Blastomyces dermatitidis
Post By: Anil Pokhrel
Blastomyces dermatitidis is a thermally dimorphic fungi originally isolated from cutaneous lesions. The teleomorph of Blastomyces dermatitidis is referred to as Ajellomyces dermatitidis. At 25°C-30°C, Blastomyces grows as a mold form with conidia and septate hyphae usually within 14 days; some strains grow more slowly. Mould form produces abundant conidia from aerial hyphae and lateral conidiophores. These conidia are spherical, ovoid, pyriform measuring 2-10 µm in diameter, resembling the macroconidia of Histoplasma capsulatum. The conidia of Blastomyces are smooth. Thick walled chlamydospores 7-18 µm in diameter also may be observed in Blastomyces dermatitidis.
B. dermatitidis reproduces in tissues by budding and the typical bud is characterized by its large size, attachment to the parent cell by a persistent wall. The persistence of the bud attachment and the tendency of the parent cell to continue budding often results in small clusters of cells with an additional bud arising from the parent cell, near the point of attachment of the first bud. Multiple budding therefore, is found in Blastomyces dermatitidis. The cells of B. dermatitidis are multinucleate, but those of H. capsulatum and Cryptococcus neoformans are uninucleate.
The yeast-like form of growth can be maintained in vitro by incubation on an enriched medium such as blood agar or even on Sabouraud’s agar at a temperature of 37°C. On Sabouraud’s agar and other similar suitable media, at 25°C-30°C, B. dermatitidis grows as a white mold. Some strains grow slowly, do not spread widely over the agar surface and sporulate sparsely, other strains spread more widely reaching a colony diameter of 5-6 cm in 2 weeks, present a powdery or granular surface and sporulate freely. Some strains become brown or produce a diffusible dark pigment.
Conidia are smooth walled, spherical or oval, measuring 2-10 µm. They are borne on lateral, slender or cylindrical conidiophores or terminally on hyphal branches. On enriched media at 37°C, B. dermatitidis grows as yeast with colonies that are folded, pasty and moist. Yeast in vitro is thick walled and spherical, they produce single bud with a characteristically broad base of attachment between the bud and the parent cell.
Benign respiratory clinical forms of blastomycosis may occur, but their frequent occurrence has not yet been proved. Mild self-limited illnesses have not been diagnosed, and the evidence one might expect from intradermal testing in survey studies is obscured in occurrence without evidence of transmission to the patient’s associates.
Blastomyces infection occurs at all ages with slightly higher incidence in 30s-50s of ages. It is seen in males more frequently than females. No association with specific occupational exposure is recognized.
This mycosis occurs mostly in North America, particularly in the St. Lawrence, Ohio, and Mississippi river valleys, and in Africa, but it is not limited to these areas; it is also reported from some parts of Asia.
Blastomycosis is relatively common in dogs and rarely other animals in endemic areas (Jawetz). Humans are infected by inhalation of conidia from colonies of mold growing as saprobes in soil or similar substrates; however, few attempts to isolate B. dermatitidis from soil have been successful, and these have not revealed a consistent pattern of distribution or a characteristic ecological association.
The conidia of B. dermatitidis are inhaled into the lungs, where the organisms transform into the yeast phase. The yeast is phagocytosed by macrophages and neutrophils (Chester’s). The fungus initially multiplies in the lung and causes pneumonia-like symptoms; however, the hallmark of blastomycosis is the development of skin lesions, which arise due to hematogenous spread. Cell-mediated immunity plays an important role in the eradication of the agent, but neutrophils also play a role in containing organisms. The histopathological changes reflect the granulomatous and pyogenic response. Yeast cells can remain viable within granulomas for years and can be a source of reactivation later on.
4.1 Blastomycosis: Definition
Blastomycosis is a chronic granulomatous and suppurative disease which originates as a respiratory infection and disseminates, usually with pulmonary, osseous, and cutaneous involvement. It’s also called Gilchrist’s disease.
