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North American blastomycosis is a pulmonary or disseminated fungal infection caused by Blastomyces dermatitidis.1,2 Although rare in children,3 the infection is often difficult to detect unless considered in the differential diagnosis. Blastomyces dermatitidis is a dimorphic fungus that exists as a mold in nature and is generally acquired through the inhalation of spores that transform to yeast in the lungs. Although isolation from natural sources has been very difficult, growth appears to occur in acidic soil in which there is decaying organic matter and high humidity. Cases of blastomycosis are reported from other countries (particularly those in central Africa), but the vast majority of cases have occurred in the Ohio and Mississippi river basins and the southeastern United States. The highest incidence of cases appears to occur in Wisconsin, Minnesota, Mississippi, Kentucky, Tennessee, and Arkansas. In endemic areas, the annual incidence of symptomatic infection is about 1 to 2 per 100,000 population. Pockets of hyperendemic regions exist where the annual incidence of symptomatic infection may approach 40 per 100,000 population.

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Although it is clear that asymptomatic infections occur, the distribution and extent have not been determined, because reliable skin tests or seroepidemiologic methods are not available. When careful immunologic studies are performed in reported outbreaks of blastomycosis, as many as 50% of infected individuals are asymptomatic. Most cases of symptomatic blastomycosis occur sporadically, but there are occasional reports of small outbreaks in communities in which as many as 15 individuals may contract infection over a short period of time. The largest reported outbreak involved 46 school-aged children and 2 adults who were infected at a camp in Wisconsin following exposure to a beaver dam and lodge. There is no seasonality to B dermatitidis infections, and infections have been reported in all age groups, including newborns.4 In large surveillance studies of confirmed cases of blastomycosis, pediatric patients age 19 years or less comprise 3% to 11% of all identified cases. However, the typical patient is a male, age 25 to 50 years, who has an outdoor lifestyle. The incubation period from exposure to primary disease is 21 to 106 days (median 45 days). However, latency with eventual reactivation disease is probable with the finding of newly recognized infection in individuals with no exposure to endemic areas for 3 or more years. Human-to-human transmission is rare.

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The lungs are the usual portal of entry for B dermatitidis conidia. Inhaled conidia elicit an inflammatory response characterized by polymorphonuclear leukocytes (PMNs). The few conidia that survive the initial PMN phagocytosis transform to yeast, which are more resistant to phagocytosis by PMNs and alveolar macrophages. Response to the replicating yeast cells results in a mononuclear infiltrate with some granulomatous component. Spread of yeast from the lungs, although rare, may seed any body organ. Development of cell-mediated immunity is believed to be the primary mechanism in prevention of progressive blastomycosis, and lymphocyte reactivity is a marker of specific cellular immunity to ...

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