Blastomycosis is a granulomatous fungal infection most commonly caused by Blastomyces dermatitidis. A second species, Blastomyces gilchristii, has been recently identified from regions in Ontario, Wisconsin, and Minnesota known to be hyperendemic for blastomycosis. Blastomycosis is rare in children, and the infection is often difficult to detect unless considered in the differential diagnosis.
PATHOGENESIS AND EPIDEMIOLOGY
Blastomyces species are thermally dimorphic fungi that exist as a mold in nature and are generally acquired through the inhalation of spores that transform to yeast in the warmer environment of the lungs. Soil exposure is likely necessary to develop blastomycosis. The broad-based budding yeast are typically 8 to 20 μm in diameter. The mycelial phase is characterized by branching septate hyphae that grow as white colonies that eventually turn light brown. 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, often in close proximity to rivers and lakes.
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 a granulomatous component. Hematogenous spread of yeast from the lungs 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 B dermatitidis.
There is no seasonality to B dermatitidis infections, and infections have been reported in all age groups, including newborns. In large surveillance studies of confirmed cases of blastomycosis, pediatric patients compose 3% to 11% of all identified cases. The incubation period from exposure to primary disease is 14 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.
Cases of blastomycosis are reported from various regions (particularly central Africa), but the vast majority of cases occur in North America. Blastomycosis is most common in the southeastern and south central United States flanking the Ohio and Mississippi River basins, the midwestern United States, the Canadian provinces surrounding the Great Lakes, and areas in Canada and the United States neighboring the Saint Lawrence Seaway. 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. There are pockets of hyperendemicity, such as northern Wisconsin, where the annual incidence of symptomatic infection may approach 40 per 100,000 population.