4.2 Clinical Types of Blastomycosis
i) Pulmonary Blastomycosis
Pulmonary blastomycosis usually begins as a mild respiratory infection which progresses with a dry cough, pleuritic pain, hoarseness, and low-grade fever. Within weeks or months, sputum increases and becomes purulent and blood-streaked, and there is increased temperature elevation, dyspnea, and loss of weight, night sweating. The disease, if untreated, usually disseminates and often progresses to death.
ii) Disseminated Blastomycosis
When disseminated, the disease is manifested in the skin, oro-nasal mucosa, and subcutaneous tissues, respiratory tract, urogenital tract, osseous system, and CNS. In contrast to paracoccidioidomycosis, the GI tract is rarely involved. Bone lesions are manifested by pain, osteomyelitis, and peritonitis. Destructive lesions of vertebrae, tibia, and femur are frequent. Compression of the spinal cord may result from vertebral mycosis; in addition, meningitis and brain abscess may occur. Granuloma and abscess of the liver and spleen are rarely found. The commonest site of involvement of the urogenital system are the prostate, epididymis, and testis, characterized by dysuria, pyuria, and hematuria.
iii) Cutaneous Blastomycosis
Cutaneous blastomycosis may originate as a subcutaneous nodule or as a papule or pustule which ulcerates. A cutaneous lesion may develop as an ulcer on an exposed skin surface at the site of injury. Even though most cutaneous lesions are the result of hematogenous spread from a pulmonary or other visceral organ lesion to a cutaneous site, the possibility that such is the result of local implantation cannot be ruled out. The cutaneous lesions develop within weeks or months into ulcerated or verrucous granulomas with an advancing border elevated to 1-3 mm with an abruptly sloping outer border. The central area of the lesion may be verrucous or ulcerated and covered with crust. The base of the lesion is granulomatous and the elevated border is indurated, dusky, red, or violaceous. Characteristic budding cells of B. dermatitidis can be demonstrated in the pus from these abscesses. Lesions slowly spread over the face, often deforming the features and leaving a scar over the central area of old lesions.
5.1 Specimens
Sputum, bronchoalveolar lavage, tracheal aspirates, lung biopsy, blood, bone marrow, lesions of mucocutaneous abscess.
5.2 Diagnostic Procedures
i) Direct Microscopy
Specimens such as concentrated sputum, pus, etc. can be examined directly after treating with KOH or calcofluor white or both for visualization of yeast.
ii) Histopathology
Histopathological examination of tissue specimens obtained from biopsy by H and E, PAS, GMS, Giemsa or Wright staining technique for pathological change and to demonstrate fungus.
iii) Culture
Specimen is inoculated on Sabouraud’s agar slant and incubated at 25°C-30°C for mold form isolation. If bacterial contaminants are suspected, media can be incorporated with antibiotics such as penicillin, streptomycin, or chloramphenicol, and for contaminating saprophytic fungi, cycloheximide can be used. For isolation of yeast form, specimen can be inoculated into glucose blood agar and incubated at 37°C. Normally sterile specimens such as CSF may be directly inoculated into blood agar, BHI agar/broth, SGA, etc. B. dermatitidis grows rather slowly, and cultures should be held for a month before they are discarded as negative.
iv) Exoantigen Detection
The exoantigen detection technique is a simple diagnostic method that detects the presence of cell-free antigens, known as exoantigen, which are produced by the mycelial form cultures. A specific exoantigen detected in the aqueous extract of a mycelial culture may identify any of the systemic dimorphic fungi.
v) Nucleic Acid Probes
The use of nucleic acid probes is an important advancement in the identification of dimorphic fungi from cultures. Probes for the identification of B. dermatitidis, H. capsulatum, and Cryptococcus immitis are commercially available.
Direct calcofluor white or KOH mount of sputum exudates and tissues can demonstrate the yeast of B. dermatitidis, which are large, spherical, and thick-walled and measure approximately 8-15 µm in diameter. The yeast cells bud singly and have a wide base of attachment between bud and parent cell. The bud of B. dermatitidis attains the same size as the parent yeast before becoming detached. When specimens are cultured at 25°C-30°C, B. dermatitidis initially produces fluffy, white colonies on routine mycological media. Some strains develop tan, glabrous colonies without conidia, and others may produce light brown colonies with concentric rings. 2-10 µm conidia measuring 2-10 µm in diameter are produced in short or long terminal or lateral hyphal branches. Conidia are typically spherical, ovoid, or pyriform. Thick-walled chlamydospores 7-18 µm in diameter also may be observed in old cultures. At 37°C, yeast forms grow as white or light brown, wrinkled colonies. In vitro or in tissue, yeast is thick-walled and spherical; they produce a single bud with a characteristically wide base of attachment between parent cell and bud, but buds with narrow attachment may also be seen.
Severe cases of blastomycosis are treated with amphotericin B. In patients with confirmed lesions, a 6-month course of itraconazole is the treatment.
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