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Blastomycosis ( Blastomyces dermatitidis and Blastomyces gilchristii )
Etiology
Blastomyces dermatitidis and Blastomyces gilchristii belong to a group of fungi that exhibit thermal dimorphism. In the soil (22-25°C [71.6-77°F]), these fungi grow as mold and produce spores, which are the infectious particles. Following soil disruption, aerosolized mycelial fragments and spores inhaled into the lungs (37°C [98.6°F]) convert into pathogenic yeast and cause infection. In addition to B. dermatitidis and B. gilchristii, 4 additional species have been recently identified including B. percursus, B. helicus, B. parvus, and B. silverae .
Epidemiology
B. dermatitidis and B. gilchristii cause disease in immunocompetent and immunocompromised children. Only 2–13% of blastomycosis cases occur in the pediatric population (average age: 9.1-11.5 yr; range: 19 days to 18 yr). Blastomycosis of newborns and infants is rare. In North America, the geographic distribution of blastomycosis cases is restricted to the Midwest, south-central, and southeastern United States and parts of Canada bordering the Great Lakes and Saint Lawrence River Valley. In these geographic regions, several areas are hyperendemic for blastomycosis (e.g., Marathon and Vilas Counties, Wisconsin; Washington Parish, Louisiana; central and south-central Mississippi; Kenora, Ontario). Outside of North America, autochthonous infections have been reported from Africa (≈100 cases) and India (<12 cases). B. dermatitidis is not considered endemic to the Middle East, Central America, South America, Europe, Asia, or Australia. In North America, Blastomyces grows in an ecologic niche characterized by forested, sandy soils with an acidic pH that have decaying vegetation and are near water. Most Blastomyces infections are sporadic; however, more than 17 outbreaks have been reported, and most of these outbreaks have involved pediatric patients. Outbreaks are associated with construction or outdoor activities (camping, hiking, fishing, rafting on a river, using a community compost pile); however, some outbreaks have no identifiable risk factors other than geography. The severity of infection is influenced by the size of the inhaled inoculum and the integrity of the patient's immune system. Those immunosuppressed by solid organ transplantation, AIDS, and tumor necrosis factor-α inhibitors are at risk for developing severe or disseminated infection.
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