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Blastomycosis is a granulomatous fungal infection caused by B. dermatitidis. B. dermatitidis is a dimorphic fungus, which occurs in two stages: as mold in soil and as yeast in tissue. On culture at 37°C and in tissue, the yeast is a round struc-ture with a double refractile wall and a single broad-based bud. This appearance helps to differentiate it from the Cryptococcus neoformans yeast, which has a narrow-based bud.On culture at 25°C, the fungus produces a mycelial growth showing typical pyriform microconidia, which measure 2–4 mm in diameter.
B. dermatitidis infection occurs by inhalation of microconidiafound in soil, its natural habitat. Inside the lungs, the conidia are transformed to the yeast phase at body temperature. The thick cell wall of the yeasts offers resistance to phagocytosis and induces expression of blastomyces adhesin 1 (BAD1), an immune-modulating virulence factor on the cell surface. Then, the yeasts replicate and may disseminate through the blood and lymphatics to other organs to cause the disease.
Pulmonary infection is the major clinical manifestation. A flu-like illness with fever, chills, myalgia, headache, and a nonproductive cough may occur, which resolves within days. In some cases, it may develop to a progressive pulmo-nary disease or may result in extrapulmonary dissemination. Extrapulmonary dissemination to other organs, more often, occurs in patients with chronic pulmonary illness and in those who are immunocompromised. The skin is the most common site of extrapulmonary blastomycosis.
Blastomycosis is documented from the United States and also from Canada and the African continent. In endemic areas, blastomycosis is a common infection among dogs. It is also reported in horse, cow, cat, bat, and lion. Microconidia are the infective stages that are present in the soil. Infection is transmitted to humans through inhalation of aerosolized conidia.
Amphotericin B and itraconazole are the drugs of choice for treatment of blastomycosis.
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