- Blastomyces dermatitidis causes blastomycosis — an endemic mycosis primarily of North America’s Great Lakes region, Mississippi and Ohio River valleys, and the Canadian provinces of Ontario, Manitoba, and Quebec.
- Unlike most other endemic mycoses, blastomycosis affects immunocompetent individuals as often as immunocompromised — a significant proportion of serious cases occur in people with no underlying immune deficiency.
- Blastomycosis has a particularly high rate of misdiagnosis: pulmonary cases are commonly treated as bacterial pneumonia for weeks to months before the correct diagnosis is established.
- Skin lesions — characteristic verrucous (wart-like) or ulcerative plaques — are the most common manifestation of disseminated blastomycosis and are often the presenting sign that leads to the correct diagnosis.
- The environmental reservoir of B. dermatitidis is incompletely understood; the organism is difficult to recover from soil despite decades of investigation, though it is associated with moist soil near waterways and decomposing organic matter.
Blastomyces dermatitidis is the causative agent of blastomycosis, a systemic fungal infection with a geographic distribution centered on the Great Lakes region and river valleys of North America. It distinguishes itself from other endemic mycoses by its willingness to cause serious disease in individuals with apparently normal immune function — making it one of the more unpredictable members of the endemic fungi group.
Taxonomy and Classification
Blastomyces dermatitidis Gilchrist and Stokes belongs to the family Ajellomycetaceae, order Onygenales, class Eurotiomycetes — the same order as Histoplasma and Coccidioides. The sexual (teleomorphic) state is Ajellomyces dermatitidis. Molecular analysis has revealed that the genus Blastomyces is more diverse than previously recognized: B. gilchristii is a sister species with an overlapping but more northern distribution; B. percursus and B. emzantsi have been described from Africa. These newly recognized species share morphological features with B. dermatitidis but can be distinguished molecularly.
Dimorphic Life Cycle
Like Histoplasma and Coccidioides, B. dermatitidis is thermally dimorphic:
- Mycelial phase (environmental, below 30°C): Produces a white to tan colony with hyphae bearing ovoid to pyriform conidia (2–10 μm) on short stalks directly from hyphae (aleurioconidia). These conidia are the infectious propagules — small enough to reach alveoli when inhaled.
- Yeast phase (at 37°C in tissue): Converts to large (8–15 μm), thick-walled yeast cells with a characteristic single broad-based bud — the “broad-based budding yeast” morphology is pathognomonic for Blastomyces and distinguishes it from Cryptococcus (which buds on a narrow neck) and Histoplasma (which is much smaller).
Geographic Distribution and Ecology
Blastomycosis is predominantly a North American disease. The highest incidence rates are in: Wisconsin, Minnesota, the Canadian provinces of Ontario, Manitoba, and Quebec, and portions of the Ohio and Mississippi River valleys. An “endemic triangle” encompassing the Great Lakes basin, the St. Lawrence River valley, and the Mississippi/Ohio River systems captures the majority of North American cases.
The environmental niche of B. dermatitidis has been difficult to characterize despite decades of investigation — the organism is rarely recovered from soil in systematic surveys. Epidemiological associations consistently implicate: moist soil near rivers, lakes, and streams; disturbed soil in construction and excavation; logging and forestry activities; and activities involving decaying wood and organic debris. Point-source outbreaks have been associated with beaver dam areas, construction sites, and hunting camps.
Clinical Presentations
- Pulmonary blastomycosis: The primary infection site in nearly all cases. Presentations range from mild, self-limited respiratory illness to severe acute respiratory distress syndrome (ARDS) — the latter carrying mortality exceeding 50% even with treatment. A significant proportion of pulmonary cases are initially treated as bacterial pneumonia with no response to antibiotics, with blastomycosis diagnosed only after weeks of clinical deterioration.
- Cutaneous blastomycosis: The most common extrapulmonary manifestation (present in 40–80% of disseminated cases). Characteristic verrucous (wart-like) plaques with a crusted surface, or ulcerative lesions, most commonly on exposed skin surfaces. Biopsy of skin lesions demonstrating broad-based budding yeast on pathology is often the route to diagnosis.
- Bone and joint involvement: Osteomyelitis (typically vertebral) and septic arthritis occur in approximately 25–50% of disseminated cases.
- CNS blastomycosis: Occurs in fewer than 5% of cases overall but in 30–40% of AIDS patients with blastomycosis; carries very high mortality.
Diagnosis and Treatment
Diagnosis is by culture (the gold standard), direct microscopy of tissue, BAL, or wound material (broad-based budding yeast with refractile cell wall), urinary antigen testing (Blastomyces antigen cross-reacts with Histoplasma antigen), and serology. Treatment is itraconazole for mild-moderate disease; amphotericin B followed by itraconazole for severe pulmonary or disseminated disease.
Frequently Asked Questions
Where is blastomycosis most common?
Blastomycosis is primarily a disease of the Great Lakes region and river valleys of North America. States with the highest incidence include Wisconsin, Minnesota, Illinois, and Mississippi; Canadian provinces with high incidence include Ontario, Manitoba, and Quebec. Lower rates of disease are reported in the southeastern United States and Central Africa. Cases outside endemic regions are almost always in travelers or former residents of endemic areas.
Can healthy people get blastomycosis?
Yes — and this distinguishes blastomycosis from many other invasive fungal diseases. A substantial proportion of serious blastomycosis cases, including ARDS-level pulmonary disease, occur in otherwise healthy individuals with no identified immune deficiency. This is in contrast to diseases like cryptococcosis or invasive aspergillosis, which almost exclusively cause serious disease in immunocompromised patients. Exposure to high inoculum (such as during point-source events at excavation sites) can overwhelm even healthy immune defenses.
What do blastomycosis skin lesions look like?
Blastomycosis skin lesions classically appear as verrucous (wart-like) plaques with a rough, crusted surface and raised, heaped-up borders, typically appearing on the face, arms, and legs. Ulcerative lesions are also common. They are painless and may be mistaken for squamous cell carcinoma, pyoderma, or other skin conditions. Biopsy is required for diagnosis and typically shows the characteristic broad-based budding yeast within tissue macrophages.
How long does blastomycosis treatment take?
Treatment duration depends on the form and severity of disease. For mild-moderate pulmonary blastomycosis, itraconazole is given for 6–12 months. For disseminated disease, treatment typically lasts 6–12 months with itraconazole following an amphotericin B induction. CNS blastomycosis and bone involvement may require 12 months or longer. Unlike coccidioidal meningitis, blastomycosis generally does not require lifelong therapy in immunocompetent individuals.
Why is blastomycosis so often misdiagnosed?
Pulmonary blastomycosis mimics bacterial pneumonia — fever, productive cough, and infiltrates on chest X-ray — leading most clinicians to initiate antibiotic therapy. Without clinical improvement, subsequent workup may include CT scan, bronchoscopy, or lung biopsy, at which point the diagnosis is often established. Additionally, outside endemic zones, clinicians may not consider blastomycosis in the differential diagnosis. Delays of months between symptom onset and diagnosis are common, during which progressive lung damage may occur.
