Blastomyces dermatitidis: General Characteristics, Pathogenesis, Clinical

Blastomyces dermatitidis: General Characteristics, Pathogenesis, Clinical Findings, Laboratory Diagnosis, Epidemiology, Prevention and Control

Introduction of Blastomyces dermatitidis

Blastomyces dermatitidis is a thermally dimorphic fungus that causes blastomycosis, a lung infection that is chronically invasive and spreads into the skin and bones, affecting both humans as well as animals.

Taxonomic Classification

(Described by van Tieghem in 1876)
Kingdom: Fungi
Phylum: Ascomycota
Class: Euascomycetes
Order: Onygenales
Family: Onygenaceae
Genus: Blastomyces

Species: Blastomyces dermatitidis

Habitat of Blastomyces dermatitidis

Naturally, B. dermatitidis are present in soil and organic matter such as animal waste, fragments of plants, leftover insects, and dust. It thrives in cold, dark areas with organic debris and a pH value of 6.0. In North America, including the USA and Canada, they are prevalent, especially in the Mississippi, Ohio, and Missouri valleys that have the highest occurrences of blastomycosis infections.

Morphology and Cultural characteristics of Blastomyces dermatitidis

Blastomyces dermatitidis are dimorphic fungi that grow both as molds and as yeast at 37 ° C, forming asexual spores known as conidia or large chlamydospores. When the fungi are growing in mycological cultures, mold forms are produced, while the yeast forms are produced when the fungi grow in host tissues and some specialized culture media. At room temperature, Blastomyces dermatitidis grows on the Sabouraud agar, developing white or brownish colonies. The branched hyphae that hold spherical, ovoid, or piriform conidia (3-5 μm in diameter) are composed of these colonies. The slender terminal or lateral conidiophores are retained by the conidia. Larger chlamydospores (7–18 μm) may also be formed in SDA media. Blastomyces dermatitidis develops as a thick-walled, multinucleated, spherical yeast (8–15 μm) forming single buds in host tissues or culture at 37 °C. With a large foundation, the bud and the parent yeast are attached, and the bud sometimes grows to the same size as the parent yeast until they become separated. The colonies of yeast are wrinkled, waxy, and soft.

Pathogenesis of Blastomyces dermatitidis

Virulence Factors of Blastomyces dermatitidis

The fungi are known to generate a known weak Blastomyces dermatitidis antigen. As blastomycin, with signs that can be identified with complement fixation. Large titers of complement fixation in patients with infections with blastomycosis are seen. The fungal infection also causes antibodies to be developed in antisera. The antigen of B. dermatitidis is known as antigen A. In causing the fungal infection, these antigens were related to the virulence of the fungi, although there is no evidence to support the levels of virulence induced by these antigens. The fungi generate small blastospores that are light and thick-walled, allowing them to be easily transported by air and thus easily inhaled. The thick-walled fungal spores cause the host tissues to be easily adhered to and colonized.

Transmission of Blastomyces dermatitidis

Transmission of B. dermatitidis spores by inhalation through the lungs, from dirty soil and debris. Spore exposure is common from excavation, building, digging, or airborne exposure is due to clearing wood. Fungal spores seldom invade wounds that are open. There is no evidence of human-to-human or animal-to-human transmission.

Clinical Features of Blastomycosis

When blastospores are inhaled into the lungs, the initial infection begins, spreading quickly to the skin and other areas of the body. Pulmonary infiltration, associated with acute lower respiratory infections, including fever, malaise, night sweats, cough, and myalgia, is the most common clinical manifestation. Chronic pneumonia may also be present in patients. Different pyogranulomatous reactions to neutrophils and non-caseating granulomas are shown by histological proof. The development of skin lesions that can develop to ulcerated verrucous granulomas with bordering and centralized scarring shows dissemination on the skin. Microabscesses with rough sloping edges fill the boundaries of the skin lesions. Diseases can also spread to the bones and genitals (prostate, epididymis, and testis) and to the central nervous system. In three clinical types, the disorder blastomycosis occurs:

cutaneous disease
pulmonary disease
disseminated disease

Laboratory Diagnosis of Blastomycosis

Specimen: It depends on the site infection and the most common specimens are sputum, pus, and tissue biopsies.

Direct Examination

Potassium hydroxide (KOH) mount: It shows the presence of yeast cells, which are 3-5 µm in diameter.

Culture Characteristics

Blastomyces dermatitidis are dimorphic fungi that grow both as molds ( 25°C) and as yeast at 37 ° C. Sabouraud Dextrose Agar (SDA) forms white or brownish colonies at room temperature ( ( 25°C) as shown above image.

LPCB preparation

LPCB preparation from the plate incubated at 37°C shows yeast-like cells which are thick-walled, multinucleated, and spherical shaped while from the plate incubated at 25°C shows one-celled, smooth-walled conidia borne on short lateral to terminal hyphal branches as shown above images.

Histological Examination

Following stains are useful for identifications of this fungus and they are-

Hematoxylin and Eosin(H &E) stain: It uses to observe neutrophils and pyogranulomatous reactions due to neutrophilic interactions of the granuloma from the host tissues. Hematoxylin stains the nuclei of cells blue to bluish-purple, and eosin stains the cellular elements in the tissues from pink to red as shown above picture.

Gomori’s methenamine silver stain (GMS): It stains the yeast cell wall deep black and the interior of the yeast cells are rose-colored, while the background is green.

Periodic acid-Schiff (PAS) Stain: It stains the yeast cells red with a pink background or light green, identified by the type of counterstain that is used. The histological stains may show small yeast cells (2-10 µm in diameter) to large yeast cells (25-40 µm in diameter) with hyaline short septate hyphae.

Immunological Diagnosis

Complement fixation test (CFT) for detecting the blastomycin antigen; high CF titers indicate the presence of Blastomyces dermatitidis antigen. Immunodiffusion is used for the detection of B. dermatitidis antigen A. Skin test for detection of blastomycin. ELISA for detection of antibodies against B. dermatitidis antigen A.

Treatment of Blastomycosis

Amphotericin B, itraconazole, or ketoconazole are the drugs of choice for treatment. Amphotericin B should be preferred particularly in immunocompromised patients while mild pulmonary blastomycosis clears spontaneously and does not require antifungal therapy. Surgery may be necessary for the drainage of large pulmonary abscesses along with antifungal therapies.

Epidemiology

In North America (the United States and Canada), the disease is primarily endemic, so it has been coined as North American Blastomycosis. However, other continents, including Africa, South America, and Asia, have discovered it. As a dimorphic fungus, B. dermatitidis develops as a mold, forming hyaline branched hyphae as a large single budding yeast at 37 ° C in the laboratory and in human tissues at room temperature in the laboratory and in the community. There are four strains that have been sequenced from Blastomyces dermatitidis, namely: Blastomyces dermatitidis SLH14081, Blastomyces dermatitidis ER-3, Blastomyces dermatitidis ATCC 18188, and Blastomyces dermatitidis ATCC 26199, with the SLH-14081 strain being the most virulent pathogen isolated from samples of immunocompetent persons. In addition, recent research has shown that B. dermatitidis in immunocompromised individuals can cause infection, so it has been identified as an emerging opportunistic pathogen.

Prevention and Control of Blastomyces dermatitidis

As this disease does not spread from person to person, there are no infection control concerns. No vaccine is available and no prophylaxis is recommended there.

Key Notes

The most commonly used dimorphic medium is brain heart infusion (BHI) agar with blood.
This organism, Blastomyces dermatitidis comes in risk group III so be careful while processing the specimens.

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