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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 ...