Is Blastomycosis a Rare Disease?
Blastomycosis is not a rare disease within its endemic regions, but it is geographically restricted and uncommon outside these areas. The disease occurs most commonly in well-defined geographic regions of North America, making it an "endemic mycosis" rather than a truly rare condition 1.
Geographic Distribution and Endemicity
Blastomycosis is endemic to specific regions of North America, which fundamentally shapes its epidemiological classification:
Endemic areas include: southeastern and south central states bordering the Mississippi and Ohio Rivers, midwestern states and Canadian provinces bordering the Great Lakes, and a small area of New York and Canada adjacent to the St. Lawrence Seaway 1.
Within endemic regions, several studies have documented areas of hyperendemicity where the rate of blastomycosis is unusually high, indicating that the disease is not rare in these specific locations 1.
Point-source outbreaks have been well-documented, associated with occupational and recreational activities along streams or rivers involving exposure to moist soil enriched with decaying vegetation 1.
Clinical Context: Frequency vs. Recognition
The characterization of blastomycosis as "rare" depends critically on geographic context:
In endemic areas: The disease is sufficiently common that it should be on the differential diagnosis for pneumonia, skin lesions, and other organ system infections 2.
Outside endemic areas: Blastomycosis is indeed rare and often difficult to diagnose because of its unusual occurrence in most geographic areas 3.
Asymptomatic infection occurs in at least 50% of infected persons during point-source epidemics, suggesting that actual exposure and infection rates are substantially higher than diagnosed symptomatic cases 1.
Epidemiological Patterns
The disease shows distinct epidemiological characteristics that argue against classification as a truly rare disease:
Epidemic cases related to point-source exposure include patients of all ages and both sexes 4, 2.
Endemic cases typically occur in young to middle-aged adults, with more men than women reported 4, 2.
Increasing recognition in immunocompromised patients: The proportion of immunocompromised patients with blastomycosis increased from approximately 3% (1956-1977) to almost 24% (1978-1991), suggesting either increased recognition or true increased incidence 5.
Clinical Implications
The practical significance is that blastomycosis should not be dismissed as "too rare to consider" in endemic regions:
The disease has been described as "the great pretender" because it manifests in a wide range of symptoms and disease severity, making it easily overlooked 6.
Pneumonia is the most common manifestation, and the lung is almost always the organ initially infected, with presentations ranging from acute bacterial pneumonia-like illness to chronic destructive lung disease mimicking tuberculosis or lung cancer 4, 2.
Extrapulmonary disease occurs in 25-40% of patients, with skin, bones, and genitourinary system being the most frequent sites 1.