Blastomycosis

Morphology and thermal dimorphism

The causative agent of the disease, Blastomyces dermatitidis, belongs to dimorphic fungi. This means that its morphological characteristics change radically depending on environmental conditions and temperature.

• Tissue (yeast) phase: forms directly within the host organism or upon cultivation on nutrient media in an incubator at 37 °C. In this phase, the fungus is represented by large round or oval cells ranging from 8 to 15 µm in size. The cells are surrounded by a thick double wall. Reproduction occurs by budding, with the crucial differential-diagnostic criterion being that the budding cell (always forming only a single bud) has a broad base (broad neck).
• Mycelial phase: forms when cultured on nutrient media at room temperature (20–25 °C). The fungus forms thin (1–3 µm) colorless, septate hyphae. This phase is characterized by sporulation with the formation of multiple lateral round or pear-shaped conidia measuring 2–10 µm. As the culture ages, the fungus begins to form large chlamydospores with a diameter of 7–18 µm.

Epidemiology and transmission routes

The natural reservoir of the infection in nature is soil, as well as various decomposing plant residues. The primary mechanism of transmission is aerogenic. Human infection occurs via the airborne-dust route as a result of inhaling microscopic fungal spores along with contaminated dust.

Geographically, the disease is endemic to Central and South America and is also frequently found in Africa, whereas it is registered significantly less often in other regions of the planet. From the moment spores enter the respiratory tract until the appearance of the first clinical signs, an incubation period elapses, lasting on average from one to four months.

Pathogenesis and clinical forms

Depending on the portal of entry and dissemination routes, two main clinical forms of the disease are distinguished:

1. Visceral form (pulmonary route): is the main variant of disease development. After inhalation, spores enter the lungs, where they transform into the aggressive yeast form. A primary pulmonary lesion develops in the form of pneumonia; pronounced infiltrates, specific granulomas, areas of suppurative inflammation and necrosis are formed, up to the formation of cavities. With further progression of the process, dissemination of the pathogen occurs via the bloodstream, leading to secondary lesions of bone tissue, the liver, spleen, central nervous system, and urogenital organs.
2. Cutaneous form (traumatic route): occurs as primary cutaneous blastomycosis when the pathogen directly enters a wound following trauma. Papular-pustular elements and nodules form on the skin. Subsequently, they transform into deep ulcers, whose distinguishing feature is overhanging edges. A characteristic bloody-purulent discharge is secreted from the affected areas. The pathological process may also spread to adjacent mucous membranes.

Laboratory diagnostic methods

Due to the high risk of aerogenic infection from inhaling spores, all work with clinical material must be carried out exclusively inside a specialized laminar flow cabinet. Pus, sputum, urine, biopsies of affected tissues, and scrapings from the edges of ulcers are collected for examination.

• Microscopic method: native or stained preparations (using hematoxylin-eosin, Gram-Weigert staining, PAS reaction, acridine orange) reveal large double-walled yeast-like cells with characteristic broad-based budding.
• Culture method: pathological material is inoculated onto Sabouraud medium. Growth of grayish-yellow cottony colonies is considered a positive result.
• Serological method: aimed at detecting specific antibodies using the complement fixation test (CFT) and other techniques. The immunofluorescence assay (IFA) is used for rapid diagnostics.
• Allergy diagnostics: includes skin allergy testing. The allergen used is blastomycin—a purified extract of fungal cells.

Antifungal drugs are used for the specific treatment of blastomycosis, with amphotericin B and itraconazole being the drugs of choice.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of the PAS reaction (periodic acid–Schiff) in detecting fungal infections
• Mechanisms of granuloma formation and necrosis areas in visceral blastomycosis
• Differential diagnosis of Blastomyces dermatitidis colonies on Sabouraud medium
• Principle of the acridine orange method in mycological diagnostics
• Pathogenesis of the transition of fungi from the mycelial to the tissue phase upon entering a macroorganism

Other topics in the section «Systemic endemic mycoses (histoplasmosis, blastomycosis, coccidioidomycosis, cryptococcosis)»

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