skin, bone, genitourinary tract, and central nervous system. As immunity develops, pyogranulomatous lesions and noncaseating granulomas form at the infected sites.

Cell-mediated immunity is the key defense that limits blastomycosis. Humoral (antibody) responses do not play a significant protective role.

Clinical signs and symptoms

Blastomycosis begins in the lungs and can then spread. The disease falls into two categories: pulmonary and extrapulmonary (disseminated).

1. Pulmonary blastomycosis
   • The lung is involved in the great majority of cases, and the presentation varies widely:
   • Asymptomatic or self-limited infection, in about half of cases, where the immune response contains the organism.
   • Acute pneumonia, with cough, sputum, chest pain, fever, and shortness of breath. This closely mimics bacterial pneumonia, so it is often misdiagnosed and treated with antibiotics that do not work, which delays the correct diagnosis.
   • Chronic pneumonia, which can mimic tuberculosis or lung cancer, with a persistent cough, weight loss, and abnormal chest imaging.
   • Severe diffuse pneumonia, which can progress to acute respiratory distress syndrome (ARDS) and is life-threatening, more often in immunocompromised patients.
2. Extrapulmonary (disseminated) blastomycosis
   • If immunity fails to contain the lung infection, the organism spreads through the blood and lymphatics to other sites:
   • Cutaneous disease: verrucous (wart-like) or ulcerative skin lesions. The skin is the most common extrapulmonary site.
   • Osseous disease: osteomyelitis with abscesses and draining sinuses.
   • Genitourinary disease: involvement of the prostate and epididymis in men.
   • Central nervous system disease: brain abscess and meningitis.

Laboratory diagnosis

Laboratory diagnosis of blastomycosis is based on culture and direct visualization of round, multinucleated yeast forms that produce daughter cells from a single broad-based bud.

Sample: Sputum, bronchoalveolar lavage, biopsy, CSF, bone marrow or pus from the abscess based on the clinical manifestations.

Microscopy and staining: KOH wet mount and hematoxylin & eosin staining are used routinely to check the presence of characteristics yeast cells in the clinical specimens. Presence of characteristics double-contoured, thick-walled, multinucleated yeast forms with single broad-based budding daughter cells confirms the diagnosis but a negative result does not exclude the possibility of blastomycosis.

Figure: Blastomyces dermatitidis in Lung Fine Needle Aspiration Assay stained with PAS. The insert shows large broad-based budding as a daughter cell is being produced.(Image source:http://thunderhouse4-yuri.blogspot.com/)

Sometimes, special staining like Periodic Acid Schiff (PAS) and Gomori Methenamine Silver Stain (GMS) may be required for the identification.

Culture

Figure: Colonies of Blastomyces dermatitidis in SDA after 7 days of growth (Image source:http://thunderhouse4-yuri.blogspot.com/)

Currently, culture is the most sensitive method for the diagnosis of blastomycosis. Specimens are inoculated into general-purpose growth media, such as Sabouraud dextrose agar, potato dextrose agar, potato flake agar, or inhibitory mold agar and are incubated at 25 to 30°C for 4 to 6 weeks.

If the specimen is likely to be contaminated with saprophytic fungi and/or bacteria, selective media containing cycloheximide and antibacterial should be used. For tissue specimen use of enrichment culture medium such as brain heart infusion agar supplemented with blood increases the yield.

Growth of the B. dermatitidis will be apparent on fungal isolation media within 10-30 days. Initially, colonies are white to off-white and glabrous or waxy in appearance, which becomes gray to brown as aerial hyphae develop with age.

Figure: “lollipop-like” conidiophore-conidia structures of B. dermatitidis (Image source:http://thunderhouse4-yuri.blogspot.com/)

Microscopic feature (of mold form):

• Delicate septate hyphae measuring 1 to 2 μm in diameter
• Oval or pyriform single-celled conidia measuring 2 to 4 μm in diameter,
• Conidia are found singly at the tips of short or long conidiophores and resemble lollipops

Figure: Slightly pyriform (tear-drop) shaped conidia  of B. dermatitidis (Image source:http://thunderhouse4-yuri.blogspot.com/)

Once sufficient growth of fungi is noted chemiluminescent DNA probe can be used to confirm the isolate as B. dermatitidis. Other ways to confirm an identification of B. dermatitidis are checking conversion of the mold to the yeast form (which is very time consuming), repetitive-sequence-based PCR and nucleic acid sequencing.

