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 appears as large round or oval cells, ranging in size from 8 to 15 µm. The cells are surrounded by a thick double wall. Reproduction occurs by budding, with the key differential diagnostic criterion being that the budding cell (always only a single bud is formed) features a broad base (broad neck).
- Mycelial phase: forms when cultured on nutrient media at room temperature (20–25 °C). The fungus produces thin (1–3 µm) colorless, septate hyphae. This phase is characterized by sporulation with the formation of numerous 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 Modes of Transmission
The natural reservoir of infection in nature is soil, as well as various decomposing plant debris. The main 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 countries in Central and South America and is also frequently found in Africa, whereas it is registered significantly less often in other regions of the globe. From the moment spores penetrate 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 pathways, two main clinical forms of the disease are distinguished:
- Visceral form (pulmonary route): is the primary variant of disease development. After inhalation, spores enter the lungs, where they transform into the aggressive yeast form. Primary lung involvement develops in the form of pneumonia; marked infiltrates, specific granulomas, areas of suppuration and necrosis are formed, up to the development of cavities. With further progression of the process, dissemination of the pathogen occurs via the bloodstream, leading to secondary involvement of bone tissue, liver, spleen, central nervous system, and urogenital organs.
- Cutaneous form (traumatic route): occurs as primary cutaneous blastomycosis upon direct entry of the pathogen into a wound following trauma. Papulopustular elements and tubercles form on the skin. Subsequently, they transform into deep ulcers characterized by overhanging edges. A characteristic bloody-purulent discharge exudes from the affected areas. The pathological process may also spread to adjacent mucous membranes.
Laboratory Diagnostics
Due to the high risk of aerogenic infection from inhaling spores, all work with clinical material must be performed exclusively inside a specialized laminar flow cabinet. Pus, sputum, urine, biopsy specimens of affected tissues, and scrapings from ulcer edges are collected for examination.
- Microscopic method: native or stained preparations (using hematoxylin-eosin, Gram–Weigert stain, PAS reaction, acridine orange) reveal large double-walled yeast-like cells with characteristic broad-based budding.
- Culture method: pathological material is inoculated onto Sabouraud agar. Growth of grayish-yellow colonies with a cotton-like texture 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.
- Allergic diagnostics: includes intradermal skin tests. Blastomycin—a purified extract of fungal cells—serves as the allergen.
Antifungal medications are used for the specific treatment of blastomycosis, with amphotericin B and itraconazole being the drugs of choice.