Blastomyces dermatitidis is a thermally dimorphic fungus that causes blastomycosis, a systemic and sometimes life-threatening infection. As a dimorphic organism, it changes its physical form depending on the surrounding temperature. In the environment, it exists as a filamentous mold. Upon entering the warm body of a human or animal host, it transforms into a thick-walled yeast form.
The Fungus and Its Environmental Habitat
Blastomyces dermatitidis thrives in environments characterized by moisture and rich organic material, such as decaying wood, leaves, and soil. It is a saprobe, meaning it obtains nutrients from dead organic matter, and prefers a slightly acidic soil environment.
The fungus is chiefly endemic to specific regions in North America, particularly the states bordering the Ohio and Mississippi River Valleys. It is also prevalent in areas around the Great Lakes, including parts of the midwestern United States and Canadian provinces like Ontario and Manitoba. Cases have also been reported in Africa and India, indicating a wider global distribution. Activities that disturb the soil, such as construction, excavation, and certain outdoor recreation, increase the risk of exposure in these endemic regions.
Mechanism of Infection
Infection begins when microscopic spores, known as conidia, are inhaled into the lungs. These conidia are released into the air when contaminated soil or decaying organic matter is disrupted. The fungal particles are small enough to reach the lower respiratory tract, where the infection process is initiated.
Once inside the warm mammalian lung, the temperature shift triggers the fungus to transition from its mold form into the pathogenic yeast form. This yeast form is significantly larger than the inhaled conidia and has a thick cell wall. This structure provides resistance against the host’s immune defenses, such as phagocytosis by alveolar macrophages, allowing the fungus to establish and spread the infection.
Person-to-person spread is extremely rare and has not been reliably demonstrated. Transmission from infected animals, most commonly dogs, to humans is also exceedingly uncommon, usually only occurring through accidental inoculation or a bite wound.
Clinical Manifestations of Blastomycosis
The clinical presentation of blastomycosis is highly variable, earning it the nickname “the great pretender” because its symptoms often mimic other conditions. Approximately half of all infected individuals may remain entirely asymptomatic, with their immune systems clearing the fungus without any noticeable illness. When symptoms do occur, the incubation period typically ranges from three weeks to three and a half months after exposure.
Pulmonary Blastomycosis
The most common form of the disease is Pulmonary Blastomycosis, as the lungs are the primary site of entry. This can manifest as an acute illness resembling bacterial pneumonia or a flu-like condition, with symptoms such as fever, chills, muscle aches, and a cough.
In severe cases, acute pulmonary infection can rapidly progress to Acute Respiratory Distress Syndrome (ARDS), a life-threatening form of respiratory failure. The infection may also develop into a chronic pulmonary disease that can be mistaken for tuberculosis or lung cancer. Symptoms in this chronic stage include persistent cough, low-grade fever, night sweats, and weight loss.
Disseminated Blastomycosis
If the fungus is not contained in the lungs, it can spread through the bloodstream and lymphatic system, leading to Disseminated Blastomycosis. Extrapulmonary dissemination occurs in about 25% to 40% of cases and can affect nearly any organ system.
The skin is the most frequent site of spread outside the lungs. Cutaneous Blastomycosis lesions are often described as verrucous, or wart-like, with raised, well-defined borders and sometimes surface crusting. Other common sites for dissemination include the bones and joints, causing osteomyelitis, and the genitourinary tract.
Diagnosis and Treatment Overview
Diagnosing blastomycosis requires a high degree of clinical suspicion, especially in non-endemic areas, due to the non-specific nature of the symptoms. Definitive diagnosis often relies on the direct visualization of the characteristic yeast form in clinical samples. This form is distinctively large (8 to 15 micrometers), featuring a thick, double-contoured wall and a broad-based bud.
Specimens like sputum, pus from skin lesions, or tissue biopsies can be examined under a microscope after staining. Fungal culture of clinical specimens is a highly sensitive method for diagnosis, though growth can take anywhere from a few days up to a month. Non-culture-based tests, such as urine antigen detection and serology, are also commonly used. The urine antigen test, which detects a component of the fungal cell wall, is useful for rapid initial screening.
Treatment for blastomycosis is always necessary, even for mild cases, to prevent the risk of spreading or recurrence. The choice of medication depends on the severity and location of the disease.
For mild to moderate infections that do not involve the central nervous system, the oral antifungal medication itraconazole is the standard treatment, typically lasting for six to twelve months. For severe or life-threatening cases, especially those with extensive lung involvement or CNS infection, intravenous amphotericin B is used first. Therapy is usually switched to oral itraconazole once the patient has stabilized to complete the long course of treatment.