Antigen Detection

A urine antigen test for B. dermatitidis is available in some reference laboratories, with high sensitivity (over 90%) but low specificity. It cross-reacts with other dimorphic fungal infections, including histoplasmosis, paracoccidioidomycosis, and talaromycosis (formerly penicilliosis), so a positive result must be interpreted alongside the clinical picture and other tests.

Antibody detection tests

Tests for the detection of antibodies to B. dermatitidis are available commercially but are not adequate.

• Complement fixation test using yeast-phase antigen: Sensitivity and specificity are too low.
• Immunodiffusion using purified B. dermatitidis A antigen: Better than complement fixation test. Sensitivity is around 65-80%.
• Enzyme immunoassays: More sensitive than immunodiffusion but less specific

Nucleic Acid Amplification

Nucleic acid amplification tests for direct detection of B. dermatitidis in clinical specimens are not in routine use.

Treatment

Treatment depends on the severity and site of infection:

• Mild to moderate disease (for example, localized pulmonary, cutaneous, or bone disease): an azole, itraconazole, is the treatment of choice.
• Severe or disseminated disease, central nervous system involvement, or immunocompromised patients: a polyene, amphotericin B, is used first, often followed by an azole (itraconazole) as step-down therapy once the patient improves.

Mild pulmonary blastomycosis can occasionally be self-limited, but treatment is generally given to prevent dissemination. Actual regimens, doses, and durations are decided by the treating clinician.

How to Remember

Broad base, think Blastomyces. The broad-based bud is the signature. A wide, flat attachment between parent and daughter cell means Blastomyces.

Wall, not capsule. The bright rim on Blastomyces is a thick, double-contoured cell wall. The capsule belongs to Cryptococcus. Broad base plus thick wall is Blastomyces; narrow base plus capsule is Cryptococcus.

BAD-1 is the bad actor. The main virulence factor is BAD-1 (formerly WI-1). It sticks the yeast to macrophages and blocks the immune response.

Lollipops in the cold, yeast in the body. The organism exhibit thermal dimorphism .In the environment (cold) the mold makes lollipop conidia. In the body (warm) it becomes broad-based budding yeast.

Itraconazole mild, amphotericin severe. Mild to moderate disease gets itraconazole. Severe, disseminated, or CNS disease gets amphotericin B.

Key exam facts

Where Students Get Confused

"Blastomyces has a thick capsule." No. It has a thick, double-contoured cell wall. The capsule is the feature of Cryptococcus. This is the single most common confusion, and the two are distinguished by broad-based budding with a wall (Blastomyces) versus narrow-based budding with a capsule (Cryptococcus).

"Broad-based and narrow-based budding are interchangeable terms." No. The width of the attachment between parent and daughter cell is a key identifier. Broad base points to Blastomyces; narrow base points to Cryptococcus or Histoplasma.

"Blastomyces forms spherules in tissue." No. It forms broad-based budding yeast. Spherules are the tissue form of Coccidioides, a different fungus.

"A negative direct microscopy rules out blastomycosis." No. Yeast cells can be sparse and hard to find. A negative smear does not exclude the diagnosis, so culture and antigen testing are still used.

"The urine antigen test confirms blastomycosis." Not by itself. It is sensitive but cross-reacts with other dimorphic fungal infections such as histoplasmosis, so it must be read with the clinical picture and other tests.

Special acknowledgement

I am grateful to yuri for the beautiful photographs of B. dermatitidis. For more images please visit http://thunderhouse4-yuri.blogspot.com/2012/12/blastomyces-dermatitidis.html

References and further readings

1. Saccente, M., and Woods, G. L. (2010). Clinical and laboratory update on blastomycosis. Clinical Microbiology Reviews, 23(2), 367–381.
2. Centers for Disease Control and Prevention. Blastomycosis. https://www.cdc.gov/blastomycosis/
3. Tille, P. M. (2022). Bailey and Scott's Diagnostic Microbiology (15th ed.). Elsevier.